Print
An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters
expand article infoRahul Mehrotra§, Manuel A. Caballer Gutiérrez|, Chad M. Scott#, Spencer Arnold#, Coline Monchanin§¤, Voranop Viyakarn, Suchana Chavanich
‡ Chulalongkorn University, Bangkok, Thailand
§ Aow Thai Marine Ecology Center, Koh Mun Nai, Thailand
| American University of Paris, Paris, France
¶ Muséum national d’Histoire naturelle, Paris, France
# Conservation Diver, Evergreen, United States of America
¤ University Paul Sabatier, Toulouse, France
Open Access

Abstract

Improved access to field survey infrastructure throughout South-East Asia has allowed for a greater intensity of biodiversity surveys than ever before. The rocky bottoms and coral reef habitats across the region have been shown to support some of the highest sea slug biodiversity on the planet, with ever increasing records. During the past ten years, intensive SCUBA surveys have been carried out at Koh Tao, in the Gulf of Thailand, which have yielded remarkable findings in sea slug biology and ecology. In this work a brief history of sea slug biodiversity research from Thailand is covered and a complete inventory of sea slugs from Koh Tao, Gulf of Thailand is provided. This inventory is based on surveys from 2012 to 2020, with previously unreported findings since 2016. Habitat specificity and species-specific ecology are reported where available with a focused comparison of coral reef habitats and deeper soft-sediment habitats. The findings contribute 90 new species records for Thai waters (92 for the Gulf of Thailand) and report a remarkable consistency in the proportional diversity found to be exclusive to one habitat type or another. Additionally, taxonomic remarks are provided for species documented from Koh Tao that have not been discussed in past literature from Thailand, and a summary of previous records in the Indo-West Pacific is given.

Keywords

Biodiversity exploration, coral reefs, Gulf of Thailand, Heterobranchia, soft sediment habitats

Introduction

Contemporary sea slug research is largely dominated by investigations into biochemistry, taxonomy, and systematics of the vast diversity of species currently known. Broader aspects remain largely understudied such as development, trophic ecology, and biogeography. Recent years have seen a dramatic increase in the abundance of biodiversity inventories, particularly from regions where much of this work had been sparse before. The importance of documenting local and regional species ranges and diversity is often overlooked despite such studies contributing to our understanding of large-scale environmental issues such as increasing ocean temperatures (Nimbs et al. 2016; Goddard et al. 2018; Ekimova et al. 2019). The problem of invasive species too relies heavily on understanding native and non-native species ranges (Zenetos et al. 2010; Nimbs and Smith 2018). It is thus vital that efforts be made to increase localised biodiversity monitoring, particularly at a time where the rate of change in terrestrial and marine environments is unprecedented and may drive significant biodiversity change and loss (Cowie et al. 2017). The documentation and understanding of baselines in localised species diversity allow for more accurate understanding of biodiversity change both spatially and across time (Nimbs and Smith 2018).

Among the earliest records of sea slugs from Thailand were provided by Bergh (1902) via the ‘Danish Expedition to Siam 1899–1900’ in which he provided records of 22 species (1 Sacoglossa, 3 Cephalaspidea, 4 Aplysiida, and 14 Nudibranchia), all from the Gulf of Thailand. Few additional records were made over the next century, notable inclusions being those of Jensen (1989, 1992), Brunckhorst (1993), and Swennen (1997). These were summarised with more records added in the first review of sea slugs from Thailand by Jensen (1998) and later numerous additional records were provided by Swennen et al. (2001). Thus, in the century since Bergh’s first documentation, the documented diversity in Thai waters reached 81 species, with 46 being recorded within the Gulf of Thailand. However, numerous species documented by Swennen et al. (2001) were only known from shells or remnants and therefore little ecological information could be gained. The waters of Thailand are extremely well suited for comparative investigations between the Indian Ocean and Western Pacific biodiversity and ecology. The Gulf of Thailand has, however, been consistently reported to host a lower diversity of marine life (i.e., Putchakarn and Sonchaeng 2004; Chanmethakul et al. 2010; Wallace et al. 2012), and is therefore often subject to less intensive sampling efforts and in fewer sampling sites. This trend extends to sea slugs with the most extensive review of nudibranch taxa in Thailand being conducted by Chavanich et al. (2013). Their work increased the known biodiversity and biogeography of sea slugs within Thai waters and documented a far greater diversity of taxa on the Andaman coast than the Gulf.

A dedicated survey effort was carried at the island of Koh Tao in the Gulf of Thailand which combined citizen science efforts with in-situ survey techniques and resulted in a dramatic increase in the documented diversity for the Gulf (Mehrotra and Scott 2016). Much of this increase was attributed to previously unexplored soft sediment habitats beyond the slope of fringing coral reefs, which appeared to host a high diversity of species that were not seen in shallower habitats nearer to coral reefs. Subsequent surveys in these habitats revealed a number of novel species descriptions and ecological features that had hitherto been overlooked (Mehrotra et al. 2017, 2019, 2020b). In the present work we summarise the findings of dedicated surveys subsequent to these studies and document a further expansion in sea slug biodiversity in Thai waters. In doing so and by providing a full inventory with known ecology of all species recorded at Koh Tao, we aim to provide a comprehensive baseline on the diversity and ecology of sea slugs in the region.

Materials and methods

Surveys and sampling

Benthic surveys were carried out using SCUBA at Koh Tao, Thailand, between January 2016 and February 2020. Roving-diver surveys were performed over both coral reef and soft sediment habitats. Belt transect surveys were also executed across both habitats following the Ecological Monitoring Protocol according to Scott (2012). The majority of surveys were done during the daytime, with a few night-time surveys achieved sporadically throughout the period. Survey intensity ranged from two to 16 people per survey and were carried out through most of the year. During the monsoon season (October–January), survey intensity was reduced to zero surveys per month in some months but were usually between ten to 20 surveys per month throughout the rest of the year. Surveys were carried out throughout the island coast and surrounding pinnacles (Fig. 1). Approximate coordinates for each site are provided in Table 1. As part of these documented surveys, approximately 9200 individual sea slug sightings were recorded across all habitats. Surveys were carried out in equal measure on the coral reef (dominated by scleractinian hard coral substrate) and soft sediment areas.

Table 1.

Surveyed sites with location codes given in each species after specimen size.

Location Code Coordinates
Leuk Bay LB 10°4'11.65"N, 99°50'34.42"E
Suan Olan Artificial Reef SO 10°4'6.70"N, 99°50'26.29"E
Coral-Aid Artificial Reef CA 10°4'20.96"N, 99°50'31.84"E
Shark Island SI 10°3'41.20"N, 99°50'40.54"E
Sai Daeng SD 10°3'49.43"N, 99°50'23.80"E
Shark Bay SB 10°3'39.75"N, 99°50'4.43"E
Chalok Bay CB 10°3'44.77"N, 99°49'30.35"E
Tao Tong TT 10°3'58.13"N, 99°49'4.76"E
Sai Nuan and Three Rocks SN 10°4'45.02"N, 99°48'45.23"E
Mae Haad MH 10°5'22.53"N, 99°49'14.07"E
Sairee Beach SRB 10°6'0.99"N, 99°49'15.89"E
Hin Pee Wee HPW 10°6'19.94"N, 99°48'47.73"E
Sattakut Wreck SW 10°6'16.97"N, 99°48'47.52"E
White Rock WR 10°6'27.94"N, 99°48'48.98"E
Hin Fai Artificial Reef HF 10°6'43.42"N, 99°49'7.18"E
Twins TW 10°7'1.93"N, 99°48'44.26"E
Green Rock GR 10°7'31.24"N, 99°48'49.57"E
Red Rock RR 10°7'19.92"N, 99°48'55.31"E
Mango Bay MB 10°7'22.52"N, 99°50'5.06"E
Hin Wong Pinnacle HWP 10°6'47.51"N, 99°51'1.95"E
Hin Wong Bay HWB 10°6'12.30"N, 99°50'58.63"E
Mao Bay North AMN 10°5'51.85"N, 99°51'7.69"E
Mao Bay AM 10°5'32.95"N, 99°51'9.29"E
Laem Thien LT 10°5'19.13"N, 99°51'17.64"E
Tanote Bay TB 10°5'1.47"N, 99°50'57.50"E
King Kong Rocks KKR 10°4'30.25"N, 99°50'46.46"E
Chumphon Pinnacle CP 10°10'20.52"N, 99°46'44.49"E
Southwest Pinnacle SWP 9°59'56.22"N, 99°46'44.28"E
Sail Rock SR 9°56'42.47"N, 99°59'26.46"E
Figure 1. 

Map of Koh Tao showing surveyed locations in reference to material examined. The inset of the Gulf of Thailand shows the location of Koh Tao.

The soft sediment habitats, located outside coral reefs, had a surface substrate composition of > 90% sand or silt particles. These are typically large areas lacking natural solid substrates and are therefore deficient in rugosity and stability over longer periods (discussed below). Areas of sandy substrate within or in close proximity to coral-dominated areas were included as coral reef (Fig. 2). In the coral reef regions, the authors surveyed in particular among coral rubble, aforementioned sandy patches, and the underside of dead Fungiidae (Anthozoa: Hexacorallia: Scleractinia) corals. Previous surveys (Mehrotra and Scott 2016) had preliminarily suggested that closer examination of these areas could yield a host of cryptic taxa. Soft sediment areas with close proximity to coral reef habitats, and those with a substrate composition of > 10% hard substrate (particle sizes or corals > ca. 5 cm), were considered as coral reef or reef edge for the surveys and were assessed visually. Thus, there was always clear separation between habitats considered to be coral reef and the deeper soft sediment habitats (referred to here simply as soft sediments). Indicators for soft sediment habitats based on our definitions included organisms that have already been found to grow exclusively in these areas such as sea pens, specific macroalgal species, and specific free-living Scleractinia among others (Mehrotra et al. 2017, 2019).

Figure 2. 

Schematic outline of benthic zones classified in the surveys conducted at Koh Tao A fringing reef slope, usually near-shore B reef edge as determined by drastic reduction in reef-building scleractinian abundance, leading to zone of no stable substrate nor any Scleractinia C soft sediment habitats characterised by sand/silt dominated substrates and colonised by organisms absent/extremely rare in zones A and B. D Deepening of soft sediment slope resulting in a drastic reduction but not absence of soft-sediment colonisers. Illustrated by Pau Urgell Plaza.

For each documented species, a small number of specimens was examined closely for taxonomic purposes, with the vast majority of subsequent specimens recorded being noted for their ecology or simply their presence. Detailed specimen examination was carried out in-situ where possible or after sampling using high-magnification underwater photography. Ex-situ examination was carried externally on live specimens which were collected by hand and subsequently returned to their original habitats. All living specimens studied are here documented as ‘material examined’. Specimens were externally identified by the authors aided by in-situ photographs based on relevant literature and contrasted with known species prevalence in Thai waters (see Table 2). Additionally, taxon validity was confirmed with the most recent literature and assisted in part with the World Register of Marine Species (WoRMS 2021) and the references contained within.

Table 2.

List of published literature between 1989–2020 in which sea slug diversity and distribution records in Thai waters are contributed.

References Gulf of Thailand diversity Total Thailand diversity
Jensen 1989 19 20
Jensen 1992 19 24
Brunckhorst 1993 19 38
Gosliner and Johnson 1994 20 39
Jensen 1998; Swennen 1997 28 63
Swennen 2001; Swennen et al. 2001 46 81
Jensen 2007; Swennen 2007; Robba et al. 2007 49 88
Swennen and Buatip 2009 49 88
Nabhitabhata 2009 50 100
Swennen 2011 51 101
Swennen and Buatip 2012 52 102
Chavanich et al. 2013 111 203
Jensen et al. 2014a 111 204
Mehrotra and Scott 2016 154 239
Martynov et al. 2019; Korshunova et al. 2019 156 241
Mehrotra et al. 2020a, b; Wang et al. 2020 160 245
Present study 256 336
Figure 3. 

Habitat specificity of the major sea slug groups at Koh Tao. Number of species exclusively found at each habitat type at Koh Tao, compared with the number of species recorded from both habitat types. Two species without a confirmed benthic habitat type were recorded as N/A.

Results

In total, 191 species of heterobranch sea slugs have been documented from Koh Tao to date across the orders Sacoglossa (14 species), Cephalaspidea (23 species), Aplysiida (5 species), Pleurobranchida (3 species), and Nudibranchia (146 species). These results more than double the known taxa from Koh Tao (new records for Koh Tao identified below by an asterisk *) and contribute a further 90 first records for Thai waters (96 for the Gulf of Thailand) from the island (not including the 32 species first documented in Mehrotra and Scott 2016). Approximately half of all recorded species (N = 92) were found exclusively within coral reef habitats, with 68 species (36%) being exclusively recorded from the soft sediment habitats and 28 species recorded in both habitats. Two species could not be attributed to a particular benthic habitat type (see remarks for Limapontiidae sp. and Scyllaea fulva). A large number of species were found exclusively or in far greater abundance under the skeletons of dead Fungiidae corals (see discussion).

Systematics

Class Gastropoda Cuvier, 1795

Subclass Heterobranchia Burmeister, 1837

Superorder Panpulmonata Jörger, Stöger, Kano, Fukuda, Knebelsberger & Schrödl, 2010

Order Sacoglossa Ihering, 1876

Family Costasiellidae Clarke, 1984

Genus Costasiella Pruvot-Fol, 1951

Costasiella cf. kuroshimae Ichikawa, 1993

Figure 4A

Material examined

Two specimens 3–6 mm, LB; two specimens 4–6 mm, SN.

Ecology

In soft sediment habitats, beyond the coral reef where it feeds predominantly on Avrainvillea longicaulis (Kützing) G. Murray & Boodle, 1889 and less commonly on Vaucheria sp. Depth 10–18 m.

Distribution

Costasiella kuroshimae is currently known from the Indo-Pacific including the Red Sea (Yonow 2015), Singapore (Jensen 2009), Indonesia (Eisenbarth et al. 2018), Japan (Ichikawa 1993), Guam (Jensen et al. 2014b), Madagascar, Tanzania, Malaysia, Papua New Guinea, Palau, and Australia (Gosliner et al. 2008). Known from the Gulf waters of Thailand (Mehrotra and Scott 2016).

Remarks

Due to the original description of the species being entirely based on external features, the identity of numerous similar species and the extent of the variability of the species has remained unclear for several years. Molecular work (Jensen et al. 2014b) has shown that specimens identified as Costasiella kuroshimae or C. cf. kuroshimae actually make up numerous distinct species that currently await description. Research carried out on specimens from Koh Tao (Mehrotra et al. 2019) indicates that this species is palatable to some scleractinian corals and, based on natural observations, may be viable prey for the free-living coral Heteropsammia cochlea (Spengler, 1781), in soft sediment habitats.

Figure 4. 

A Costasiella cf. kuroshimae 6 mm B Costasiella usagi 6 mm C Limapontiidae sp. 3 mm D Plakobranchus noctisstellatus 21 mm (photograph by Pau Urgell Plaza) E Plakobranchus ocellatus 32 mm F Plakobranchus papua 27 mm G Plakobranchus papua 21 mm on Holothuria edulis H Elysia aowthai 12 mm I Elysia asbecki 15 mm J Elysia cf. marginata 70 mm K Elysia mercieri 12 mm L Elysia obtusa 9 mm.

Costasiella usagi Ichikawa, 1993

Figure 4B

Material examined

Three specimens 2–6 mm, LB.

Ecology

In soft sediment habitats beyond the coral reef where it feeds predominantly on Avrainvillea longicaulis and less commonly on Vaucheria sp. An individual of the species has been observed to be naturally captured and ingested by the free-living coral Heteropsammia cochlea with subsequent investigations suggesting the species may represent viable prey for the coral in soft sediment habitats (Mehrotra et al. 2019). Depth 10–18 m.

Distribution

Across the Indo-Pacific including India (Dixit et al. 2017), Singapore (Jensen 2009), Japan (Ichikawa 1993), Malaysia, the Philippines, Papua New Guinea, Australia, and Guam (Gosliner et al. 2008). Known from the Gulf waters of Thailand (Mehrotra and Scott 2016).

Family Limapontiidae Gray, 1847

Limapontiidae sp.

Figure 4C

Material examined

One specimen 3 mm, location unknown.

Ecology

Local ecology is unknown.

Distribution

Unknown.

Remarks

Similar to Ercolania translucens Jensen, 1993 or Stiliger sp. 7 in Gosliner et al. (2018) in having cerata with elongated white apices and green pigment on the head not extending to the white rhinophores. The specimen was observed in a holding tank several days after a broad sampling effort of multiple algal species from Koh Tao. Algae were collected from several locations at the south of the island and although they had been checked carefully, the cryptic nature of the species allowed it to be overlooked. The species was recorded only once in April 2015 and has not been recorded since. The internal anatomy of the specimen was not studied and thus it could not be identified to genus level.

Suborder Plakobranchacea Gray, 1840

Superfamily Plakobranchoidea Gray, 1840

Family Plakobranchidae Rang, 1829

Genus Plakobranchus van Hasselt, 1824

Plakobranchus noctisstellatus Mehrotra, Caballer, Scott, Arnold, Monchanin & Chavanich, 2020

Figure 4D

Material examined

One specimen 28 mm, SN; two specimens 26–31 mm, TT.

Ecology

From deeper soft sediments outside coral reef habitats. Depth 15–25 m.

Distribution

Vanuatu, Indonesia, Papua New Guinea (Gosliner et al. 2008, 2015) and the Gulf of Thailand (Mehrotra et al. 2020b).

Plakobranchus ocellatus van Hasselt, 1824

Figure 4E

Material examined

Three specimens 25–32 mm, CB.

Ecology

From shallow soft sediments to sandy areas along the reef edge. Rarely in deeper soft sediment habitats beyond the reef edge. Depth 0.5–11 m.

Distribution

Plakobranchus ocellatus and P. cf. ocellatus are currently considered widespread across the Indo-Pacific including Kenya, Zanzibar, the Red Sea, Maldives, Seychelles, Réunion (Yonow 2012), India (Sheeja and Padma Kumar 2014), the Philippines (Christa et al. 2012), Indonesia (Eisenbarth et al. 2018; Yonow and Jensen 2018), Japan (Maeda et al. 2012), Australia, Papua New Guinea (Yonow and Jensen 2018), Guam (Wägele et al. 2011), Vanuatu (Krug et al. 2013), Hawaii (Wade and Sherwood 2016), Tanzania, Madagascar, Malaysia and Palau (Gosliner et al. 2008). Specimens considered as P. ocellatus have been previously recorded from the Andaman and Gulf waters of Thailand (Jensen 1992; Nabhitabhata 2009).

Remarks

Specimens from Koh Tao were recently reviewed by Mehrotra et al. (2020b).

Plakobranchus papua Meyers-Muñoz & van der Velde in Meyers-Muñoz et al., 2016

Figure 4F, G

Material examined

Three specimens 19–30 mm, SN.

Ecology

Abundant in shallow soft sediment habitats and among the corals and soft sediments of the reef edge. Uncommon, but present in dense coral reef habitats. Rare in deeper soft sediment habitats outside the coral reef. Has been observed being ingested naturally by the scleractinian coral Pleuractis paumotensis (Stutchbury, 1833) but is mostly considered unpalatable by such corals (Mehrotra et al. 2015, 2019). During daytime surveys, a single observation was made of P. papua crawling upon the sea cucumber Holothuria edulis Lesson, 1830 (Fig. 4G), which may have been considered unremarkable were it not for the findings of Mercier and Hamel (2005). Depth 1–19 m.

Distribution

Known only from the Philippines, Malaysia, Indonesia, and Papua New Guinea (Meyers-Muñoz et al. 2016; Yonow and Jensen 2018). Known from Gulf waters of Thailand (Mehrotra et al. 2020b).

Remarks

Specimens from Koh Tao were recently reviewed (Mehrotra et al. 2020b).

Elysia aowthai Mehrotra, Caballer, Scott, Arnold, Monchanin & Chavanich, 2020

Figure 4H

Material examined

One specimen 14 mm, LB; one specimen 16 mm, TT.

Ecology

From deeper soft sediments outside coral reef habitats. Depth 10–24 m.

Distribution

Guam, Australia, and the Gulf of Thailand (Mehrotra et al. 2020b).

Genus Elysia Risso, 1818

Elysia asbecki Wägele, Stemmer, Burghardt & Händeler, 2010

Figure 4I

Material examined

One specimen 15 mm, HF; one specimen 23 mm, TW.

Ecology

In coral reef habitats throughout the island. Depth 3–18 m.

Distribution

Australia, Samoa (Wägele et al. 2010), the Philippines, Indonesia, Papua New Guinea, Japan, Guam, and Hawaii (Gosliner et al. 2008; Wägele et al. 2010). Known from the Gulf waters of Thailand (Mehrotra and Scott 2016).

Elysia cf. marginata (Pease, 1871)

Figure 4J

Material examined

Three specimens 65–82 mm, CB.

Ecology

Mostly recorded from specimens inhabiting a shallow, isolated patch of Halimeda macroloba Decaisne, 1841 in soft sediment habits, although presumably feeding on other nearby algae. Also observed from shallow coral reef habitats, rarely. Depth 0.5–6 m.

Distribution

Elysia marginata is at present recorded from the Indo-Pacific including Myanmar (Sanpanich and Duangdee 2019), Vietnam (Martynov and Korshunova 2012), Indonesia (Yonow and Jensen 2018), Australia (Nimbs and Smith 2016), Japan, Guam, French Polynesia, Vanuatu, and Hawaii (Krug et al. 2013). Specimens from South Africa, Madagascar, and Réunion (Gosliner et al. 2008 as Elysia ornata) also are likely to correspond to this complex. Known from the Andaman waters of Thailand (Jensen 1992), here representing a first record for the Gulf of Thailand.

Remarks

Recent molecular investigations (Krug et al. 2013) have indicated up to four possible clades making up the species Elysia marginata, that was separated from its Caribbean counterpart Elysia ornata (Swainson, 1840), which was formerly considered circumtropical. Yonow and Jensen (2018) further discuss the challenges in assigning all specimens with the ‘characteristic’ orange and black marginal bands on the parapodia to E. marginata as similar species such as E. faustula Bergh, 1871 and E. grandifolia Kelaart, 1858 were described and illustrated with comparable features. Both aforementioned species differ in ground colour or the presence/absence of denticulation on radular teeth, and both of these features have been shown to be variable within a single species and often a single specimen (Mehrotra et al. 2020b). Therefore, all indications point to a need for a comprehensive analysis integrating morphology, ecology, and molecular data to delineate species in this complex.

Elysia mercieri (Pruvot-Fol, 1930)

Figure 4K

Material examined

One specimen 12 mm, SO.

Ecology

Found upon concrete artificial reefs in soft sediment habitats that formed part of coral restoration efforts. Depth 11–14 m.

Distribution

Across the Indo-Pacific including the Red Sea (Yonow 2015), Indonesia (Eisenbarth et al. 2018), Japan (Trowbridge et al. 2011), Mariana Islands (Carlson and Hoff 2003), Malaysia, Papua New Guinea, New Caledonia, and Guam (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

Elysia mercieri is known to be predated upon by the nudibranch Gymnodoris okinawae Baba, 1936 (Nakano and Hirose 2011).

Elysia obtusa Baba, 1938

Figure 4L

Material examined

One specimen 9 mm, CB; one specimen 12 mm, TW.

Ecology

Among rubble, particularly found underneath the skeletons of dead Fungiidae corals, in shallow coral reef habitats. No association with prey was observed but is known to be part of a group of species feeding on the alga Bryopsis (Krug et al. 2016). Depth 3–8 m.

Distribution

Across the Indo-Pacific including India (Apte et al. 2010), Taiwan (Huang et al. 2016), Hong Kong (Jensen 2003), Japan (Trowbridge et al. 2011), Australia (Nimbs and Smith 2016), Samoa (Wägele et al. 2010), Madagascar, Malaysia, Papua New Guinea, the Philippines, Korea, Guam, Marshall Islands, and Hawaii (Gosliner et al. 2008 as Elysia flava Verrill, 1901). Here representing a first record for Thai waters.

Remarks

Both individuals recorded were found adhering to the underside of dead fungiid skeletons. The species is locally rare and here included as a first record for the Gulf of Thailand and Thai waters in general.

Elysia pusilla (Bergh, 1871)

Figure 6A

Material examined

Three specimens 3–7 mm, CB; one specimen 4 mm, SRB.

Ecology

Feeds on Halimeda macroloba, on which it is highly cryptic. Host and prey found in soft sediment habitats near coral reefs between 0.5 and 9 m depth. Multiple individuals may be found feeding on a single prey item. Populations of the host algae H. macroloba have been found at only two locations at the island, 6–9 m depth at SRB and a small intertidal patch at site CB at 0.5–1.5 m. The abundance of E. pusilla has been found to be greater at CB than on the larger but less dense population of H. macroloba at site SRB.

Distribution

Widespread across the Indo-Pacific including the Red Sea (Yonow 2008), Réunion (Bourjon et al. 2018), India (Sreeraj et al. 2012), Singapore (Jensen 2009), Vietnam (Martynov and Korshunova 2012), Indonesia (Eisenbarth et al. 2018), Australia (Nimbs and Smith 2016), Japan, Guam (Vendetti et al. 2012), Mexico (Goddard and Hermosillo 2008), Costa Rica (García-Méndez and Camacho-García 2016), South Africa, Tanzania, Madagascar, New Caledonia, and Hawaii (Gosliner et al. 2008). Known from the Andaman waters of Thailand (Jensen 1992), and here representing a first record for the Gulf of Thailand.

Remarks

The status of Elysia pusilla and its taxonomic implications for the genus needs closer investigation (Jensen 2009, 2015; Krug et al. 2016). Recent observations carried out by Mehrotra et al. (2019) indicate that E. pusilla specimens from Koh Tao are considered palatable to opportunistic scleractinian coral predators and are readily consumed by these reef building corals. However, to date, no instances of natural prey capture of E. pusilla by these corals has been documented.

Elysia cf. tomentosa Jensen, 1997

Figure 6B

Material examined

One specimen 37 mm, CB.

Ecology

In soft sediment habitats outside the coral reef. Observed feeding on a pinnate form of Caulerpa racemosa (Forsskål) J. Agardh, 1873 which is found chiefly in soft sediment habitats beyond the coral reef. A more lenticular/globular form of the algae can be abundant in some shallow, degraded reef habitats; however, specimens from Koh Tao not been observed associated with this variety. Depth 12–20 m.

Distribution

At present Elysia tomentosa is considered widespread across the Indo-Pacific (but see remarks below) including Iran (Oladi et al. 2018), Madagascar, Malaysia (Gosliner et al. 2008), Australia (Jensen 1997), Réunion, India, Singapore, the Philippines, Indonesia, Japan, New Caledonia, and Hawaii (Jensen 2015). Likely introduced to the Mediterranean alongside the highly invasive Caulerpa spp. (Zenetos et al. 2010). Here representing a first record for Thai waters.

Remarks

Recent molecular investigations have found that specimens recognised as Elysia tomentosa likely correspond to a complex of at least six species (Krug et al. 2013). Discussions about the identity of specimens identifiable by black marginal lines along the parapodia and the similar species Elysia expansa (O’Donoghue, 1924) have yet to be resolved (Rudman 2009a; Krug et al. 2013). Additionally, Oladi et al. (2018) recently documented a species in this complex from Iran that matches a sequence of E. cf. tomentosa from the Andaman coast of Thailand, by Cornelius Swennen (GenBank accession number KC573755.1); however, no record of this species from the waters of Thailand has been found in the literature to date. Greater sampling efforts from more locations, with an assessment of internal characters, are needed to clarify this complex, including specimens from Koh Tao.

Genus Thuridilla Bergh, 1872

Thuridilla cf. gracilis (Risbec, 1928)

Figure 6C

Material examined

1 specimen 18 mm, HWB; 2 specimens 8–15 mm, LB.

Ecology

Found in coral reef habitats throughout the island. Depth 2–25 m.

Distribution

Thuridilla gracilis sensu lato is known from Maldives, Seychelles (Yonow 2012), India (Apte 2009), Singapore (Jensen 2009), Myanmar (Sanpanich and Duangdee 2019), Indonesia (Yonow and Jensen 2018), Vietnam (Martynov and Korshunova 2012), Taiwan (Huang et al. 2016), Japan (Trowbridge et al. 2011), Australia (Nimbs and Smith 2016), Madagascar, Malaysia, the Philippines, Papua New Guinea, Palau, New Caledonia, Guam, and Fiji (Gosliner et al. 2008). Known from the Andaman and Gulf waters of Thailand (Jensen 1992; Nabhitabhata 2009).

Remarks

The taxonomic status of Thuridilla gracilis is at present unclear. Recent works (Yonow and Jensen 2018; Papu et al. 2020) have highlighted the significant variability of specimens ascribed to the species and questioned the synonymisation of multiple white-striped species of Thuridilla (Gosliner 1995; Rudman 2000b) under the single species T. gracilis. Indications suggest that the breadth of specimens currently considered as T. gracilis encompass a complex of species (Händeler and Wägele 2007; Yonow and Jensen 2018; Papu et al. 2020) that requires a comprehensive analysis utilising both morphological and molecular means. Specimens from Koh Tao do not possess any blue markings, instead being closer to the original description and illustration of T. gracilis in possessing a thin orange-red marginal band along the parapodia that meets medially and having the white lines often extending to the tips of the rhinophores. Papu et al. (2020) provided photographs highlighting the external variability on the heads of specimens from Bangka, Indonesia suggesting possible diagnostic value. As such, we have emulated this and provided the range of variation visible in the heads of adult specimens from Koh Tao (Fig. 5).

Figure 5. 

Variation in the heads of Thuridilla cf. gracilis at Koh Tao. Specimens 14 mm (A), 18 mm (B), 9 mm (C), 16 mm (D).

Order Cephalaspidea P. Fischer, 1883

Superfamily Cylichnoidea H. Adams & A. Adams, 1854

Family Colinatydidae Oskars, Bouchet & Malaquias, 2015

Genus Colinatys Ortea, Moro & Espinosa, 2013

Colinatys sp.

Figure 6D

Material examined

One specimen 3 mm, CB.

Ecology

Endobenthic in sand in coral reef habitats. Depth 3–11 m.

Distribution

The genus is currently recognised to be monospecific with Colinatys alayoi (Espinosa & Ortea Rato, 2004) known from the Bahamas, Cuba, Florida, and Martinique Island (Romani et al. 2015; Ortea Rato and Buske 2018). Indo-Pacific records are limited to specimens from Hawaii (Oskars et al. 2015) and Japan (SSW 2017) with species identity yet to be confirmed. Here representing a first record of the family from Thai waters.

Figure 6. 

A Elysia pusilla 7 mm B Elysia cf. tomentosa 37 mm C Thuridilla cf. gracilis 18 mm D Colinatys sp. 3 mm E Aliculastrum cylindricum 30 mm F Aliculastrum debilis 28 mm G A. debilis (shell 12 mm) captured alive and predated by Haptosquilla cf. nefanda H Roxaniella multistriata 9 mm I Atys semistriatus 8 mm J Atys sp. 11 mm K Diniatys dentifer 5 mm L Diniatys dubius 6 mm.

Superfamily Haminoeoidea Pilsbry, 1895

Family Haminoeidae Pilsbry, 1895

Genus Aliculastrum Pilsbry, 1896

Aliculastrum cylindricum (Helbling, 1779)

Figure 6E

Material examined

Two specimens 25–30 mm, LB.

Ecology

In soft sediment habitats outside the coral reef where it is found associated with cyanobacterial mats on the benthos. May also be found near the reef edge when cyanobacteria abundances increase, indicating a possible seasonal influence in abundance. Often found together with other Haminoeidae species from Koh Tao, apart from Atys sp. It may be a probable prey species for the mantis shrimp Haptosquilla cf. nefanda (Kemp, 1911) (see ecology of Aliculastrum debilis below). Depth 8–18 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Macnae and Kalk 1958), Tanzania, Madagascar, the Philippines, Papua New Guinea, New Caledonia (Gosliner et al. 2008), South Africa, Seychelles, Mauritius, India, Japan, China, Fiji, and Tahiti (Too et al. 2014). Known from the Gulf waters of Thailand (Nabhitabhata 2009).

Aliculastrum debilis (Pease, 1860)

Figure 6F, G

Material examined

Two specimens 30 mm, LB; one specimen 28 mm, TB.

Ecology

Extremely similar to A. cylindricum (see above). Often found together with other Haminoeidae species from Koh Tao, apart from Atys sp. As part of the present surveys, observations were made of hunting and capture of specimens of A. debilis by the stomatopod Haptosquilla cf. nefanda (Fig. 6G). Captured animals were taken into burrows made by the crustacean. While active feeding was not observed, it should be noted that shells of A. debilis, A. cylindricum, and rarely Atys semistriatus Pease, 1860 can be found in high abundance around the holes of H. cf. nefanda and other mantis shrimps in the soft sediment habitats. These shells are often broken, but not always, and are likely indications of predation by these crustaceans, which are abundant in these habitats. Depth 8–18 m.

Distribution

Across the western Pacific including the Philippines, Guam, Tahiti, Fiji, and Hawaii (Too et al. 2014). Known from the Gulf waters of Thailand (Mehrotra and Scott 2016).

Genus Roxaniella Monterosato, 1884

Roxaniella multistriata (Schepman, 1913)

Figure 6H

Material examined

Two specimens 5–9 mm, LB.

Ecology

Very similar to those of Aliculastrum spp. though more regularly found immersed within/under cyanobacterial mats than on top. Often found together with other Haminoeidae species from Koh Tao, apart from Atys sp. Depth 8–18 m.

Distribution

Known across the Indo-Pacific including Tanzania, the Philippines, Fiji (Gosliner et al. 2008), Indonesia, Palau, Guam, Tahiti (Too et al. 2014), and Hawaii (Kay 1979). Here representing a first record from Thai waters.

Genus Atys Montfort, 1810

Atys semistriatus Pease, 1860

Figure 6I

Material examined

Two specimens 8–12 mm, LB.

Ecology

Very similar to R. multistriata, with which it is often found. A possible prey species for the mantis shrimp Haptosquilla cf. nefanda (see ecology of Aliculastrum debilis). Depth 8–18 m.

Distribution

Across the Indo-Pacific including Japan (Kuroda and Habe 1952), Madagascar, Malaysia, the Philippines, Papua New Guinea, Guam, Samoa (Gosliner et al. 2008), Indonesia, New Caledonia, Tahiti, Hawaii (Too et al. 2014), and the Red Sea (Heller and Thompson 1983; Yonow 2008). Here representing a first record for Thai waters.

Atys sp.

Figure 6J

Material examined

One specimen 11 mm, AMB.

Ecology

In soft sediment habitats outside the coral reef. Depth 23 m.

Distribution

Atys sp. 6 (Gosliner et al. 2018) is currently known from the Philippines, Vanuatu, and Indonesia.

Diniatys Iredale, 1936

Diniatys dentifer (A. Adams, 1850)

Figure 6K

Material examined

Three individuals 2–5 mm, LB.

Ecology

Very similar to the other soft-sediment associated Haminoeidae such as Aliculastrum spp., R. multistriata etc., which are often found together. Depth 8–18 m.

Distribution

Diniatys dentifer is known from Madagascar, the Philippines, Japan, Indonesia, Papua New Guinea, Guam, Hawaii, French Polynesia (Too et al. 2014), and the Red Sea (Yonow 2008). Here documented as a first record for Thai waters.

Diniatys dubius (Schepman, 1913)

Figure 6L

Material examined

Three individuals 3–6 mm, LB.

Ecology

Very similar to D. dentifer. Depth 8–18 m.

Distribution

Diniatys dubius is known from the Philippines, Indonesia, Papua New Guinea, Guam, Hawaii (Too et al. 2014), and the Red Sea (Yonow 2008). Here documented as a first record for Thai waters.

Haloa Pilsbry, 1921

Haloa sp.

Figure 7A

Material examined

Three individuals 2–5 mm, LB.

Ecology

While rarer than most other soft sediment associated Haminoeidae spp., from Koh Tao, the strong association with cyanobacterial mats is a shared feature across these species. Depth 8–18 m.

Figure 7. 

A Haloa sp. 5 mm B Lamprohaminoea ovalis 18 mm C L. ovalis 9 mm (photograph by Elouise Haskin) D L. ovalis 4 mm E Chelidonura cf. castanea 62 mm F Chelidonura punctata 32 and 34 mm G Niparaya sp. 4 mm H Philinopsis speciosa 18 mm IPhilinopsiscoronata 35 mm (photograph by Phannee Mccarthy) J Tubulophilinopsis lineolata 38 mm K Tubulophilinopsis pilsbryi 35 mm L Tubulophilinopsis reticulata 30 mm (photograph by Kirsty Magson).

Distribution

Unknown.

Lamprohaminoea Habe, 1952

Lamprohaminoea ovalis (Pease, 1868)

Figure 7B, C, D

Material examined

Six individuals 9–35 mm, SB; eight individuals 6–21 mm; LB.

Ecology

White morphs (Fig. 7B) among rubble in shallow coral reef habitats at depths 4–8 m. Purple/red morphs in soft sediment habitats outside the coral reef where it is found strongly associated with mats of cyanobacteria on the benthos. Purple/red morphs (Fig. 7C, D) may also be found near the reef edge when cyanobacteria abundances increase, indicating a possible seasonal influence in abundance. Often found together with other Haminoeidae species from Koh Tao, apart from Atys sp. White and purple/red morphs not found together suggesting a fundamental division in local ecology (i.e., diet), potentially contributing to the difference in colouration. Specimens from Koh Tao have been observed to be ingested, and sometimes rejected post-ingestion, by the wrasses Cheilinus fasciatus (Bloch, 1791) and Thalassoma lunare (Linnaeus, 1758). This method of prey rejection has been suggested as a vector for prey dispersal of sea slugs onto opportunistic predatory scleractinian corals (Mehrotra et al. 2019). Depth 8–18 m.

Distribution

Lamprohaminoea ovalis known from the Red Sea, Oman, Philippines, Vanuatu, Guam, French Polynesia, Hawaii (Oskars and Malaquias 2020), Australia (Nimbs and Smith 2016), Mariana Islands (Carlson and Hoff 2003), Japan (Hori 2017), Marshall Islands (Marcus and Burch 1965), and invasive in the Mediterranean Sea (Fernández-Vilert et al. 2018). It is here recorded for the first time from Thai waters.

Remarks

With the recent comprehensive review of the genus Lamprohaminoea (Oskars and Malaquias 2020), specimens from Koh Tao were identified as L. ovalis. This identification was based on shell morphology and external colouration, the variability of both now being well documented. Despite specimens from Koh Tao being from the same species, the different morphs have been found to exhibit distinct ecological characteristics.

Superfamily Philinoidea Gray, 1850 (1815)

Family Aglajidae Pilsbry, 1895 (1847)

Genus Chelidonura A. Adams, 1850

Chelidonura cf. castanea Yonow, 1994

Figure 7E

Material examined

Two specimens 62–74 mm, TT.

Ecology

In soft sediment habitats outside the coral reef. Depth 22–26 m.

Distribution

Chelidonura castanea is currently known only from the Maldives (Yonow 1994). Unconfirmed sightings have also been made from Mozambique and Myanmar (iNaturalist 2011; TSS 2020). Here representing a first record for Thai waters (but see below).

Remarks

Specimens from Koh Tao differ from those originally described by lacking orange spots across the dorsum, instead having only two tiny orange spots on the anterior portion of the head, on either side of the mouth. Additionally, the body is uniformly deep reddish brown with a thin white line on the upper margin of the cephalic shield. In the larger specimen (74 mm), both orange spots and the white line were markedly less distinct. The presence and absence of yellow/orange spots in such Aglajids has been shown to be an unreliable character for species delimitations (Turner and Wilson 2012). To date, C. castanea is only known from the Indian Ocean; however, a distribution from Thailand was recorded, without reference to any source, by Gosliner et al. (2008) but omitted in later versions (Gosliner et al. 2018). Therefore, we hereby provide details of a similar species from the Gulf of Thailand waters as a first record.

Chelidonura punctata Eliot, 1903

Figure 7F

Material examined

Four specimens 32–39 mm, TT.

Ecology

In soft sediment habitats outside the coral reef. Depth 22–26 m.

Distribution

Chelidonura punctata is currently known from Kenya (Mangubhai 2007), Mozambique (Tibiriçá and Malaquias 2016), Zanzibar, Mauritius, the Chagos Islands, the Maldives (Yonow 2012), India (Apte 2009), and Myanmar (Sanpanich and Duangdee 2019). Chelidonura punctata has been recorded from the Andaman sea of Thailand (Gosliner et al. 2008; Nabhitabhata 2009) and is here recorded for the first time from the Gulf of Thailand.

Genus Niparaya Zamora-Silva & Malaquias, 2018

Niparaya sp.

Figure 7G

Material examined

Two specimens 4 mm, CB; one specimen 3 mm, TT.

Ecology. Among rubble in coral reef habitats and soft sediments near the reef edge. Depth 4–8 m.

Distribution

Niparaya sp. 3 is currently known only from eastern Malaysia (Gosliner et al. 2018).

Genus Philinopsis Pease, 1860

Philinopsis speciosa Pease, 1860

Figure 7H

Material examined

Two specimens 18–26 mm, SB; one specimen 14 mm, TT; one specimen 11 mm, SN.

Ecology

In soft sediment habitats outside the coral reef. Depth 14–26 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá and Malaquias 2016), Maldives (Yonow 1994), Vietnam (Martynov and Korshunova 2012), Australia (Nimbs and Smith 2016), Guam, Hawaii, the Galapagos Islands (Zamora-Silva and Malaquias 2018), South Africa, Tanzania, the Philippines, Indonesia, Papua New Guinea, Japan, Panama (Gosliner et al. 2008), and Red Sea (Yonow 1990). Previously documented from Thai waters (Nabhitabhata 2009) but a specific location was not given. Here confirmed from the Gulf waters of Thailand.

‘Philinopsis’ coronata (Gosliner, 2011)

Figure 7I

Material examined

One specimen 35 mm, SRB.

Ecology

In soft sediment habitats outside the coral reef. Depth 10 m.

Distribution

Philinopsiscoronata is known from the Philippines (Gosliner 2011) and Indonesia (Gosliner et al. 2018). Here recorded for the first time from Thai waters.

Remarks

The taxonomic validity of this species name remains unresolved after it was designated the type species for the recently erected genus Spinophallus by Zamora-Silva and Malaquias (2018), which is a junior homonym of Spinophallus A. Riedel, 1962 [Gastropoda, Pristilomatidae] (MolluscaBase 2020). To date, no replacement name has been proposed leaving us to retain the use of the earlier but incorrect genus designation for this species.

Genus Tubulophilinopsis Zamora-Silva & Malaquias, 2018

Tubulophilinopsis lineolata (H. Adams & A. Adams, 1854)

Figure 7J

Material examined

One specimen 45 mm, LB; two specimens 32–38 mm, TT.

Ecology

In soft sediment habitats outside the coral reef. Depth 12–18 m.

Distribution

Currently known only from Australia (Nimbs and Smith 2016), Japan and the Philippines (Gosliner et al. 2018). Recorded from the Gulf waters of Thailand (Jensen 1998).

Tubulophilinopsis pilsbryi (Eliot, 1900)

Figure 7K

Material examined

Two specimens 35–39 mm, SN; two specimens 25–42 mm, TT; one specimen 19 mm, MB.

Ecology

Abundant in soft sediment habitats outside the coral reef. A single individual of the species was observed being ingested by the scleractinian coral Heteropsammia cochlea (Mehrotra et al. 2019). Depth 12–28 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá and Malaquias 2016), Myanmar (Sanpanich and Duangdee 2019), Vietnam (Martynov and Korshunova 2012), Australia (Nimbs and Smith 2016), the Philippines, Vanuatu (Zamora-Silva and Malaquias 2018), Madagascar, Malaysia, Indonesia, Papua New Guinea, Palau, Guam, Marshall Islands, and Hawaii (Gosliner et al. 2008). Documented from the Gulf of Thailand (Mehrotra and Scott 2016).

Tubulophilinopsis reticulata (Eliot, 1903)

Figure 7L

Material examined

One specimen 30 mm, SN.

Ecology

In soft sediment habitats outside the coral reef. Depth 14–16 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá and Malaquias 2016), Vietnam (Martynov and Korshunova 2012), Taiwan (Huang et al. 2016), Australia (Nimbs and Smith 2016), Marshall Islands (Zamora-Silva and Malaquias 2018), Tanzania, South Africa, Madagascar (Gosliner et al. 2008), and the Red Sea (Yonow 1990). Here documented as a first record for Thai waters.

Genus Migaya Ortea, Caballer & Espinosa, 2014

Migaya sp.

Figure 8A

Material examined

One specimen 3 mm, LB.

Ecology

In soft sediment habitats outside the coral reef. Depth 24 m.

Distribution

Currently known only from the Gulf of Thailand, documented here for the first time.

Remarks

Ortea Rato et al. (2014) described the genus Migaya to hold all the Caribbean and Indo-Pacific cephalaspideans that were found to cluster (subclade B.2.) with Aglaja felis Er. Marcus & Ev. Marcus, 1970 in the molecular phylogeny inferred by Camacho-García et al. (2013). These authors transferred A. felis to the genus Nakamigawaia Kuroda & Habe, 1961 based on the apparently wide distribution of A. felis in the Indo-Pacific, assuming that they could only belong to the Japanese genus Nakamigawaia because of the similarities in their external morphology and colouration, but they did not include representatives of the type species of the genus, N. spiralis Kuroda & Habe, 1961, in their study, nor any other co-generic species coming from Japan. Ortea Rato et al. (2014) compared the shells N. spiralis with those of A. felis in the context of a wide-range shell comparison including all the Aglajidae, and concluded that both species belonged to different genera, consequently describing the genus Migaya. Afterwards, Zamora-Silva and Malaquias (2018) published a new molecular phylogeny based on a wider taxonomical sampling within the Aglajidae in which they synonymised the genus Migaya and transferred A. felis to the genus Nakamigawaia. Again, these authors did not include representatives of N. spiralis from Japan, but similar species from Australia and Papua New Guinea, without checking their internal anatomies. For these reasons, given the high rate of endemicity of the Japanese sea slugs, and after the study of the shell of the specimen from Thailand (bearing a similar shell to that of M. felis), we prefer to maintain the genus Migaya until representatives of N. spiralis from Japan are sequenced and compared in a phylogenetic context.

Figure 8. 

A Migaya sp. 3 mm B Siphopteron makisig 3 mm C Siphopteron sp. 3 mm (photograph by Will Malsukum) D Philine orca 3 mm E Aplysia kurodai 30 mm (photograph by Geoffrey Chamayou) F Aplysia nigrocincta 9 mm G Bursatella cf. ocelligera 65 mm (photograph by Elouise Haskin) H Stylocheilus longicauda 45 mm (photograph by Kirsty Magson) I Stylocheilus striatus 29 mm J Berthella cf. caledonica 10 mm K Berthella martensi 60 mm (photograph by Paddy Steele) L Pleurobranchus forskalii 130 mm (photograph by Tine Kvamme).

Family Gastropteridae Swainson, 1840

Genus Siphopteron Gosliner, 1989

Siphopteron makisig Ong & Gosliner, 2017

Figure 8B

Material examined

One individual 3 mm, SO.

Ecology

In soft sediment habitats outside the coral reef. Depth 12–16 m.

Distribution

Currently known only from the Philippines, Indonesia, and Australia (Ong et al. 2017). Here documented as a first record for Thai waters.

Siphopteron sp.

Figure 8C

Material examined

One individual 3 mm, RR.

Ecology

In soft sediment habitats outside the coral reef. Depth 20 m.

Distribution

Unknown

Family Philinidae Gray, 1850 (1815)

Genus Philine Ascanius, 1772

Philine orca (Pease, 1860)

Figure 8D

Material examined

Two individuals 2–4 mm, CB; one individual 3 mm, SB.

Ecology

Observed exclusively under dead Fungiidae coral skeletons where it is extremely cryptic, although it may be abundant. The only cephalaspidean species recorded exclusively from the coral reef habitat at Koh Tao. Depth 3–8 m.

Distribution

Widespread across the Indo-Pacific including Japan (Baba 1990), Australia (Nimbs and Smith 2016), Madagascar, Malaysia, the Philippines, Indonesia, Papua New Guinea, Hawaii, and the Galapagos Islands (Gosliner et al. 2008). Here documented as a first record for Thai waters.

Order Aplysiida

Superfamily Aplysioidea Lamarck, 1809

Family Aplysiidae Lamarck, 1809

Genus Aplysia Linnaeus, 1767

Aplysia kurodai Baba, 1937

Figure 8E

Material examined

Two specimens 60–81 mm, SN; one specimen 30 mm, MH.

Ecology

In soft sediment habitats, occasionally found in aggregations, although more often observed as solitary. Depth 8–22 m.

Distribution

Currently known only from China (Guang-Yu and Tchang 1965), Korea (Lee et al. 2014), and Japan (Baba 1937). Documented from the Gulf of Thailand (Mehrotra and Scott 2016).

Aplysia nigrocincta von Martens, 1880

Figure 8F

Material examined

One specimen 9 mm, SB.

Ecology

Under a dead Fungiidae coral among rubble in shallow coral reef habitats. Depth 6 m.

Distribution

Across the Indo-Pacific including Mozambique, Mauritius, the Philippines, Indonesia, Papua New Guinea, Vanuatu (Golestani et al. 2019), Maldives (Yonow 1994 as Aplysia fasciata), and the Red Sea (Yonow 1990 as Aplysia cf. parvula.). Here documented as a first record for Thai waters.

Genus Bursatella Blainville, 1817

Bursatella cf. ocelligera (Bergh, 1902)

Figure 8G

Material examined

One specimen 65 mm, SB.

Ecology

In soft sediment habitats where it grazes on cyanobacterial mats on the benthos. Depth 18–25 m.

Distribution

Bursatella ocelligera is known only from the Philippines (Bazzicalupo et al. 2020) and the Gulf of Thailand (Bergh 1902). Bursatella leachii is circumtropical (Bazzicalupo et al. 2020) including Brazil (Galvão Filho et al. 2015), Guadeloupe (Ortea Rato et al. 2012), Spain (González-Wangüemert et al. 2014), Italy (Travaglini and Crocetta 2019), Tunisia (Zakhama-Sraieb 2009), Morocco (Selfati et al. 2017), Ghana (Bebbington 1969), Iran (Rezai et al. 2016), India (Sethi et al. 2015), Vietnam (Martynov and Korshunova 2012), Australia (Nimbs and Smith 2016), New Zealand (Appleton et al. 2002), South Africa, Madagascar, and Hawaii (Gosliner et al. 2008).

Remarks

Eales and Engel (1935) synonymised all species of Bursatella into the single circumtropical species B. leachii. Recent evidence (Bazzicalupo et al. 2020) has supported the presence of a second species, B. ocelligera, based on internal morphological differences such as an unarmed penis and supported by molecular data. This name was attributed to specimens described from the Gulf of Thailand by Bergh (1902), who was the first to describe specimens with an unarmed penis. At present, there is no reliable way of discerning between the two species based on external morphology alone, and no available molecular data for specimens from the Gulf of Thailand. The internal anatomy of specimens from Koh Tao could not be investigated as part of the present study; however, the proximity to the type locality (Koh Chang) supports the need for comprehensive analyses of specimens from the Gulf of Thailand in particular, given the wide geographical range of B. leachii and the recent separation of other ‘circumtropical’ aplysiids.

Genus Stylocheilus Gould, 1852

Stylocheilus longicauda (Quoy & Gaimard, 1825)

Figure 8H

Material examined

One specimen 45 mm, SRB; one specimen 30 mm, TT.

Ecology

In soft sediment habitats rarely and upon mooring ropes where it grazes on cyanobacteria. Far less common than S. striatus. The association with mooring ropes is believed to be driven by its pelagic lifestyle, as these ropes act as mid-water substrates for cyanobacterial growth. Depth 5–18 m.

Distribution

Circumtropical including Brazil (Galvão Filho et al. 2015), Gulf of Oman (Fatemi and Attaran 2015), Red Sea (Yonow 2008), India (Chinnadurai et al. 2014), Australia (Nimbs and Smith 2016), Tanzania, the Philippines, and Hawaii (Gosliner et al. 2008). Documented from the Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Recent work (Yonow 2012; Bazzicalupo et al. 2020), has indicated the need for morphological and molecular examination of S. longicauda across their range with the taxonomic validity of the species being questioned. In the present work, S. longicauda is treated as distinct from S. striatus based on differences in external morphology (consistently shorter papillae and yellow/lime-green colouration in S. longicauda) and ecology. We here retain the use of the compound noun used in the original description of the species (see Nimbs et al. 2017).

Stylocheilus striatus (Quoy & Gaimard, 1832)

Figure 8I

Material examined

One specimen 25 mm, SRB; one specimen 22 mm, CB; one specimen 29 mm, TT.

Ecology

From shallow and deep soft sediment habitats grazing of mats of cyanobacteria on the benthos. Depth 1–18 m.

Distribution

Circumtropical including Brazil (Galvão Filho et al. 2015), Mexico (Ortigosa et al. 2015), Guadeloupe (Ortea Rato et al. 2012), the Azores (Malaquias et al. 2009), Mozambique (Jochum and Favre 2017), India (Apte 2009), Vietnam (Martynov and Korshunova 2012), Indonesia (Eisenbarth et al. 2018), Australia (Nimbs and Smith 2016), French Polynesia (Horwitz et al. 2017), Fiji (Thaman et al. 2017), the Caribbean, Galapagos, South Africa, Red Sea, Hawaii and California (Gosliner et al. 2008). Documented from the Gulf of Thailand (Mehrotra and Scott 2016), and from Andaman Sea (as S. longicauda) by Jensen (1998).

Clade Nudipleura Wägele & Willan, 2000

Order Pleurobranchida Deshayes, 1832

Superfamily Pleurobranchoidea Gray, 1827

Family Pleurobranchidae Gray, 1827

Genus Berthella Blainville, 1824

Berthella cf. caledonica (Risbec, 1928)

Figure 8J

Material examined

Three specimens 10–15 mm, TW.

Ecology

Under coral rubble in shallow coral reef habitats. Depth 6–8 m.

Distribution

Berthella caledonica is known from New Caledonia (Risbec 1928), Mariana Islands (Carlson and Hoff 2003), Hawaii (Johnson 2002a), Marshall Islands (Johnson 2002b), Australia (Cobb 2009), and Japan (Bolland 2002).

Remarks

Overall colour variable from pink to light or dark brown, with numerous small, low tubercles across the dorsal surface, often surrounded by a brown ring, and with dark brown apices. A prominent brown mark surrounded by a diffuse ring of translucent white is located centrally on the dorsal surface. While specimens from Koh Tao resemble the description of Berthella caledonica (Risbec, 1928) rather well, records of Berthella africana (Pruvot-Fol, 1956) have also been made from Thailand, with an unclear locality (Nabhitabhata 2009). Both species share external similarities, in particular the presence of a brown spot or ‘hole’ found centrally on the dorsum, and the need for clarification between both species has been noted (Gosliner et al. 2008; Rudman 2009b). The present species is most similar to Berthella sp. 1 of Gosliner et al. (2018). With the Gulf of Thailand being distant from the type localities of both species (Morocco for B. africana and New Caledonia for B. caledonica), and the Pacific range currently known for B. caledonica, the present species is treated as potentially distinct until such a time as closer examinations can be made.

Berthella martensi (Pilsbry, 1896)

Figure 8K

Material examined

One specimen 60 mm, LB.

Ecology

Exclusively recorded from soft sediment habitats outside coral reefs. Depth 11–21 m.

Distribution

Widespread throughout the Indo-Pacific including the Red Sea (Yonow 2015), Mozambique (Tibiriçá et al. 2017), India (Sreeraj et al. 2012), Maldives (Yonow 1994), Tanzania, Mauritius, Indonesia, the Philippines, Taiwan, Australia, Papua New Guinea, Solomon Islands, Hawaii, and the Pacific coast of Mexico (Gosliner et al. 2008).

Remarks

Individuals from Koh Tao have a dark, almost black mantle with numerous inconspicuous black spots. Berthella martensi was recorded by Nabhitabhata (2009) based on a local record (in Thai), but the location(s) of this record is unknown. Therefore, while B. martensi is known from the Gulf of Thailand, its presence along the Andaman coast of Thailand is unconfirmed.

Genus Pleurobranchus Cuvier, 1804

Pleurobranchus forskalii Rüppell & Leuckart, 1828

Figure 8L

Material examined

One specimen 265 mm, SN; one specimen 55 mm, TT.

Ecology

Exclusively recorded from soft sediment habitats outside coral reefs. Observed feeding on colonies of the tunicate Didemnum molle Herdmann, 1886. Depth 11–21 m.

Distribution

Widespread throughout the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Gulf of Oman (Fatemi and Attaran 2015), India (Apte and Bhave 2014), Tanzania, the Red Sea, the Philippines, Indonesia, Japan, Australia, Papua New Guinea, and Fiji (Gosliner et al. 2008). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Order Nudibranchia Cuvier, 1817

Suborder Doridina Odhner, 1934

Superfamily Doridoidea Rafinesque, 1815

Family Actinocyclidae O’Donoghue, 1929

Genus Hallaxa Eliot, 1909

Hallaxa iju Gosliner & Johnson, 1994

Figure 9A

Material examined

One specimen 10 mm, SO.

Ecology

Among rubble in coral reef and reef edge habitats. Associated with an unidentified pale/creamy white sponge. Depth 4–12 m.

Distribution

Across the Pacific including Australia (Nimbs and Smith 2016), the Philippines, Papua New Guinea, Japan, the Marshall Islands (Gosliner and Johnson 1994), Hong Kong, and Hawaii (Gosliner et al. 2008). Here representing a first record for Thai waters.

Figure 9. 

A Hallaxa iju 10 mm B Hallaxa indecora 8 mm C Cadlinella ornatissima 30 mm (photograph by Mati Pauner) D Ceratosoma tenue 85 mm E Chromodoris mandapamensis 50 mm (photograph by Tine Kvamme) F Chromodoris cf. mandapamensis 7 mm G Chromodoris cf. balat 25 mm (photograph by Elouise Haskin) H Diversidoris aurantionodulosa 30 mm I Diversidoris crocea 8 mm (photograph by Pau Urgell Plaza) J Doriprismatica atromarginata 60 mm K Glossodoris cf. cincta 40 mm L Goniobranchus cf. albonares 5 mm.

Hallaxa indecora (Bergh, 1905)

Figure 9B

Material examined

One specimen 8 mm, SRB; one specimen 8 mm, CB.

Ecology

Among rubble and under dead fungiid coral skeletons in coral reef habitats. Occasionally near isolated unattached colonies of sponge in deeper soft sediment habitats. Associated with an unidentified pale/creamy white sponge. Depth 4–20 m.

Distribution

Red Sea (Yonow 2008) and across the Indo-Pacific including Australia (Nimbs and Smith 2016), the Gulf Aden, the Philippines, Indonesia, Japan, New Caledonia, and American Samoa (Gosliner et al. 2008). Here representing a first record for Thai waters.

Family Chromodorididae Bergh, 1891

Genus Cadlinella Thiele, 1931

Cadlinella ornatissima (Risbec, 1928)

Figure 9C

Material examined

Two specimens 12–30 mm, TT.

Ecology

On rocks and among corals at offshore pinnacles. Depth 8–25 m.

Distribution

Widespread in the Indo-Pacific including New Caledonia (Risbec 1928), Japan (Baba 1949), Mozambique (Tibiriçá et al. 2017), Chagos Islands (Yonow et al. 2002), India (Ramakrishna et al. 2010), Sri Lanka, Gulf of Oman, Réunion Island (Yonow 2012), Red Sea (Yonow 2008), Myanmar (Sanpanich and Duangdee 2019), Australia (Nimbs and Smith 2016), Taiwan, Hong Kong (Gosliner et al. 2008), Singapore (Lim and Chou 1970), the Gulf of Thailand, and the Andaman coast of Thailand (Chavanich et al. 2013).

Genus Ceratosoma A. Adams & Reeve, 1850

Ceratosoma tenue Abraham, 1876

Figure 9D

Material examined

One specimen 85 mm, AM.

Ecology

Soft sediment habitat. Depth 26 m.

Distribution

Widespread throughout the Indo-Pacific including Japan (Baba 1949), Indonesia (Yonow 2001), South Africa, Mozambique, Malaysia, Australia, New Caledonia, Hawaii (Gosliner et al. 2008), and the Red Sea (Yonow 2008). Here representing a first record for Thai waters.

Genus Chromodoris Alder & Hancock, 1855

Chromodoris mandapamensis Valdés, Mollo & Ortea, 1999

Figure 9E

Material examined

One specimen 50 mm, CP.

Ecology

Among corals and coral rubble at offshore pinnacle sites. 9–20 m.

Distribution

Widespread throughout the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), India (Valdés et al. 1999), South Africa, Solomon Islands, Papua New Guinea, Myanmar (Gosliner et al. 2008), and Gulf of Thailand (Chavanich et al. 2013).

Remarks

While externally matching the original description of the species completely, the internal anatomy was not analysed to verify this, the importance of which is particular to this and other similar species (Layton et al. 2018). See ‘Remarks’ for Chromodoris cf. mandapamensis below.

Chromodoris cf. mandapamensis Valdés, Mollo & Ortea, 1999

Figure 9F

Material examined

Three specimens 5–10 mm, CB.

Ecology

Among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Depth 2–8 m.

Distribution

Goniobranchus pruna (Gosliner, 1994) is known from Madagascar and South Africa (Gosliner 1994) and Mozambique (Tibiriçá et al. 2017). Chromodoris sp. 15 is known from Philippines and New Caledonia (Gosliner et al. 2018).

Remarks

Very similar to Chromodoris aff. mandapamensis (Layton et al. 2018; Bonomo and Gosliner 2020), Chromodoris sp. 15 (Gosliner et al. 2018), and Goniobranchus pruna (Gosliner 1994). While C. mandapamensis has been suggested as a possible synonym of G. pruna as discussed in Tibiriçá et al. (2017), the present species is kept separate from C. mandapamensis due to difference in the local ecology of both species. Chromodoris mandapamensis is locally recorded only from sparse observations at offshore pinnacles, with no confirmed records for the past five years, whereas C. cf. mandapamensis is regularly found in surveys in shallow reef areas near the shore. While it is possible that this species undergoes its juvenile stages closer to shore before moving out towards the offshore pinnacles, no observations have been made of this species in the intervening deeper soft sediment habitats between the two. Additionally, no individuals larger than approximately 15 mm have been recorded and no individuals that externally match C. mandapamensis have yet been recorded near the island.

Chromodoris cf. balat Bonomo & Gosliner, 2020

Figure 9G

Material examined

One specimen 25 mm, CB.

Ecology

Among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Depth 4–8 m

Distribution

Chromodoris balat is known only from the Philippines (Bonomo and Gosliner 2020) and a similar species is recorded here for the first time from Thai waters.

Remarks

Similar to Chromodoris balat in having a striated dorsum with numerous large blotches and a broken yellow-orange marginal line. This species was differentiated from the similar Chromodoris striatella Bergh, 1877 based on these and other features (Layton et al. 2018; Bonomo and Gosliner 2020). Our specimen is differentiated from C. balat by lacking small yellow, orange, and red spots on the white parts of the dorsal surface, the pale tan-coloured gills with orange spots instead of red-brown as seen in C. balat, and by the blotches being pale grey-brown and indistinct instead of dark and pronounced. The indistinct blotches on the dorsum, the broken yellow-orange marginal band, and the colouration of rhinophores and gills do share a resemblance with C. mandapamensis and C. cf. mandapamensis (the latter of which may be found living alongside C. cf. balat at Koh Tao). Given the difficulties of relying on external features for species delineation in many of these striped and spotted species of Chromodoris (Layton et al. 2018; Bonomo and Gosliner 2020) we refrain from committing to a species identification until specimens from Koh Tao can be investigated further. Chavanich et al. (2013) recorded C. striatella from both Gulf and Andaman coasts of Thailand; however, in the absence of specimen details from both areas, this distribution record may be called into question in light of the recent findings regarding the complex surrounding C. striatella. Further documentation of Chromodoris species from both coasts may clarify this.

Genus Diversidoris Rudman, 1987

Diversidoris aurantionodulosa Rudman, 1987

Figure 9H

Material examined

One specimen 30 mm, SI.

Ecology

Found upon its pink host sponge, Darwinella sp., at deeper reef and pinnacle sites, and in muck habitats. Depth 12–30 m.

Distribution

Red Sea (Yonow 2015), South Africa, Tanzania, Australia, and Hong Kong (Gosliner et al. 2008). Here representing a first record for Thai waters

Remarks

This species was mistakenly identified as Ardeadoris averni (Rudman, 1985) by Mehrotra and Scott (2016) based on limited photographic data. Specimens found several years after initial second-hand observations have permitted reidentification, leaving A. averni remaining currently unrecorded in Thai waters.

Diversidoris crocea (Rudman, 1986)

Figure 9I

Material examined

One specimen 8 mm, TT.

Ecology. Coral reef habitats. Usually cryptic on its sponge, a yellow Darwinella sp. (Rudman 2005a).

Distribution. Widespread throughout the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Philippines, Indonesia, Japan, Papua New Guinea (Gosliner et al. 2008), Guam (Carlson and Hoff 2003), and Australia (Rudman 1986). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Genus Doriprismatica d’Orbigny, 1839

Doriprismatica atromarginata (Cuvier, 1804)

Figure 9J

Material examined

One specimen 60 mm, CP.

Ecology

On rocks and among corals at offshore pinnacles. Depth 8–25 m.

Distribution

Widespread throughout the Indo-Pacific including South Africa, Red Sea, French Polynesia, Solomon Islands, China, Philippines (Gosliner 1987), Papua New Guinea, Australia (Rudman 1986), Myanmar (Sanpanich and Duangdee 2019), Japan (Baba 1949), Indonesia (Yonow 2001), Gulf of Thailand (Jensen 1998), Mauritius (Yonow and Hayward 1991), and the Andaman coast of Thailand (Chavanich et al. 2013).

Genus Glossodoris Ehrenberg, 1831

Glossodoris cf. cincta (Bergh, 1888)

Figure 9K

Material examined

One specimen 40 mm, HWB.

Ecology

Coral reefs throughout the island. Depth 5–15 m.

Distribution

Papua New Guinea, the Philippines and Madagascar (Matsuda and Gosliner 2018). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Remarks

Previously recorded as Glossodoris cincta (Bergh 1888), recent work by Matsuda and Gosliner (2018) has shown that Southeast Asian/western Pacific species may be distinct. However, this cannot be verified until specimens of G. cincta are analysed from the type locality of Mauritius. Therefore, the present species, which externally matches the description by Matsuda and Gosliner, is separated from the true Indian Ocean Glossodoris cincta for now.

Genus Goniobranchus Pease, 1866

Goniobranchus cf. albonares (Rudman, 1990)

Figure 9L

Material examined

Three specimens 4–6 mm, CB.

Ecology

Observed under dead fungiid coral skeletons and occasionally among rubble in shallow coral reef. Depth 3–8 m

Distribution

Goniobranchus albonares is known from Australia (Rudman 1990), Japan (Gosliner et al. 2008), Madagascar (Rassat 2016), and Mozambique (Tibiriçá et al. 2017).

Remarks

Externally resembling both Goniobranchus albonares (Rudman, 1990) and Goniobranchus rubrocornutus (Rudman, 1985), the present species differs from the former by possessing a broken submarginal band of deep red and from the latter by the presence of completely white rhinophore clubs and gills as opposed to red. There is significant overlap in the range of both species, with G. rubrocornutus known from Australia, Hong Kong, and Japan (Rudman 1985). A comprehensive comparison of the three species is needed.

Goniobranchus aureopurpureus (Collingwood, 1881)

Figure 10A

Material examined

One specimen 45 mm, SN.

Ecology

Locally rare, known only from soft sediment habitats outside the coral reef. Depth 12–16 m.

Distribution

Across the Indo-Pacific including Myanmar (Sanpanich and Duangdee 2019), Australia (Nimbs and Smith 2016), the Philippines, Indonesia, China, Japan, Papua New Guinea, and New Caledonia (Gosliner et al. 2008). Known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Figure 10. 

A Goniobranchus aureopurpureus 45 mm B Goniobranchus fidelis 12 mm C Goniobranchus geometricus 15 mm (photograph by Kirsty Magson) D, E Goniobranchus sinensis different morphs, 34 mm (D) and 55 mm (E) F Goniobranchus tumuliferus 10 mm G Goniobranchus verrieri 10 mm (photograph by Khumron Waipaka) H Goniobranchus sp. 55 mm (photograph by Phannee Mccarthy) I Hypselodoris cerisae 4 mm J Hypselodoris confetti 24 mm (photograph by Pau Urgell Plaza) K Hypselodoris decorata 18 mm L Hypselodoris infucata 12 mm.

Goniobranchus fidelis (Kelaart, 1858)

Figure 10B

Material examined

Two specimens 6–18 mm CB; one specimen 15 mm, SB; one specimen 12 mm, SD.

Ecology

Coral reef habitats throughout the region. Depth 2–25 m.

Distribution

Widespread throughout the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Maldives (Yonow 1994), India (Ramakrishna et al. 2010), Myanmar (Sanpanich and Duangdee 2019), Madagascar, Red Sea, Philippines, Japan, Australia, New Caledonia (Gosliner et al. 2008), and Indonesia (Yonow 2001). Known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Goniobranchus geometricus (Risbec, 1928)

Figure 10C

Material examined

One specimen 15 mm, KKR.

Ecology

Locally rare, known only from the reef edge and soft sediment habitats outside the coral reef. Depth 12–25 m.

Distribution

Widespread throughout the Indo-Pacific including Myanmar (Sanpanich and Duangdee 2019), Tanzania, Madagascar, Guam, Japan, Papua New Guinea (Gosliner et al. 2008), Maldives (Yonow 1994), Indonesia (Yonow 2001), Philippines (Debelius 1996), and Australia (Nimbs and Smith 2016). Known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Goniobranchus preciosus (Kelaart, 1858)

Material examined

None found presently.

Ecology

Soft sediment habitats outside the coral reef. Depth 18–22 m.

Distribution

Across the Indo-Pacific including Indonesia (Scott 2005), Malaysia, the Philippines, Papua New Guinea, Australia, New Caledonia. Recorded from Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Remarks

The species has historically been recorded from Koh Tao (Mehrotra and Scott 2016) based on a citizen science effort, but no material has been found in surveys to date.

Goniobranchus sinensis (Rudman, 1985)

Figure 10D, E

Material examined

Three specimens 20–55 mm, CP.

Ecology

Among rocks and corals at offshore rocky pinnacles. Depth 9–22 m.

Distribution

Across the Indo-Pacific including the Gulf of Oman (Fatemi and Attaran 2015), India (Sreeraj et al. 2012), Malaysia, Indonesia, China, and Japan (Gosliner et al. 2008). Recorded from the Gulf waters of Thailand (Chavanich et al. 2013).

Remarks

Incorrectly identified as Goniobranchus trimarginatus (Winckworth, 1946) by Mehrotra and Scott (2016). Subsequent observations indicate that specimens from Koh Tao are Goniobranchus sinensis, with individuals being recorded with marginal and submarginal bands ranging from complete to broken.

Goniobranchus tumuliferus (Collingwood, 1881)

Figure 10F

Material examined

Two specimens 10–15 mm, CP.

Ecology

Predominantly found among coral and rock at an offshore submerged pinnacle site. Also sparsely recorded from the deeper soft sediment habitats near the island. Depth 11–25 m.

Distribution

Across the western Pacific including Vietnam (Martynov and Korshunova 2012), the Philippines, Japan, Australia, and New Caledonia (Gosliner et al. 2008). Known from the Gulf of Thailand (Jensen 1998; Chavanich et al. 2013).

Goniobranchus verrieri (Crosse, 1875)

Figure 10G

Material examined

One specimen 10 mm, GR.

Ecology

Coral reefs. Depth 5–10 m.

Distribution

Widespread throughout the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Tanzania, New Caledonia (Rudman 1985), Australia (Nimbs and Smith 2016), South Africa, Madagascar, Indonesia, Philippines, Hawaii (Gosliner et al. 2008), and Red Sea (Yonow 1989). Here representing a first record for Thai waters.

Remarks

Locally known only from a single individual.

Goniobranchus sp.

Figure 10H

Material examined

One specimen 55 mm, CP.

Ecology

Among rocks and corals at offshore rocky pinnacles. Depth 18 m.

Distribution

Goniobranchus sp. 5 (Soong et al. 2020) is known from the Philippines, Indonesia, Japan, and Australia. A similar species is known from the Gulf of Thailand, incorrectly recorded as Goniobranchus reticulatus by Mehrotra and Scott (2016).

Remarks

Red reticulated specimens of Goniobranchus have been known to represent a complex of species often attributed to G. reticulatus or G. tinctorius. Recent molecular work by Soong et al. (2020) concluded that none of the five distinct lineages identified corresponded exactly with the description of either of the aforementioned species. Due to the overlapping ranges and external variability of some of these, a further morphological investigation into these lineages is needed to define the species.

Genus Hypselodoris Stimpson, 1855

Hypselodoris cerisae Gosliner & Johnson, 2018

Figure 10I

Material examined

One specimen 4 mm, CB.

Ecology

Among reef rubble in shallow coral reef. Depth 4 m.

Distribution

Japan, Malaysia, and Taiwan (Epstein et al. 2018). Here representing a first record for Thai waters.

Remarks

Though a small individual, it is identified as H. cerisae based on pink and purple pigmentation and dark brown lines with white spots. An absence of orange/burnt orange pigmentation separates it from H. krakatoa Gosliner & Johnson, 1999.

Hypselodoris confetti Gosliner & Johnson, 2018

Figure 10J

Material examined

Two specimens 12–24 mm, SB.

Ecology

Locally found exclusively from deeper soft sediment habitats of the island. Depth 14–25 m.

Distribution

Philippines, Papua New Guinea, probably Indonesia and Hong Kong (Epstein et al. 2018). Here representing a first record for Thai waters.

Hypselodoris decorata (Risbec, 1928)

Figure 10K

Material examined

Two specimens 8–18 mm, CB.

Ecology

Abundant among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Rare in other habitats. Depth 2–12 m.

Distribution

Widespread throughout the Indo-Pacific including Malaysia, Philippines, Indonesia, Papua New Guinea, New Caledonia, Vanuatu, and the Marshall Islands (Epstein et al. 2018). Here representing a first record for the Gulf of Thailand. Hypselodoris decorata was recorded as Hypselodoris maculosa (Pease, 1871) from the Andaman coast by Chavanich et al. (2013).

Hypselodoris infucata (Rüppell & Leuckart, 1830)

Figure 10L

Material examined

One specimen 25 mm, SRB; one specimen 12 mm, CB; one specimen 8 mm, SB.

Ecology

Juveniles and smaller individuals common under dead fungiid corals and reef rubble in shallow coral reef areas, making up some of the most abundant nudibranch taxa in some areas. Larger individuals rarer. Throughout reef and deeper soft sediment habitats. Depth 2–25 m.

Distribution

Widespread and abundant in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Oman, South Africa, the Philippines, Australia (Debelius 1996), Red Sea (Yonow 1989), Madagascar, Mediterranean Sea, Indonesia, Japan, Papua New Guinea, and Hawaii (Gosliner et al. 2008). Distribution within Thailand currently unclear (see below), previously recorded from the Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Chavanich et al. (2013) recorded Hypselodoris infucata from Andaman and Gulf coasts, and Hypselodoris obscura from the Gulf of Thailand. While images were not presented alongside these records, review of the original data hints at a possible clarification. Hypselodoris obscura is known to be a sub-tropical species from eastern Australia (Epstein et al. 2018) while H. infucata is known to be widespread across the Indo-Pacific; therefore, records from the Gulf of Thailand by Chavanich et al. (2013) are likely to be H. infucata. Review of photographs used in the initial identifications supports this clarification, but also suggest that the species called Hypselodoris infucata found to be present on both coasts may or may not be the true H. infucata. A closer investigation on the distribution of Hypselodoris species across both coasts is needed to confirm its range in Thai waters.

Hypselodoris cf. juniperae Gosliner & Johnson, 2018

Figure 11A

Material examined

One specimen 6 mm, CB.

Ecology

Observed under a dead fungiid coral skeleton in shallow coral reef. Depth 6 m.

Distribution

Hypselodoris juniperae is currently known from Madagascar (Epstein et al. 2018) and potentially South Africa, Sri Lanka, and Réunion Island (Bidgrain 2005; Ogden 2005; Houben 2007). A similar species represents a first record for Thai waters.

Remarks

Externally similar to both Hypselodoris maculosa (Pease, 1871) and Hypselodoris juniperae Gosliner & Johnson, 2018 in bearing thin longitudinal white lines, dark purple spots, an opaque orange marginal band and white gills with red apices. Identified as the former species in Mehrotra and Scott (2016). Observed living sympatrically with H. decorata, readily distinguished by bearing two red rhinophoral rings and purple rather than reddish brown body colouration. Given the close relationship between all three species, a much closer study is required. Only a single individual has been observed in the area to date.

Figure 11. 

A Hypselodoris cf. juniperae 6 mm B Hypselodoris cf. kanga 40 mm C Hypselodoris katherinae 15 mm D Hypselodoris cf. lacuna 10 mm E, F Hypselodoris cf. maritima 20 mm (E) and 25 mm (F) G Hypselodoris tryoni, right specimen 35 mm (photograph by Kirsty Magson) H Mexichromis mariei 15 mm (photograph by Pau Urgell Plaza) I Mexichromis multituberculata 30 mm (photograph by Pau Urgell Plaza) J Mexichromis trilineata 8 mm K Verconia cf. hongkongiensis 6 mm L Asteronotus cespitosus 120 mm (photograph by Emily Palmer).

Hypselodoris cf. kanga Rudman, 1977

Figure 11B

Material examined

One specimen 40 mm, TW; two specimens 10–25 mm, SB.

Ecology

Locally found exclusively from deeper soft sediment habitats of the island. Depth 14–25 m.

Distribution

Misidentified by Chavanich et al. (2013) as H. kanga, with a recorded distribution in both Andaman Sea and Gulf coasts of Thailand.

Remarks

Hypselodoris kanga bears blueish purple lines across its dorsum which are absent in this species, instead replaced by deep blue, almost black spots. These are more abundant towards the margin where they diffuse outwards turning into blue streaks closer to the edge. Given the historic confusion surrounding H. kanga (see Epstein et al. 2018), this difference in colouration merits the need for a closer investigation.

Hypselodoris katherinae Gosliner & Johnson, 2018

Figure 11C

Material examined

Two specimens 5–15 mm, CB.

Ecology

Observed under dead fungiid coral skeletons and on rocks in shallow coral reef. Depth 3–15 m.

Distribution

Indonesia, Eastern Malaysia, and the Philippines (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

Recorded as undescribed from Koh Tao (Mehrotra and Scott 2016: fig. 2G) now identified as a first record for Thai waters.

Hypselodoris cf. lacuna Gosliner & Johnson, 2018

Figure 11D

Material examined

One specimen 10 mm, CB.

Ecology

Observed under dead fungiid coral skeletons and on rocks in shallow coral reef. Depth 5 m.

Distribution

Hypselodoris lacuna is known from the Philippines, Indonesia, Japan, Papua New Guinea, Vanuatu, and Aldabra Atoll (Gosliner et al. 2008). Here representing a first record for Thai waters is a similar species.

Remarks

Dorsum centrally translucent grey with a network of opaque white lines. Gills grey with light grey apices, rhinophore stalks translucent, clubs white basally turning red with white tips. Mantle edge pale yellow areas with alternating blue spots. Differentiated from H. lacuna by having a mostly translucent grey dorsal surface rather than just isolated circles and by the pale yellow areas between the marginal ring of blue spots. Similar to Hypselodoris sp. 8 in Gosliner et al. (2018).

Hypselodoris cf. maritima (Baba, 1949)

Figure 11E, F

Material examined

Three specimens 14–30 mm, TT; two specimens 15–25 mm, SWP; one specimen 20 mm, SR.

Ecology

On rocks and rubble within coral reef. Depth 5–25 m.

Distribution

Hypselodoris maritima is recorded from Japan (Baba 1949), Vietnam (Martynov and Korshunova 2012), Taiwan (Su et al. 2009), the Philippines, Indonesia, Hong Kong, Papua New Guinea, and Australia (Gosliner et al. 2008). The species is also recorded as being present in Thailand by Gosliner et al. (2008) but no confirmed records of this or similar species outside of Koh Tao have been found in the literature.

Remarks

Initially recorded as H. maritima from Koh Tao (Mehrotra and Scott 2016), further observations have shown variation in external morphology that diverges from the original description. In general, the dorsal surface is always white with scattered and slightly raised spots. While many individuals bear the deep blue to black ‘longitudinal streaks’ along the central dorsal surface, in others these are broken lines or even entirely disconnected spots of varying sizes. Rhinophore clubs range from entirely orange to white with orange apices, stalks always translucent white. The blue marginal band is always separated from the yellow submarginal band by the same white as the dorsum, and both bands are often broken or rows of pigmented spots, matching variation in the dark pigmentation. There are always deep blue-black spots between marginal and submarginal bands/rows, that may vary in size and often extend to the mantle edge. While closer examination may reveal individuals from Koh Tao and the nearby pinnacles to be more than one species, the population is here treated as a single variable species.

Hypselodoris tryoni (Garrett, 1873)

Figure 11G

Material examined

Two specimens 30–35 mm, CB.

Ecology

On rocks and rubble within coral reef. Depth 1–30 m.

Distribution

Across the Indo-Pacific including Singapore (Toh 2016), Vietnam (Martynov and Korshunova 2012), Australia (Nimbs and Smith 2016), Malaysia, the Philippines, Indonesia, Japan, Palau, Papua New Guinea, Vanuatu, the Marshall Islands (Gosliner et al. 2008), and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Genus Mexichromis Bertsch, 1977

Mexichromis mariei (Crosse, 1872)

Figure 11H

Material examined

One specimen 15 mm, SB.

Ecology

In deep soft sediment habitats. Depth 20 m. Feeding on Dysidea sp. sponge.

Distribution

Widespread throughout the Indo-Pacific including India (Patel and Apte 2014), Malaysia (Ho 1989), Australia (Nimbs and Smith 2016), South Africa, Madagascar, the Philippines, Indonesia. Japan, Papua New Guinea, and New Caledonia (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

Locally rare with only a single individual observed in the present surveys. Sharing the same habitat and prey preference as M. multituberculata.

Mexichromis multituberculata (Baba, 1953)

Figure 11I

Material examined

One specimen 30 mm, SB, one specimen 8 mm, SN.

Ecology

Observed in deep soft sediment habitats throughout the island, though uncommon. Often found associated with or actively feeding on Dysidea sp. sponge which grows unattached on the benthos. Depth 14–25 m.

Distribution

Widespread throughout the Indo-Pacific including, India (Kumar et al. 2011), Myanmar (Sanpanich and Duangdee 2019), Vietnam (Martynov and Korshunova 2012), Taiwan (Huang et al. 2015), Hong Kong (Rudman and Darvell 1990), China (Lin 1990), the Philippines, Indonesia, and Japan (Gosliner et al. 2008). Recorded from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Mexichromis trilineata (A. Adams & Reeve, 1850)

Figure 11J

Material examined

Three specimens 5–8 mm, CB; two specimens 5 mm, TW.

Ecology

Usually found immersed in prey sponge Dysidea sp. under rubble and dead fungiid coral skeletons in shallow coral reef habitats. Uncommon, though multiple individuals may be observed together. Depth 3–8 m.

Distribution

Across the western Pacific including Indonesia (Yonow 2001; 2017), the Philippines, Palau, Papua New Guinea, and Australia (Gosliner et al. 2008). Recorded from the Gulf of Thailand (Mehrotra and Scott 2016). Very variable in pattern.

Genus Verconia Pruvot-Fol, 1931

Verconia cf. hongkongiensis (Rudman, 1990)

Figure 11K

Material examined

One specimen 6 mm, CB; one specimen 4 mm, TW.

Ecology

Observed under dead fungiid coral skeleton and rubble in shallow coral reef. Depth 3–8 m.

Distribution

Verconia hongkongiensis is known from Japan and Hong Kong (Gosliner et al. 2008, 2018). The present Verconia cf. hongkongiensis is a first record for Thai waters.

Remarks

Initially recorded as Hypselodoris bullockii (Collingwood, 1881) from a single small specimen by Mehrotra and Scott (2016), further observations have concluded that the species from Koh Tao is not H. bullockii but one superficially resembling Verconia hongkongiensis (Rudman 1990). Similarities between the two species are the thin marginal white line and pale variations in colour of the dorsum. However, rather than reddish tips, gills appear to be uniformly red (more orange in some specimens) and rhinophores appear pigmented throughout, basally red, sometimes with a paler median band, ending in red/orange tips, with all individuals recorded from Koh Tao being smaller than 6 mm.

Family Discodorididae Bergh, 1891

Genus Asteronotus Ehrenberg, 1831

Asteronotus cespitosus (van Hasselt, 1824)

Figure 11L

Material examined

One specimen 120 mm, CB.

Ecology

Locally found exclusively in soft sediment habitats. Depth 1–16 m.

Distribution

Widespread Indo-Pacific including Australia, Indonesia, Mauritius (Gosliner et al. 2008), Red Sea (Yonow 1990), Hawaii (Kay and Young 1969), Mozambique (Tibiriçá et al. 2017), and Gulf of Thailand (Chavanich et al. 2013).

Genus Atagema Gray, 1850

Atagema intecta (Kelaart, 1859)

Figure 12A

Material examined

One specimen 25 mm, CB.

Ecology

Coral reefs. Depth 5–10 m.

Distribution

Widespread Indo-Pacific including the Red Sea (Yonow 2008), Mozambique (Tibiriçá et al. 2017), Sri Lanka (Kelaart 1859), Indonesia (Debelius 1996), Australia (Nimbs and Smith 2016), Japan, Papua New Guinea, and Hawaii (Gosliner et al. 2008). Here representing a first record for Thai waters.

Figure 12. 

A Atagema intecta 25 mm (photograph by Nick Tringham) B Atagema spongiosa 95 mm C Atagema sp. 32 mm D Carminodoris cf. bifurcata 9 mm E Discodoris cebuensis 30 mm (photograph by Kirsty Magson) F Discodoris boholiensis 55 mm G Halgerda bacalusia (unknown size, photograph by Stephan Pelletier) H Jorunna funebris 87 mm I Jorunna sp. 10 mm J Peltodoris murrea 30 mm K, L Platydoris cf. formosa 35 mm, dorsal and ventral views (photographs by Kirsty Magson).

Atagema spongiosa (Kelaart, 1858)

Figure 12B

Material examined

One specimen 95 mm, CB.

Ecology

Observed upon prey sponge Dysidea sp. in deeper soft sediment habitats. Depth 14–24 m.

Distribution

Gulf of Oman (Fatemi and Attaran 2015), Red Sea (Yonow 2008), India (Apte et al. 2010), Sri Lanka (Kelaart 1858), Australia (Nimbs and Smith 2016), Fiji (Brodie and Brodie 1990), Madagascar, Singapore, the Philippines, South Korea, and Papua New Guinea (Gosliner et al. 2008). Here representing a first record for Thai waters.

Atagema sp. (Kelaart, 1858)

Figure 12C

Material examined

One specimen 32 mm, CB.

Ecology

Observed among rubble in shallow coral reef habitats at night with no observed association with prey. Depth 4–6 m.

Distribution

Atagema sp. 8 (Gosliner et al. 2018) is known only from the Philippines.

Remarks

Externally distinct from Atagema spongiosa (Kelaart, 1858) in being completely translucent pale grey, including the circular pits, separated by ridges, along the mantle. Similar to Atagema sp. 8 (Gosliner et al. 2018). Individuals from Koh Tao also differ from A. spongiosa in ecology, being observed in shallow reef environments instead of deeper soft sediment habitats.

Genus Carminodoris Bergh, 1889

Carminodoris cf. bifurcata Baba, 1993

Figure 12D

Material examined

Two specimens 9–17 mm, HF.

Ecology

Among rubble in shallow coral reef habitats. Depth 3–8 m.

Distribution

Carminodoris bifurcata is recorded across the Indo-Pacific including from Mozambique (Tibiriçá et al. 2017), the Red Sea (Yonow 2008), Korea (Koh 2006), the Philippines, Japan, and Hawaii (Gosliner and Fahey 2011) and Carminodoris flammea is recorded from Indonesia (Gosliner and Fahey 2011). Neither species has been previously recorded from Thai waters.

Remarks

Specimens from Koh Tao resemble both Carminodoris bifurcata Baba 1993 and Carminodoris flammea (Gosliner and Fahey 2011). They differ from the descriptions of the former by lacking any black spots on the dorsum and having a brown rather than grey ground colour and they differ from the latter in having brown rather than grey gill leaves, with white tips, and a tan median colouration instead of the bright red for which the species is named. A very similar looking specimen from Vietnam was identified as Hoplodoris bifurcata by Martynov and Korshunova (2012: pl. 31E). However, see discussions regarding Carminodoris pustulata (Abraham, 1877) by Jensen (1994), Yonow et al. (2002), and Yonow (2017) and indeed by Baba in the original description.

Genus Discodoris Bergh, 1877

Discodoris cebuensis Bergh, 1877

Figure 12E

Material examined

One specimen 30 mm, SN.

Ecology

Locally found exclusively in soft sediment habitats. Depth 14–18 m.

Distribution

Across the Indo-Pacific including the Red Sea (Yonow 2008), South Africa (Gosliner 1987), Seychelles (Eliot 1910), Tanzania, the Philippines, Indonesia, Japan, Papua New Guinea, and Hawaii (Dayrat 2010). Here representing a first record for Thai waters.

Discodoris boholiensis Bergh, 1877

Figure 12F

Material examined

One specimen 55 mm, SD.

Ecology

Found exclusively in soft sediment habitats. Depth 12–24 m.

Distribution

Widespread across the Indo-Pacific including India (Rao 1960), Vietnam (Risbec 1956), Singapore (Lim and Chou 1970), New Caledonia (Risbec 1928), Vanuatu (Coleman 2001), Madagascar, Indonesia, the Philippines, Palau, Papua New Guinea, Australia (Dayrat 2010), and the Gulf of Thailand (Chavanich et al. 2013).

Genus Halgerda Bergh, 1880

Halgerda bacalusia Fahey & Gosliner, 1999

Figure 12G

Material examined

No specimen collected.

Ecology

Shallow coral reef habitats. Depth 5–10 m.

Distribution

The range of this species appears to be very limited thus far including only Myanmar (Gosliner et al. 2018) and Thailand, from both the Andaman coast (Fahey and Gosliner 1999) and Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Recorded at Koh Tao from a single individual in 2011 and not recorded since. The included figure (Fig. 12G) represents the only evidence of the species from the Gulf of Thailand. This largely agrees with the observations of the authors that specimens of the genus Halgerda are thus far exceptionally rare from within the Gulf of Thailand.

Genus Jorunna Bergh, 1876

Jorunna funebris (Kelaart, 1859)

Figure 12H

Material examined

Two specimens 15–25 mm, LB; one specimen 30 mm, SI; one specimen 87 mm, CB.

Ecology

Abundant throughout corals and rubble in both nearshore reefs and offshore pinnacles. Rarely observed in soft sediment habitats. Depth 2–35 m; preys on blue Xestospongia sp. (Huang et al. 2016).

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2008), India (Apte 2009), Sri Lanka (Kelaart 1859), Indonesia (Yonow 2011), Australia (Debelius 1996) Mauritius, Madagascar, Philippines, Japan, Palau, New Caledonia (Camacho-García and Gosliner 2008a), Malaysia (Ho 1989), Vietnam (Risbec 1956), and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Jorunna sp.

Figure 12I

Material examined

Two specimens 10–15 mm, SB.

Ecology

Found exclusively in deeper soft sediment habitats. Depth 16–25 m.

Distribution

Jorunna sp. 7 recorded in the Philippines (Gosliner et al. 2018).

Remarks

An undescribed species covered in numerous long caryophyllidia, similar to Jorunna sp. 7 in Gosliner et al. (2018). Cream coloured dorsum with dark brown patches, white pigment on most caryophyllidia, dark brown rhinophore clubs with lamellae edged in white and white apices. Gills cream with some white. Here representing a first record for Thai waters.

Genus Peltodoris Bergh, 1880

Peltodoris murrea (Abraham, 1877)

Figure 12J

Material examined

One specimen 30 mm, SR.

Ecology

Documented here from a single record found upon submerged concrete artificial substrate at an offshore pinnacle site. Depth 30 m.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2008), New Caledonia, Australia, Japan (Dayrat 2010), Indonesia, and Malaysia (Yonow 2017). Here representing a first record for Thai waters.

Genus Platydoris Bergh, 1877

Platydoris cf. formosa (Alder & Hancock, 1864)

Figure 12K, L

Material examined

One specimen 20 mm, CA; two specimens 35–40 mm, SI.

Ecology

Among rubble at the coral reef edge. Depth 8–16 m.

Distribution

Platydoris formosa is known from Australia (Nimbs and Smith 2016), Tanzania, India, Indonesia, the Philippines, Samoa, New Caledonia, and Hawaii (Gosliner et al. 2008). Platydoris cinereobranchiata Dorgan et al., 2002 is known from the Philippines, Australia, the Solomon Islands (Gosliner et al. 2008), and Indonesia (Yonow 2011). Neither species has yet been documented in Thai waters.

Remarks

Externally, the specimens bear similarity to Platydoris formosa, as detailed by Dorgan et al. (2002) although differ in numerous ways. The rhinophores of the present species are yellow to pale brown, sometimes with red spots near the apex, with no black spots between them. Rhinophores have 33–35 lamellae. The gills are grey with dark brownish red lines running along the rachises, with no black spots anterior to the gill sheath. A white ring is present around the rims of the rhinophore and gill sheaths. Most of the caryophyllidia covering the dorsum are a dark reddish colour with a few white patches randomly distributed. Larger bright red patches are more numerous and also randomly distributed across the dorsal surface. Ventrally white with large red spots which become more diffuse and concentrated towards the foot. Based on these characteristics, it appears that the specimens from Koh Tao bear characteristics of both P. formosa and P. cinereobranchiata.

Genus Rostanga Bergh, 1879

Rostanga sp.

Figure 13A

Material examined

Two specimens 6–18 mm, CB.

Ecology

Exclusively found under coral rubble and the skeletons of dead Fungiidae corals. Cryptic on its pink-red sponge. Depth 3–8 m.

Distribution

Unknown.

Remarks

The present species is only identified based on external morphology and as such has not been identified to species level. Chavanich et al. (2013) documented Rostanga orientalis Rudman & Avern, 1989 from the Gulf of Thailand, which bears some external similarities to the present specimens.

Figure 13. 

A Rostanga sp. 18 mm B Sebadoris fragilis 70 mm C Thordisa sp. 12 mm D Doriopsis cf. granulosa 7 mm E Doriopsis pecten 6 mm F Doriopsis viridis 4 mm G Doris cf. immonda 5 mm H, I Dorididae sp. 15 mm (H photograph by Jeremy Coz) and 8 mm (I) J Goniodoridella sp. 1 6 mm K Goniodoridella sp. 2 3 mm (photograph by Khumron Waipaka) L Trapania cf. gibbera 5 mm.

Genus Sebadoris Er. Marcus & Ev. Marcus, 1960

Sebadoris fragilis (Alder & Hancock, 1864)

Figure 13B

Material examined

One specimen 70 mm, SB.

Ecology

Found under coral rubble and the skeletons of dead Fungiidae corals. Depth 3–8 m.

Distribution

Across the Indo-Pacific including Madagascar (Gosliner et al. 2008), Red Sea (Yonow 2008), Kuwait (Nithyanandan 2012), India (Bhave and Apte 2011), Indonesia, the Philippines, Papua New Guinea (Yonow 2017), and known from the Gulf of Thailand (Chavanich et al. 2013).

Genus Thordisa Bergh, 1877

Thordisa sp.

Figure 13C

Material examined

One specimen 12 mm, TT.

Ecology

Found in soft sediment habitats at 16 m depth.

Distribution

Unknown.

Remarks

Dorsal colour deep red, with white rhinophore clubs and gills. The most distinctive feature of the species appears to be a pair of distinct, elongated, white, conical papillae surrounded by 5–7 white extensions radiating out at the base of each papilla, on either side of the mid-dorsal ridge. Besides these are a few smaller but still elongated white papillae randomly distributed. Additionally, the base of the white gills and translucent red rhinophore stalks is surrounded by a thin white line which similarly can be found around the margin of the mantle. The most similar known species might be T. sanguinea Baba, 1955 which may be distinguished based on the ground colour, dorsal ornamentation (covered in papillae), and the colour of the rhinophores and gills.

Family Dorididae Rafinesque, 1815

Genus Doriopsis Pease, 1860

Doriopsis cf. granulosa (Pease, 1860)

Figure 13D

Material examined

One specimen 7 mm, HF.

Ecology

Under reef rubble and on rocks in coral reef habitats. Cryptic on its yellow prey sponge. Depth 6–12 m.

Distribution

Doriopsis granulosa is found across the Indo-Pacific including India (Apte and Bhave 2014), Seychelles (Gosliner et al. 2008), Tanzania (Edmunds 1971), Japan (Baba and Hamatani 1961), Hong Kong (Orr 1981), Australia (Nimbs and Smith 2016), and Hawaii (Kay and Young 1969). Here representing a first record for Thai waters.

Remarks

While clearly resembling Doriopsis granulosa (Pease, 1860) by an overall yellow dorsum, six gill leaves arranged horizontally in an almost transverse line, and numerous low, rounded tubercles, it bears differences from other descriptions (see Baba and Hamatani 1961; Valdés 2002; Apte and Bhave 2014). These include the absence of any brown spots along the dorsum and rhinophores that are not uniformly yellow-orange but instead have translucent stalks and abruptly transition from a yellow base to a white apex, separated by a thin brown median line. These subtle differences may be found to be within the variability of the species and cannot be confirmed without a closer investigation.

Doriopsis pecten Collingwood, 1881

Figure 13E

Material examined

Two specimens 6 mm, CB.

Ecology

Among rubble in coral reef habitats. Depth 3–6 m.

Distribution

Across the Indo-Pacific including Taiwan (Collingwood 1881), Vietnam (Martynov and Korshunova 2012), the Philippines, Indonesia (Yonow 2017), New Caledonia (Risbec 1953), South Africa, Madagascar, and Hawaii (Gosliner et al. 2008). Previously documented from the Gulf of Thailand (Chavanich et al. 2013).

Doriopsis viridis (Pease, 1861)

Figure 13F

Material examined

Two specimens 4–6 mm, CB.

Ecology

Among rubble in coral reef habitats. Depth: 3–6 m.

Distribution

Known from China (Lin 1990), Vietnam (Risbec 1956), Tahiti, Hawaii, and western Mexico (Gosliner et al. 2008). Here representing a first record for Thai waters.

Genus Doris Linnaeus, 1758

Doris cf. immonda Risbec, 1928

Figure 13G

Material examined

One specimen 5 mm, HF.

Ecology

Under reef rubble and on rocks in coral reef habitats. Cryptic on its orange prey sponge. Depth 6–12 m.

Distribution

Doris immonda is known across the Indo-Pacific including Japan (Rudman 2000a), Australia (Nimbs and Smith 2016), Papua New Guinea, New Caledonia, Hawaii (Gosliner et al. 2008), and the Pacific coast of Costa Rica (Camacho-García and Gosliner 2008b). Here representing a first record for Thai waters.

Remarks

Broadly matches the description and variations highlighted by Valdés (2002); however, like specimens of D. cf. granulosa, specimens documented from Koh Tao differ in colouration from Doris immonda. The dorsal colour is a bright orange with numerous rounded or conical tubercles covering the surface, many capped in white. The Y-shaped marking between the rhinophores to in front of the gills is made up of purple tubercles and the rhinophore club is a pale brown with white edges of lamellae. As above, this variation in colouration may yet be considered to be within what can be found in Doris immonda Risbec, 1928.

Dorididae sp.

Figure 13H, I

Material examined

Three specimens 6–15 mm, SN.

Ecology

In soft sediment habitats beyond the coral reef.

Distribution

Unknown. Currently only documented from Koh Tao.

Remarks

A small dorid with a dark mantle ranging from grey to dark brown, covered in numerous small, clearly separated pustules. Gills arranged circularly, pinnate, dark brown. The lamellate rhinophores are basally dark brown with translucent white clubs and reddish brown apices with white tips. A much more in-depth analysis of this species is needed to ascertain its placement.

Superfamily Onchidoridoidea Gray, 1827

Family Goniodorididae H. Adams & A. Adams, 1854

Genus Goniodoridella Pruvot-Fol, 1933

Goniodoridella sp. 1

Figure 13J

Material examined

One specimen 5 mm, TT; two specimens 6–8 mm, CA.

Ecology

Rare and cryptic within coral reef habitats. Depth 8–24 m.

Distribution

Similar to Goniodoridella sp. 2 (Gosliner et al. 2008) which is known only from the Philippines and Papua New Guinea. First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Goniodoridella sp. 2

Figure 13K

Material examined

One specimen 3 mm, SI.

Ecology

Rare and cryptic within coral reef habitats. Depth 14 m.

Distribution

Similar to Goniodoridella sp. 10 (Gosliner et al. 2018) which is known only from the Indonesia. Here documented as a first record for Thai waters.

Genus Trapania Pruvot-Fol, 1931

Trapania cf. gibbera Gosliner & Fahey, 2008

Figure 13L

Material examined

One specimen 5 mm, CB.

Ecology

Under rubble in shallow coral reefs. Depth 4–6 m.

Distribution

Trapania gibbera is known from Indonesia, Japan, and Papua New Guinea (Gosliner and Fahey 2008). Here representing a first record for Thai waters.

Remarks

Specimens from Koh Tao differ slightly from Trapania gibbera Gosliner and Fahey 2008 in colouration. The ‘hump’ located anterior to the gills, for which T. gibbera was named, is also seen in specimens from Koh Tao. The dorsal colouration is tan rather than white with numerous reddish brown patches spread over the body, sometimes giving animals a reticulated appearance. Rhinophore clubs have nine lamellae that are mostly white with some edges being red, with translucent red stalks and red apices. A deep red mark can be seen at the anterior margin of the head that extends to the oral tentacles, which turn orange-brown and have a single distinctive white spot on the dorsal surface of each. Ventrally the oral tentacles are entirely orange.

Trapania miltabrancha Gosliner & Fahey, 2008

Figure 14A, B

Material examined

Three specimens 8–25 mm, LB.

Ecology

Among colonies of Didemnid tunicates in soft sediment habitats. Depth 12–15 m.

Distribution

Known only from Indonesia (Gosliner and Fahey 2008) and Japan (Uyeno and Nagasawa 2012). Here representing a first record for Thai waters

Remarks

While known to be predators of Entoprocta (Gosliner et al. 2018), the present individuals were all found directly upon Didemnum molle tunicate colonies in soft sediment habitats. While active feeding could not be confirmed, it is of interest to note that all 12 individuals recorded were initially found crawling upon the tunicates and not upon the benthos which may indicate the presence of a prey source associated with the tunicates but feeding investigations were not carried out.

Figure 14. 

A, B Trapania miltabrancha 25 mm (A) two specimens on Didemnum molle (B) C Dendrodoris coronata 30 mm (photograph by Kirsty Magson) D Dendrodoris krusensternii 45 mm E Dendrodoris elongata 29 mm F Dendrodoris fumata 55 mm G Dendrodoris nigra 16 mm H Dendrodoris tuberculosa 88 mm I Phyllidia coelestis specimens 25–35 mm J Phyllidia elegans 45 mm (photograph by Pau Urgell Plaza) K Phyllidia exquisita 30 mm L Phyllidia ocellata 60 mm.

Superfamily Phyllidioidea Rafinesque, 1814

Family Dendrodorididae O’Donoghue, 1924 (1864)

Genus Dendrodoris Ehrenberg, 1831

Dendrodoris coronata Kay & Young, 1969

Figure 14C

Material examined

One specimen 30 mm, LT.

Ecology

On rocks and among corals in shallow reefs. Depth 2–6 m.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2008), Mozambique (Tibiriçá et al. 2017), Japan, Australia, Palau, Papua New Guinea, New Caledonia, Marshall Islands (Gosliner et al. 2008), probably Indonesia (Yonow 2017), and Hawaii (Kay and Young 1969). Here representing a first record for Thai waters.

Dendrodoris krusensternii (Gray, 1850)

Figure 14D

Material examined

Three specimens 8–45 mm, TT; one specimen 22 mm, SB.

Ecology

Exclusively recorded from soft sediment habitats beyond the fringing coral reef. Depth 14–26 m.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2015), Mozambique (Tibiriçá et al. 2017), South Africa, Hawaii (Gosliner 1987), Singapore, the Philippines, Indonesia, Korea, Papua New Guinea, New Caledonia, New Zealand (Gosliner et al. 2008), Japan (Baba 1949), Australia (Angas 1864) and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Dendrodoris elongata Baba, 1936

Figure 14E

Material examined

Three specimens 29–42 mm, CB.

Ecology

Found under coral rubble and the skeletons of dead Fungiidae corals. Depth 3–8 m.

Distribution

Across the Indo-Pacific including the Red Sea (Yonow 2008), India (Vadher and Kardani 2018), Vietnam (Risbec 1956), China (Lin 1990), Malaysia, the Philippines, Australia, New Caledonia, Fiji (Gosliner et al. 2008), and Indonesia (Yonow 2017). Here representing a first record for Thai waters.

Dendrodoris fumata (Rüppell & Leuckart, 1830)

Figure 14F

Material examined

One specimen 7 mm, SB; one specimen 55 mm, CB.

Ecology

Found under coral rubble and the skeletons of dead fungiid corals. Depth 3–8 m

Distribution

Widespread in the Indo-Pacific including South Africa, Tanzania, Madagascar, Malaysia, Palau, Vanuatu, New Caledonia, Hawaii (Gosliner et al. 2008), Red Sea (Yonow 2008), Mauritius, Socotra, Persian Gulf, La Réunion (Yonow 2012), Hong Kong, Papua New Guinea, Fiji, Australia (Brodie et al. 1997), Indonesia (Yonow 2017), and known from the Gulf of Thailand (Chavanich et al. 2013).

Dendrodoris nigra (Stimpson, 1855)

Figure 14G

Material examined

Two specimens 8–16 mm, LT; one specimen 25 mm, MH.

Ecology

Found under coral rubble and the skeletons of dead Fungiidae corals. More abundant towards the edge of the reef, less abundant but present in soft sediment habitats outside of the coral reef. Depth 2–25 m.

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Mauritius (Yonow and Hayward 1991), Red Sea (Yonow 1990), South Africa (Gosliner 1987), Socotra, Maldives, Zanzibar, Gulf of Oman, Seychelles, La Réunion, (Yonow 2012), India (Apte 2009), Gulf of Oman (Fatemi and Attaran 2015), Indonesia (Yonow 2017), Vietnam (Risbec 1956), Japan (Stimpson 1855), Australia (Burn 2006), Hawaii (Kay and Young 1969), and known from both Andaman and Gulf waters of Thailand (Jensen 1998; Chavanich et al. 2013).

Dendrodoris tuberculosa (Quoy & Gaimard, 1832)

Figure 14H

Material examined

Two specimens 24–88 mm, TT.

Ecology

Found in soft sediment habitats outside the coral reef alongside D. krusensternii. Incorrectly classified as a coral reef-associated species from Koh Tao (Mehrotra and Scott 2016) based on a single observation in shallower waters nearer the coral reef.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2008), Tanzania, South Africa, Maldives, Malaysia, Philippines, Korea, Papua New Guinea, Australia, Solomon Islands, Marshall Islands (Gosliner et al. 2008), Mozambique (Tibiriçá et al. 2017), India (Apte 2009), Chagos (Yonow et al. 2002), Vietnam (Risbec 1956), Japan (Baba 1949), Hawaii (Kay and Young 1969), and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013; Mehrotra and Scott 2016).

Family Phyllidiidae Rafinesque, 1814

Genus Phyllidia Cuvier, 1797

Phyllidia coelestis Bergh, 1905

Figure 14I

Material examined

Three specimens 25–35 mm, GR.

Ecology

Abundant in coral reef habitats. Depth 3–30 m

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), South Africa (Brunckhorst 1993), Madagascar, Seychelles (Yonow 2012), Tanzania (Edmunds 1972), Sri Lanka (Yonow 1984), Chagos Islands (Yonow et al. 2002), India (Apte 2009), the Philippines, Japan, Papua New Guinea (Gosliner et al. 2008), Australia (Nimbs and Smith 2016), China (Lin 1990), Indonesia (Yonow 2011), and known from both Andaman and Gulf waters of Thailand (Brunckhorst 1993; Chavanich et al. 2013).

Phyllidia elegans Bergh, 1869

Figure 14J

Material examined

One specimen 45 mm, LT; one specimen 30 mm, RR; one specimen 45 mm, SP.

Ecology

Abundant in coral reef habitats. Depth 3–30 m

Distribution

Widespread in the western Pacific including), Indonesia, Taiwan, Australia, Guam, Solomon Islands (Brunckhorst 1993), Myanmar, Malaysia, the Philippines, Japan, Fiji, Papua New Guinea, Vanuatu (Gosliner et al. 2008), Vietnam (Risbec 1956), China (Lin 1990), Singapore (Lim and Chou 1970) and known from both Andaman and Gulf waters of Thailand (Brunckhorst 1993; Chavanich et al. 2013).

Phyllidia exquisita Brunckhorst, 1993

Figure 14K

Material examined

One specimen 30 mm, SI.

Ecology

Rare, found in coral reef habitats, Depth 15 m.

Distribution

Known from the Maldives (Yonow 2012), Vietnam (Martynov and Korshunova 2012), Hong Kong (Orr 1981), Malaysia, Indonesia, Philippines, Palau, Japan (Gosliner et al. 2008), Australia, Papua New Guinea, Fiji, the Marshall Islands, and the Andaman sea of Thailand (Brunckhorst 1993). Here representing a first record for the Gulf of Thailand.

Phyllidia ocellata Cuvier, 1804

Figure 14L

Material examined

One specimen 30 mm, AMN; one specimen 65 mm, SP; three specimens 45–60 mm, HF.

Ecology

Abundant in coral reef, reef edge, and soft sediment habitats. Depth 3–30 m.

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Oman, Sri Lanka, the Philippines (Debelius 1996), India (Ramakrishna et al. 2010), Myanmar (Sanpanich and Duangdee 2019), Indonesia (Yonow 1996), South Africa, Mauritius, Madagascar, Tanzania, Japan, Papua New Guinea, Australia, Guam, Fiji, Vanuatu (Gosliner et al. 2008) and known from both Andaman and Gulf waters of Thailand (Brunckhorst 1993; Chavanich et al. 2013).

Phyllidia picta Pruvot-Fol, 1957

Figure 15A

Material examined

Two specimens 28 mm, CB; one specimen 33 mm, TB.

Ecology

Abundant in coral reef habitats. Depth 3–30 m

Distribution

Known mostly from South-East Asia and the western Pacific including Malaysia, the Philippines, Indonesia, Hong Kong, Papua New Guinea, Australia, Japan, Fiji, Vanuatu, the Solomon Islands (Gosliner et al. 2008) and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013). Indian Ocean records are limited but include Socotra Island and Christmas Island (Yonow 1996, 2012).

Remarks

External identification of some specimens yielded the incorrect inclusion of Phyllidia marindica (Yonow & Hayward, 1991) by Mehrotra and Scott (2016) from Koh Tao. More intensive surveys have yielded no specimens that externally match P. marindica, which appears to be limited to the Indian Ocean. Brunckhorst (1993) recorded this species from the Andaman coast of Thailand and later Chavanich et al. (2013) included the species from the Gulf of Thailand. Collections of P. marindica from the Gulf of Thailand are needed to firmly establish its presence within the Gulf and therefore indicating its range as one not exclusive to the Indian Ocean.

Figure 15. 

A Phyllidia picta specimens 28 mm B Phyllidia varicosa 55 mm (photograph by Pau Urgell Plaza) C Phyllidiella nigra 50 mm D Phyllidiella cf. pustulosa 35 mm E Phyllidiopsis loricata 28 mm F Gymnodoris cf. alba 25 mm (photograph by Tine Kvamme) G Gymnodoris ceylonica 35 mm H Gymnodoris impudica 60 mm I Gymnodoris inornata 28 mm (photograph by Pau Urgell Plaza) J Gymnodoris sp. 1 25 mm (photograph by Pim Bontenbal) K, L Gymnodoris sp. 2 14 mm, same specimen feeding on Dendrodoris elongata.

Phyllidia varicosa Lamarck, 1801

Figure 15B

Material examined

One specimen 32 mm, HWB; one specimen 55 mm, SR.

Ecology

Abundant in coral reef habitats. Depth 3–30 m.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 1996), South Africa (Gosliner 1987), Tanzania (Edmunds 1971), Mozambique (Tibiriçá et al. 2017), Madagascar (Risbec 1928), Socotra, Kenya, Mauritius, Zanzibar, Seychelles, Maldives (Yonow 2012),Chagos Islands (Yonow et al. 2002), Sri Lanka, Hawaii (Debelius 1996), India (Apte 2009), Myanmar (Sanpanich and Duangdee 2019), Philippines, Vietnam (Brunckhorst 1993), Indonesia (Yonow 2011), Australia (Nimbs and Smith 2016), Palau, Japan, Papua New Guinea (Gosliner et al. 2008), and known from both Andaman and Gulf waters of Thailand (Brunckhorst 1993; Chavanich et al. 2013).

Genus Phyllidiella Bergh, 1869

Phyllidiella nigra (van Hasselt, 1824)

Figure 15C

Material examined

One specimen 50 mm, AMN; one specimen 50 mm, TB.

Ecology

Abundant in coral reef habitats. Depth 3–30 m.

Distribution

Widespread in the Indo-Pacific including the Philippines, Australia, Papua New Guinea (Brunckhorst 1993), Myanmar (Sanpanich and Duangdee 2019), Malaysia, Vietnam, Indonesia, Japan, Palau, Guam (Gosliner et al. 2008), and known from both Andaman and Gulf waters of Thailand (Swennen et al. 2001; Chavanich et al. 2013).

Phyllidiella cf. pustulosa (Cuvier, 1804)

Figure 15D

Material examined

Two specimens 35–40 mm, TT; one specimen 35 mm, CB.

Ecology

Abundant in coral reef habitats. Depth 3–30 m.

Distribution

Phyllidiella pustulosa’ is known from India (Apte 2009), the Red Sea, Indonesia, the Philippines, Japan, Papua New Guinea, Australia, Fiji (Brunckhorst 1993), Hawaii (Gosliner et al. 2008), both Andaman and Gulf waters of Thailand (Brunckhorst 1993; Chavanich et al. 2013), and an externally different morphotype referred to as ‘Phyllidiella cf. pustulosa’ is known from Mozambique (Tibiriçá et al. 2017).

Remarks

Morphological and molecular work (Chang and Willan 2015; Stoffels et al. 2016) has indicated that P. pustulosa is composed of a number of cryptic species. Specimens recorded from Koh Tao externally look similar to those illustrated in Brunckhorst (1993: pl. 5E, F) and Stoffels et al. (2016: figs 11I, J, 12F, 13B) but dissimilar from specimens in Mozambique (Tibiriçá et al. 2017: figs 20C, D).

Genus Phyllidiopsis Bergh, 1876

Phyllidiopsis loricata (Bergh, 1873)

Figure 15E

Material examined

One specimen 28 mm, SB; one specimen 15 mm, CP.

Ecology

Under and among rocks and coral rubble. Depth 3–30 m.

Distribution

Across the Indo-Pacific including Australia, Guam, Marshall Islands, Tahiti (Brunckhorst 1993), western Indian Ocean of Réunion, Papua New Guinea, Hawaii (Gosliner et al. 2008) and Singapore (Lim and Chou 1970). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Superfamily Polyceroidea Alder & Hancock, 1845

Family Polyceridae Alder & Hancock, 1845

Genus Gymnodoris Stimpson, 1855

Gymnodoris cf. alba (Bergh, 1877)

Figure 15F

Material examined

One specimen 25 mm, SI.

Ecology

Among coral and rubble in coral reef habitats. Depth 4–18 m.

Distribution

Gymnodoris alba is currently believed to be found across the Indo-Pacific, from the Red Sea (Yonow 2008), Australia (Nimbs and Smith 2016), South Africa, the Philippines, Japan, Papua New Guinea, and Hawaii (Gosliner et al. 2008). Gymnodoris alba is also known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Remarks

Similar to Gymnodoris alba (Bergh, 1877) but more observations and material are needed to confidently ascertain an identification. A clarification of the G. alba species complex and indeed a revision of the genus is sorely needed to provide a biogeographical and taxonomic understanding of Gymnodoris in the Indo-Pacific. This species was first recorded as Gymnodoris sp. by Mehrotra and Scott (2016: fig. 2E) and is distinct from specimens of G. alba as recorded in both Andaman and Gulf waters by Chavanich et al. (2013).

Gymnodoris ceylonica (Kelaart, 1858)

Figure 15G

Material examined

Two specimen 35 mm, TB.

Ecology

Exclusively recorded from soft sediment habitats beyond the fringing coral reef. Depth 14 m.

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), the Red Sea (Yonow 1990), Indonesia (Debelius 1996), India (Apte 2009), Taiwan (Huang et al. 2015), Singapore (Lim and Chou 1970), Réunion Island, Seychelles, Sri Lanka, the Philippines, Japan, Papua New Guinea, Australia, Guam, Marshall Islands (Gosliner et al. 2008), and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013; Mehrotra and Scott 2016).

Gymnodoris impudica (Rüppell & Leuckart, 1830)

Figure 15H

Material examined

One specimen 45 mm, TT; two specimens 65 mm, HF; one specimen 60 mm, TW.

Ecology

In coral reef, rubble and soft sediment habitats throughout the island and nearby offshore pinnacles. Depth 5–25 m.

Distribution

Widespread in the Indo-Pacific including the Red Sea (Yonow 2008), Mozambique (Tibiriçá et al. 2017), India (Ramakrishna et al. 2010), Singapore (Lim and Chou 1970), Japan (Baba 1949), South Africa, Tanzania, the Philippines, Indonesia, New Caledonia, Hawaii (Gosliner et al. 2008) and known from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Gymnodoris inornata (Bergh, 1880)

Figure 15I

Material examined

One specimen 28 mm, HF.

Ecology

Among coral and rubble in coral reef habitats. Depth 6–12 m.

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), China (Lin 1990), Hong Kong (Orr 1981), Japan (Baba 1949), Australia (Nimbs and Smith 2016), South Africa, Tanzania, Red Sea, Indonesia, Philippines, New Caledonia, and Hawaii (Gosliner et al. 2008). Here representing a first record for Thai waters.

Gymnodoris sp. 1

Figure 15J

Material examined

One specimen 25 mm, TT.

Ecology

On rocks and among coral and rubble in coral reef habitats. Depth 4–18 m.

Distribution

First documented as Gymnodoris sp. from the Gulf of Thailand by Mehrotra and Scott (2016: fig. 2D). Similar Gymnodoris species (Gosliner et al. 2008: sp. 5; Gosliner et al. 2018: sp. 21) known from Japan and Papua New Guinea only.

Gymnodoris sp. 2

Figure 15K, L

Material examined

One specimen 14 mm, CB.

Ecology

Among rubble in shallow coral reef habitats, observed feeding on Dendrodoris elongata by progressively feeding on the extended mantle around the animal. Depth 6 m.

Distribution

Unknown.

Remarks

Resembling multiple species considered undescribed according to Gosliner et al. (2008, 2018). Gills white, pinnate, overall body relatively cylindrical, translucent white with numerous very small yellow-orange spots throughout the dorsum, internal viscera visible throughout. Rhinophores triangular, pale yellow.

Gymnodoris sp. 3

Figure 16A

Material examined

One specimen 12 mm, HF.

Ecology

Among rubble in coral reef habitats. Depth 6–12 m.

Distribution

Gymnodoris sp. 36 (Gosliner et al. 2018) is known from the Philippines and Papua New Guinea.

Remarks

Smooth, translucent white dorsum with club shaped gill leaves. Observed feeding on an unknown nudibranch species (Fig. 16A).

Figure 16. 

A Gymnodoris sp. 3 12 mm B Gymnodoris nigricolor 5 mm C Gymnodoris cf. nigricolor 6 mm D Gymnodoris sp. 4 9 mm E Plocamopherus tilesii 90 mm F Polycera sp. 12 mm G Tambja amakusana 6 mm (photograph by Richard Brinke) H Tambja pulcherrima 40 mm (photograph by Liam Kelly) I Tambja sp. 4 mm (photo by Kaitlyn Harris) J Thecacera sp. 6 mm K Anteaeolidiella sp. 15 mm L Aeolidiopsis harrietae 17 mm.

Gymnodoris nigricolor Baba, 1960

Figure 16B

Material examined

Two specimens 5–7 mm, SN.

Ecology

Recorded exclusively from deeper soft sediment habitats, crawling on the substrate. Depth 19–27 m.

Distribution

Gymnodoris nigricolor is known from the Philippines, Japan, New Caledonia, and the Solomon Islands (Gosliner et al. 2008). Here recorded as a first for Thai waters.

Remarks

The present species closely resembles externally the description by Baba (1960) possessing a black (sometimes translucent black) dorsum and rhinophores, and a translucent foot. The gills match the description in being black, small, and club-shaped and arranged in a semi-circle. It should also be noted that the plate supplementing the original description of the species shows gills in a circular arrangement and seemingly plumose rather than reduced and club shaped. While the species is known to be parasite of gobies (Osumi and Yamasu 1994), all observations from Koh Tao were not in the vicinity of any demersal fish, including any in the family Gobiidae. Ongoing searches of gobies from both soft sediment and coral reef habitats have thus far revealed no observations of parasitic nudibranchs.

Gymnodoris cf. nigricolor Baba, 1960

Figure 16C

Material examined

One specimen 6 mm, SB; one specimen 7 mm, LB.

Ecology

Recorded exclusively from deeper soft sediment habitats, crawling on the substrate. Depth 19–27 m.

Distribution

Unknown

Remarks

Strikingly similar to Gymnodoris nigricolor from Koh Tao in sharing habitat, size, and much of their external morphology except the arc of gills. Both observed specimens, several months and kilometres apart, were found to possess white club-shaped gills instead of black. As such we treat this species as likely distinct.

Gymnodoris sp. 4

Figure 16D

Material examined

One specimen 9 mm, TW.

Ecology

In soft sediment habitats beyond fringing coral reefs. Depth 12–16 m.

Distribution

So far only recorded in the Gulf of Thailand.

Remarks

Incorrectly identified as Gymnodoris bicolor (Alder & Hancock, 1864) by Mehrotra and Scott (2016) due to low quality images, additional material has shown several unique differences, including translucent club-shaped gills with yellow tips in an arc and a prominently raised area between the eyes, behind the rhinophores.

Genus Plocamopherus Rüppell & Leuckart, 1828

Plocamopherus tilesii Bergh, 1877

Figure 16E

Material examined

Two specimens 102 mm, TT.

Ecology

Exclusively recorded from soft sediment habitats beyond the fringing coral reef. Depth 18–25 m.

Distribution

Plocamopherus tilesii is known from Turkey (Yokeş et al. 2012), Vietnam (Martynov and Korshunova 2012), the Philippines (Gosliner et al. 2008), Hong Kong (Jensen 2000), Japan (Nakano 2017), China (Gosliner and Vallès 2006), Australia (Nimbs and Smith 2016), and Korea (Song et al. 2017). Here documented as a first record for Thai waters.

Genus Polycera Cuvier, 1816

Polycera sp.

Figure 16F

Material examined

One specimen 12 mm, SN.

Ecology

Exclusively recorded from soft sediment habitats beyond the fringing coral reef where it feeds on Bugulidae spp. arborescent bryozoans. Depth 12–25 m.

Distribution

Polycera sp. 1 (Gosliner et al. 2008, 2018) is known from Indonesia only. Here representing a first record for Thai waters.

Genus Tambja Burn, 1962

Tambja amakusana Baba, 1987

Figure 16G

Material examined

One specimen 6 mm, CP.

Ecology

Among rocks and corals on offshore pinnacle sites.

Distribution

Widespread in the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Maldives (Yonow 2012), Japan (Baba 1987), Australia (Marshall and Willan 1999), Papua New Guinea (Pola et al. 2006), Vanuatu and Hawaii (Gosliner et al. 2008). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Tambja pulcherrima Willan & Chang, 2017

Figure 16H

Material examined

One specimen 40 mm, SWP.

Ecology

Locally recorded exclusively in deep soft sediment habitats at an offshore pinnacle. Depth 25–30 m.

Distribution

Known from South Korea, Japan, Taiwan, Malaysia, Papua New Guinea Australia, and New Zealand (Willan and Chang 2017). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Remarks

Externally, the present species matches the species recently described by Willan and Chang (2017) and was initially documented as an unidentified species of Tambja by Mehrotra and Scott (2016: fig. 2H).

Tambja sp.

Figure 16I

Material examined

One specimen 4 mm, TT.

Ecology

Recorded from soft sediment habitats beyond the fringing coral reef where it was found on Bugulidae spp. Depth 12–25 m.

Distribution

Tambja sp. 2 (Gosliner et al. 2018) is known from the Philippines and Indonesia. Here representing a first record for Thai waters. All available photographs show this species on the arborescent bryozoans of the family Bugulidae.

Genus Thecacera J. Fleming, 1828

Thecacera sp.

Figure 16J

Material examined

Two specimens 6–16 mm, SB; one specimen 12 mm, KKR.

Ecology

Exclusively recorded from soft sediment habitats beyond the fringing coral reef where it feeds on Bugulidae spp. arborescent bryozoans. Depth 12–25 m.

Distribution

Similar species are known from Mozambique (Tibiriçá et al. 2017), Maldives (Yonow 1994), Malaysia, Indonesia, and Japan (Gosliner et al. 2008).

Remarks

While similar to Thecacera cf. picta Baba, 1972 from Mozambique (Tibiriçá et al. 2017) our preliminary investigations indicate that this species is distinct from both Thecacera picta Baba, 1972, Thecacera vittata Yonow, 1994, and the supposedly pan-tropical/sub-tropical species Thecacera pennigera (Montagu, 1813) which was incorrectly documented from Koh Tao by Mehrotra and Scott (2016). This species is also distinct from Thecacera pennigera previously recorded from both Andaman and Gulf waters of Thailand by Chavanich et al. (2013).

Suborder Cladobranchia Willan & Morton, 1984

Superfamily Aeolidioidea Gray, 1827

Family Aeolidiidae Gray, 1827

Genus Anteaeolidiella Miller, 2001

Anteaeolidiella sp.

Figure 16K

Material examined

One specimen 15 mm, CA.

Ecology

Among corals and rubble in coral reef habitats. Depth 6–10 m.

Distribution

Anteaeolidiella cacaotica (Stimpson, 1855) is known from Australia and Japan (Carmona et al. 2014b) and Anteaeolidiella sp. 2 (Gosliner et al. 2018) is known from Indonesia. Here representing a first record for Thai waters.

Genus Aeolidiopsis Pruvot-Fol, 1956

Aeolidiopsis harrietae Rudman, 1982

Figure 16L

Material examined

Two specimens 21–29 mm, LB; one specimen 17 mm, SRB.

Ecology

Cryptic on their prey, Palythoa sp. which has thus far only been recorded growing on isolated pieces of rubble or artificial substrate (i.e., discarded plastic) in deeper soft sediment habitats outside the coral reef.

Distribution

Known from Japan (Ono 2004), Australia (Rudman 1982), the Philippines and Papua New Guinea (Gosliner et al. 2008). Previously recorded from the Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Specimens from Koh Tao have papillate rhinophores (similar to Carmona et al. 2014a: fig. 3D) with distinctly white tips, have between 4–6 cerata per row which are the same brown colour as the dorsum with distinctive yellowish to pale brown cnidosacs.

Aeolidiopsis ransoni Pruvot-Fol, 1956

Figure 17A, B

Material examined

Two specimens 25–30 mm, SB; one specimen 23 mm, CB.

Ecology

Exclusively recorded on its prey species, the zoanthid Palythoa tuberculosa (Esper, 1805), on which it is extremely cryptic. Depth 1–18 m.

Distribution

Across the Pacific including Japan (Ono 2004), French Polynesia (Pruvot-Fol 1956), Hawaii (Carmona et al. 2014a), the Philippines, Australia, and Papua New Guinea (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

Separated from Aeolidiopsis harrietae and A. palythoae (Gosliner, 1985) by the presence of smooth rhinophores and highly elongate body with between 17–23 pairs of cerata. Unlike specimens described by Carmona et al. (2014b) but similar to those described by Rudman (1982), specimens from Koh Tao have rhinophores that lack white tips.

Figure 17. 

A, B Aeolidiopsis ransoni 23 mm (A) and 25 mm (B) C Baeolidia salaamica 10 mm D Cerberilla ambonensis 20 mm E Cerberilla asamusiensis 8 mm (photograph by Pau Urgell Plaza) F, G Cerberilla cf. incola variants 12 mm (F) and 7 mm (G) H Cerberilla sp. 22 mm I Limenandra confusa 12 mm J Caloria indica 26 mm K Favorinus sp. 1 5 mm L Favorinus sp. 2 5 mm

Genus Baeolidia Bergh, 1888

Baeolidia salaamica (Rudman, 1982)

Figure 17C

Material examined

One specimen 10 mm, CA.

Ecology

Among corals and rubble in coral reef habitats. Depth 6–10 m.

Distribution

Across the Indo-Pacific including Tanzania (Rudman 1982), Japan (Ono 2004), Korea (Koh 2006), Hawaii (Carmona et al. 2014a), Philippines, and Papua New Guinea (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

Rhinophores with numerous small white knobs leading to white apices, a faint white ring visible on the head, anterior to the rhinophores. Foot white and wide. Oral tentacles basally translucent with white tips, often held curled closer to the head (Fig. 17C) but longer than rhinophores when extended. Cerata moderately long, brown with white speckles with very distinctive white cnidosacs. A similar brown colour (though often more orange) is the main colouration of the dorsum, head, and rhinophores, under the aforementioned white pigmentation, with some faint white pigmentation anterior and posterior to the pericardium. This brownish orange colouration appears to be the main externally obvious difference between the specimens from Koh Tao and those described by Carmona et al. (2014b), but orange pigmentation was described in the original description of the species, particularly in relation to the rhinophores.

Genus Cerberilla Bergh, 1873

Cerberilla ambonensis Bergh, 1905

Figure 17D

Material examined

One specimen 10 mm, TT; one specimen 20 mm, SN.

Ecology

Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.

Distribution

Across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), India (Ramakrishna et al. 2010), Indonesia, Australia, and the Solomon Islands (Gosliner et al. 2008). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Cerberilla asamusiensis Baba, 1940

Figure 17E

Material examined

One specimen 8 mm, TT.

Ecology

Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.

Distribution

Across the Indo-Pacific including Australia (Nimbs and Smith 2016), China (Lin 1990), South Korea (Koh 2006), Japan (Baba 1957), and Indonesia (Gosliner et al. 2018). First documented from the Gulf of Thailand by Mehrotra and Scott (2016).

Cerberilla cf. incola Burn, 1974

Figure 17F, G

Material examined

Two specimens 7–12 mm, TT.

Ecology

Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.

Distribution

Cerberilla incola is known from Australia (Burn 1974) and Réunion Island (Bachel 2010). First documented from the Gulf of Thailand by Chavanich et al. (2013).

Remarks

Cerberilla incola as described by Burn (1974) represents a largely brown animal with dark brown rhinophores, dark brown lines across oral tentacles and along lateral and central lines of the dorsum with central cerata tipped with dark arrow-shaped marks. A later observation from near the type locality of South-East Australia (Cobb 2010) highlighted a light brown to nearly white specimen with pale blue lines along oral tentacles, a distinctive yellow-orange band along the anterior portion of the head, and with most cerata bearing parallel lines ranging from light to dark brown. A further observation from Réunion Island in the Indian Ocean (Bachel 2010) was also considered to be C. incola by Rudman (2010) bearing the same parallel lines along the cerata but lacking in the yellow-orange band of the earlier observation. All animals appear to have dark brown-grey rhinophores and the same dark colour anterior to the pericardium. Specimens from Koh Tao have been observed to show external variability (Fig. 17F, G) and further investigation is required.

Cerberilla sp.

Figure 17H

Material examined

One specimen 22 mm, SN.

Ecology

Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.

Distribution

Cerberilla sp. 4 (Gosliner et al. 2018) is currently known from Indonesia.

Genus Limenandra Haefelfinger & Stamm, 1958

Limenandra confusa Carmona, Pola, Gosliner & Cervera, 2014

Figure 17I

Material examined

One specimen 4 mm, LT; one specimen 7 mm, LB; two specimens 6–12 mm, CB.

Ecology

On rocks and under rubble, including skeletons of dead fungiid corals, in coral reef habitats. Depth 2–14 m.

Distribution

Until recently, known only from the Pacific including Costa Rica (Camacho-García et al. 2005), Gulf of California (Bertsch 1972), Hawaii (Kay 1979), Australia (Nimbs and Smith 2016), Mexico, and the Philippines (Gosliner et al. 2008). Recorded in the Indian Ocean from Mozambique (Tibiriçá et al. 2017) and the Red Sea (Yonow 2015). Here documented as a first record for Thai waters.

Family Facelinidae Bergh, 1889

Genus Caloria Trinchese, 1888

Caloria indica (Bergh, 1896)

Figure 17J

Material examined

One specimen 26 mm, TW; two specimens 31–39 mm, SWP; two specimens 18–25 mm, HF.

Ecology

Abundant in coral reef habitats, particularly at the numerous coral reef restoration sites at Koh Tao and at rocky pinnacle sites nearshore and offshore. Depth 2–30 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), India (Sreeraj et al. 2012), Maldives (Yonow 1994), Myanmar (Sanpanich and Duangdee 2019), Papua New Guinea (Baine and Harasti 2007), South Africa, Tanzania, Indonesia, Australia, Japan, Hawaii (Gosliner 1987), Oman, Seychelles, New Caledonia, Solomon Islands, Fiji (Gosliner et al. 2008) and documented from both Andaman and Gulf waters of Thailand by Chavanich et al. (2013).

Genus Favorinus Gray, 1850

Favorinus sp. 1

Figure 17K

Material examined

One specimen 5 mm, CB; one specimen 5 mm, MH.

Ecology

Documented in habitats ranging from shallow reef rubble and soft sediments to deeper soft sediment habitats. Depth 5–18 m.

Distribution

Favorinus sp. 8 (Gosliner et al. 2018) is known from Papua New Guinea and Palau. Here documented as a first record for Thai waters.

Remarks

Similar to Favorinus sp. 8 in Gosliner et al. (2018) which they record as feeding on the eggs of Stylocheilus striatus under ex-situ conditions. While the prey of the present species has yet to be documented, its range almost directly overlaps with the range of S. striatus from Koh Tao, which grazes on cyanobacteria in shallow and deeper soft sediment habitats at sites CB and SRB.

Favorinus sp. 2

Figure 17L

Material examined

Two specimens SB, 5 mm.

Ecology

Documented from deeper soft sediment habitats only. Depth 18–25 m.

Distribution

Favorinus sp. 4 and Favorinus sp. 12 (Gosliner et al. 2018) are known from the Philippines. Here documented as a first record for Thai waters.

Myja cf. longicornis Bergh, 1896

Figure 18A

Material examined

One specimen GR, 9 mm.

Ecology

Found exclusively on its prey hydroid Pennaria disticha (Goldfuss, 1820) uncommonly found in both coral reef and deeper soft sediment habitats. Depth 8–25 m.

Distribution

Myja longicornis is known from Indonesia (Bergh 1896), Australia (Nimbs and Smith 2016), Japan, and Papua New Guinea (Gosliner et al. 2008). A similar species Myja cf. longicornis was recently documented from the Gulf of Thailand (Martynov et al. 2019).

Remarks

This genus was recently reviewed and expanded based on specimens from Japan and the Gulf of Thailand (Martynov et al. 2019). In that study, authors concluded that the studied specimens from the Gulf of Thailand have a close resemblance to the description of M. longicornis by Bergh, with some internal and external differences that would require examination of specimens from the type locality of Ambon. The single specimen found at Koh Tao closely resembles other specimens found in the Gulf of Thailand, and thus its species designation remains unresolved in the absence of contemporary studies of the genus from closer to the type locality.

Figure 18. 

A Myja cf. longicornis 9 mm (photograph by Phannee Mccarthy) B, C Noumeaella sp. 1 5 mm D, E Noumeaella sp. 2 5 mm (photographs by Tony Myshlyaev) F Phidiana militaris 35 mm (photograph by Pau Urgell Plaza) G Phidiana sp. 7 mm H Phyllodesmium magnum 30 mm I Phyllodesmium cf. magnum 35 mm (photograph by Pau Urgell Plaza) J Phyllodesmium opalescens 35 mm, (photograph by Guillaume Gandoin) K Phyllodesmium sp. 55 mm (photograph by Pau Urgell Plaza) L Armina occulta 65 mm.

Genus Noumeaella Risbec, 1937

Noumeaella sp. 1

Figure 18B, C

Material examined

One specimen 5 mm, CB.

Ecology

Found under rocks and coral rubble in shallow coral reef habitats. Depth 4–8 m.

Distribution

Noumeaella sp. 4 is known from the Philippines (Gosliner et al. 2018). Noumeaella rehderi Er. Marcus, 1965 is known from Madagascar, Tanzania, Australia, Papua New Guinea, Philippines, Palau, Marshall Islands, and Hawaii (Gosliner et al. 2008). Here documented as a first record for Thai waters.

Noumeaella sp. 2

Figure 18D, E

Material examined

One specimen 5 mm, SN.

Ecology

Recorded from soft sediment habitats beyond the fringing coral reef. Depth 15–20 m.

Distribution

Unknown. Here documented as a first record for Thai waters.

Remarks

Generally translucent throughout with some patches of white or light grey, and cerata edged in white with white apices. Bears resemblance to a number of species within the genus and requires a larger sampling effort to identify further.

Genus Phidiana Gray, 1850

Phidiana militaris (Alder & Hancock, 1864)

Figure 18F

Material examined

Two specimens 30–35 mm, SO; two specimens 30–40 mm, HF.

Ecology

Occasionally found among rubble and corals in reef habitats. Most abundant at some artificial reef sites at the island, which were initially constructed from steel rebar or concrete to support coral restoration efforts. It is likely that these substrates, while promoting scleractinian conservation, also support different potential prey items than might be found in coral reef or soft sediment habitats. Depth 8–25 m.

Distribution

Across the Indo-Pacific including the Gulf of Oman (Fatemi and Attaran 2015), Red Sea (Yonow 2008), Malaysia (Ho 1989), United Arab Emirates, India, Singapore, and the Philippines (Gosliner et al. 2008). Known to have been introduced into the Mediterranean from the Red Sea (Rothman et al. 2017). Known from Andaman and Gulf waters of Thailand (Gosliner et al. 2008; Mehrotra and Scott 2016).

Phidiana sp.

Figure 18G

Material examined

One specimen 7 mm, HWP.

Ecology

Among rocks and coral rubble. Depth 5–16 m.

Distribution

Unknown.

Remarks

Bearing some similarities to Phidiana anulifera (Baba, 1949) and Phidiana semidecora (Pease, 1860).

Family Myrrhinidae Bergh, 1905

Genus Phyllodesmium Ehrenberg, 1831

Phyllodesmium magnum Rudman, 1991

Figure 18H

Material examined

Two specimens 30–40 mm, TT.

Ecology

In coral reef habitats where it feeds on the octocoral Sinularia sp. Depth 5–18 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), South Africa (Fraser 2001), the Red Sea (Koretz 2005), Hong Kong, New Caledonia, Australia, Marshall Islands (Rudman 1991), Japan (Ono 2004), Tanzania, Philippines, Indonesia, and Papua New Guinea (Gosliner et al. 2008). Documented from both Andaman and Gulf waters of Thailand by Chavanich et al. (2013).

Phyllodesmium cf. magnum Rudman, 1991

Figure 18I

Material examined

One specimen 15 mm, TT; one specimen 35 mm, SB.

Ecology

In soft sediment habitats outside coral reef habitats, found regularly on or in the immediate vicinity of Dendronephthya octocorals, though active feeding has not yet been confirmed. Depth 16–28 m.

Distribution

Unknown

Remarks

The present species externally appears to match Phyllodesmium magnum Rudman, 1991, though smaller with relatively short cerata and no brown pigment on the animal, having pale yellow tips to rhinophores and oral tentacles. All individuals recorded to date were separately found on or very close to Dendronephthya spp., distant from any colonies of Sinularia which at Koh Tao grow only on rocks in shallower coral reef habitats. While it is possible that some animals recorded at Koh Tao are indeed P. magnum with differences in external colouration reflecting a drastically different diet from that known for P. magnum, the present species is provisionally treated as distinct due to its unique ecology.

Phyllodesmium opalescens Rudman, 1991

Figure 18J

Material examined

One specimen 35 mm, CP.

Ecology

Among coral, rocks, and discarded fishing equipment at an offshore pinnacle site. Depth 16 m.

Distribution

Known from the Philippines, Hong Kong, Korea, and Japan (Gosliner et al. 2008). Here documented as a first record for Thai waters.

Phyllodesmium sp.

Figure 18K

Material examined

Two specimens 40–55 mm, HWB.

Ecology

In shallow coral reef habitats where it observed feeding on a different species of Sinularia than P. magnum. Depth 2–8 m.

Distribution

Phyllodesmium sp. 2 (Gosliner et al. 2018) is known from Indonesia.

Remarks

Similar to Phyllodesmium sp. 2 by Gosliner et al. (2018) which in turn is similar to P. magnum. External differences here are pale cerata that are less curved and held more upright than those of P. magnum, with digestive glands clearly visible and lacking any blueish purple pigment. Some purple-grey pigment is seen on the head.

Superfamily Arminoidea Iredale & O’Donoghue, 1923 (1841)

Family Arminidae Iredale & O’Donoghue, 1923 (1841)

Genus Armina Rafinesque, 1814

Armina occulta Mehrotra, Caballer & Chavanich, 2017

Figure 18L

Material examined

Two specimens 65–72 mm, SN.

Ecology

Exclusively known from soft sediment habitats outside the coral reef where it feeds on the sea pen Virgularia sp. Depth 14–22 m.

Distribution

Known from the western Pacific including Indonesia (Yonow 2017), the Philippines (Koehler 2002), Australia (Hatton 2019), Papua New Guinea (Adams 2000), and Palau (Gosliner et al. 2008). Known from the Gulf of Thailand (Mehrotra et al. 2017). Koh Tao is the type locality of this species.

Armina scotti Mehrotra, Caballer & Chavanich, 2017

Figure 19A

Material examined

Two specimens 35–49 mm, SB; one specimen 41 mm, TT.

Ecology

Exclusively known from soft sediment habitats outside the coral reef where it feeds on the sea pen Virgularia sp. One individual observed feeding on partially decomposed Pteroeides sp. (Octocorallia: Pennatulidae). Depth 12–29 m.

Distribution

Known from the western Pacific including Japan (Okutani 2000), Singapore, the Philippines, and Indonesia (Gosliner et al. 2008). Known from the Andaman and Gulf waters of Thailand (Chavanich et al. 2013; Mehrotra et al. 2017). Koh Tao is the type locality of this species.

Remarks

Specimens identified as Armina semperi (Bergh, 1866) from coasts of Thailand correspond to this species (Mehrotra et al. 2017).

Figure 19. 

A Armina scotti 41 mm B Dermatobranchus caeruleomaculatus 55 mm C Dermatobranchus dendronephthyphagus 40 mm D Dermatobranchus fortunatus 11 mm E Dermatobranchus semilunus 33 mm (photograph by Pau Urgell Plaza) F Dermatobranchus cf. striatus 14 mm G, H Dermatobranchus sp. 1 32 mm (G) and 28 mm (H) I, J Dermatobranchus sp. 2 27 mm K Bornella hermanni 12 mm L Bornella johnsonorum 35 mm.

Genus Dermatobranchus van Hasselt, 1824

Dermatobranchus caeruleomaculatus Gosliner & Fahey, 2011

Figure 19B

Material examined

Two specimens 55–67 mm, SN.

Ecology

Exclusive to the soft sediment habitats outside the coral reef. Depth 14–24 m.

Distribution

Known from the western Pacific including Malaysia, Indonesia, and the Philippines (Gosliner et al. 2008), Papua New Guinea (Coleman 2008), and Japan (Nakano and Fujii 2014). Here documented as a first record for Thai waters.

Dermatobranchus dendronephthyphagus Gosliner & Fahey, 2011

Figure 19C

Material examined

Two specimens 40–50 mm, SB.

Ecology

Exclusive to the soft sediment habitats outside the coral reef where it may be found feeding on Dendronephthya sp. or partially buried in the nearby silt/sand. Depth 14–24 m.

Distribution

Known from the West Pacific including Japan and Australia (Rudman 2005b; Nimbs and Smith 2016). Here documented as a first record for Thai waters.

Remarks

Recorded as Dermatobranchus sp. by Mehrotra and Scott (2016: fig. 3C).

Dermatobranchus fortunatus (Bergh, 1888)

Figure 19D

Material examined

Two specimens 7–11 mm, CB; one specimen 9 mm, SB.

Ecology

On rocks and under rubble, in particular under skeletons of dead Fungiidae corals, in shallow coral reef habitats. Depth 1–18 m.

Distribution

Across the Indo-Pacific including the Indian Ocean of Java (Bergh 1888), India (Dixit et al. 2017), Japan (Bolland 2003), Australia (Marshall and Willan 1999), the Seychelles, the Philippines, Malaysia, Indonesia, and Papua New Guinea (Gosliner and Fahey 2011). Documented from the Gulf waters of Thailand by Mehrotra and Scott (2016).

Dermatobranchus semilunus Gosliner & Fahey, 2011

Figure 19E

Material examined

One specimen 33 mm, TB.

Ecology

Exclusive to the soft sediment habitats outside the coral reef. Observed on Dendronephthya sp. octocoral, though active feeding was not observed. Depth 14–24 m.

Distribution

Known from the West Pacific including Malaysia, the Philippines, Indonesia, and Papua New Guinea (Gosliner and Fahey 2011). Here documented as a first record for Thai waters.

Remarks

Specimens from Koh Tao are sometimes found with a pale yellow-pink margin to the oral veil. A similar trait is known from the closely related Dermatobranchus fasciatus Gosliner and Fahey 2011; however, the present specimens externally match D. semilunus based on other characteristics. Additionally, similar pigment appears to be visible in a photograph of a specimen in the original description of the species (Gosliner and Fahey 2011: fig. 74C).

Dermatobranchus cf. striatus van Hasselt, 1824

Figure 19F

Material examined

Two specimens 9–12 mm, SI; two specimens 11 mm, HPW; one specimen 14 mm, SW.

Ecology

On rocks in coral reef habitats where it feeds on the octocoral Briareum stechei (Kükenthal, 1908). Occasionally observed among colonies of B. stechei growing on artificial substrates (i.e., discarded fishing nets) in deeper soft sediment habitats. Depth 6–22 m.

Distribution

Dermatobranchus striatus is known from Indonesia, Papua New Guinea, and Japan (see Gosliner and Fahey 2011).

Remarks

This species was recorded as Dermatobranchus striatus van Hasselt, 1824 by Mehrotra and Scott (2016). Due to taxonomic uncertainty between this and similar species (see Gosliner and Fahey 2011), and with D. striatus being the type species for the genus, we refer to this species as D. cf. striatus.

Dermatobranchus sp. 1

Figure 19G, H

Material examined

Three specimens 28–40 mm, SB.

Ecology

Exclusive to the soft sediment habitats outside the coral reef where it feeds on colonies of the octocoral Dendronephthya sp. Depth 14–24 m.

Distribution

Unknown.

Remarks

A species that vaguely resembles but is distinct from Dermatobranchus semilunus is regularly recorded from the same habitats and locations as other soft-sediment dwelling members of the genus. Specimens of Dermatobranchus sp. 1 externally appear to have characteristics of D. fasciatus and D. semilunus. All specimens have prominent longitudinal ridges on a generally white dorsum, with numerous black spots of varying sizes distributed along the ridges and margin of the oral veil. The oral veil always has patches of grey and the margin is sometimes pigmented with a yellow-pink band which is often pale or completely absent in some specimens. The dorsal surface usually has a single horizontal diffuse band approximately one third of the total animal length. The foot is pale pink to white, sometimes with numerous small black spots. The rhinophores have white tips, dark clubs with white lines along the lamellae, and white stalks with dark grey pigment along the inner edge of the stalks often forming a dark grey band in between and anterior to the rhinophores.

Dermatobranchus sp. 2

Figure 19I, J

Material examined

One specimen 27 mm, TT.

Ecology

Exclusive to the soft sediment habitats outside the coral reef. Depth 21 m.

Distribution

Unknown.

Remarks

Dermatobranchus sp. 2 is characterised by prominent pale yellow marginal sacs that are very visible as conical papillae. The dorsal longitudinal ridges and grooves are white to pale brown and scattered sparsely with small brown spots of varying sizes. These spots extend to the oral veil, which is noticeably whiter than the dorsum. The rhinophore stalks are translucent white followed by a sharp black band at the base of the rhinophore club. This fades into brown, becoming paler apically with a translucent white apex. The anterior foot corners are blunt and short, with the foot being white. This species resembles D. fasciatus but differs in lacking any marginal pigmentation on either mantle or foot, and possessing prominent marginal papillae that are not seen in D. fasciatus.

Superfamily Dendronotoidea Allman, 1845

Family Bornellidae Bergh, 1874

Genus Bornella Gray, 1850

Bornella hermanni Angas, 1864

Figure 19K

Material examined

Two specimens 12–18 mm, HWB.

Ecology

Among corals, rocks and under rubble in shallow coral reef habitats. Depth 2–12 m.

Distribution

Across the Indo-Pacific including Christmas Island in the Indian Ocean, Malaysia, the Marshall Islands (Pola et al. 2009), Korea (Koh 2006), Japan (Baba 1949), and Australia (Angas 1864). First recorded from the Gulf of Thailand by Mehrotra and Scott (2016).

Bornella johnsonorum Pola, Rudman & Gosliner, 2009

Figure 19L

Material examined

One specimen 35 mm, CB.

Ecology

On rocks and under rubble, in particular under skeletons of dead fungiid corals, in shallow coral reef habitats. Depth 2–8 m.

Distribution

Bornella johnsonorum is known from the Marshall Islands in the Pacific (Pola et al. 2009) and maybe from Réunion Island and the Red Sea and in the western Indian Ocean (Bidgrain 2009; Yonow 2015). Here documented as a first record for Thai waters.

Remarks

The present species matches the external description of the species (Pola et al. 2009) very closely, although lacking any signs of orange reticulation. Specimens from Koh Tao have six paired dorsal processes and a single, extremely small, unpaired dorsal process near the tip of the tail.

Bornella stellifera (A. Adams & Reeve in A. Adams, 1848)

Figure 20A

Material examined

One specimen 31 mm, CB.

Ecology

On rocks and under rubble, in particular under skeletons of dead Fungiidae corals, in shallow coral reef habitats. Depth 2–8 m.

Distribution

Widespread across the Indo-Pacific including the Red Sea (Vayssière 1912), Mozambique (Tibiriçá et al. 2017), South Africa (Gosliner 1987), Japan (Baba 1949), Singapore, Madagascar, India, Malaysia, the Philippines, Australia, Papua New Guinea, Fiji, Hawaii (Pola et al. 2009), Indonesia, Taiwan and New Caledonia (Gosliner et al. 2008). Recorded from the Andaman and Gulf waters of Thailand (Jensen 1998; Pola et al. 2009).

Figure 20. 

A Bornella stellifera 31 mm B Lomanotus vermiformis 25 mm C Scyllaea fulva 45 mm D Melibe viridis 97 mm E Melibe sp. 22 mm F Eubranchus ocellatus 22 mm G Eubranchus sp. 7 mm H Coryphellina exoptata 15 mm (photograph by Tine Kvamme) I, J Coryphellina cf. lotos variants 40 mm (I) and 35 mm (J) K Samla bicolor 12 mm L Phestilla fuscostriata 15 mm.

Family Lomanotidae Bergh, 1890

Genus Lomanotus Vérany, 1844

Lomanotus vermiformis Eliot, 1908

Figure 20B

Material examined

Two specimens 15–25 mm, SN; one specimen 17 mm, TW; one specimen 30 mm, SI.

Ecology

Cryptic on its host hydroid Macrorhynchia sp., colonies of which are found exclusively in soft sediment habitats outside the coral reef. Depth 12–34 m.

Distribution

Circumtropical, recorded from Florida, the Bahamas (Valdés et al. 2006), Panama (Collin et al. 2005), the Red Sea (Vayssière 1912), India (Dixit et al. 2017), Malaysia (Mayes 2008), the Philippines (Koehler 2005), Australia (Nimbs and Smith 2016), Indonesia, and Papua New Guinea (Gosliner et al. 2008). Recorded from the Andaman and Gulf waters of Thailand (Koehler 1999; Mehrotra and Scott 2016).

Family Scyllaeidae Alder & Hancock, 1855

Genus Scyllaea Linnaeus, 1758

Scyllaea fulva Quoy & Gaimard, 1824

Figure 20C

Material examined

One specimen 45 mm, CB.

Ecology

Recorded from a single individual on floating algae Sargassum oligocystum (Montagne, 1845).

Distribution

Across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Réunion Island (Cadet 2012), Japan (Baba 1949), the Philippines, Papua New Guinea (Pola et al. 2012), and maybe the Mediterranean (Yokes 2002; Pola et al. 2012). Here documented as a first record for Thai waters.

Family Tethydidae Rafinesque, 1815

Genus Melibe Rang, 1829

Melibe viridis (Kelaart, 1858)

Figure 20D

Material examined

One specimen 12 mm, TT; one specimen 122 mm, SB; one specimen 97 mm, SN.

Ecology

Recorded from the soft sediment habitats outside the coral reef; however, individuals have rarely been observed swimming near the surface closer to shore. It is likely that these individuals were disturbed as no individuals have been recorded in shallower reef or sandy habitats after five years of survey. Depth 14–24 m.

Distribution

Across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), Tanzania (Eliot 1904), the Red Sea (Yonow 2015), India (Parasharya and Patel 2014), Vietnam, the Philippines, Japan, and Australia (Gosliner et al. 2008). It has also recently moved into the waters of the Mediterranean (Mastrototaro et al. 2004). Recorded from the Gulf waters of Thailand (Mehrotra and Scott 2016).

Melibe sp.

Figure 20E

Material examined

Two specimens 15–22 mm, TT.

Ecology

In soft sediment habitats beyond the coral reef, grazing upon the substrate. Depth 17–22 m.

Distribution

Melibe engeli Risbec, 1937 is known across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), the Red Sea (Burghardt and Wägele 2014), the Philippines, Indonesia, Japan, New Caledonia, and Hawaii (Gosliner et al. 2008). Melibe sp. 1 (Gosliner et al. 2008) is known from Indonesia only. Recorded from Koh Tao as Melibe sp. 1 by Mehrotra and Scott (2016: fig. 3D, E).

Remarks

Externally similar to Melibe engeli and Melibe sp. 1 (Gosliner et al. 2008, 2018) in shape and some morphology, with similarities between the two also being noted by Yonow (2017). Specimens recorded from Koh Tao range in colour from near colourless to strongly golden brown, although always at least slightly transparent. The body is covered in numerous small papillae, the oral hood is able to stretch to at least half of the length of the remaining body, which have four or five pairs of cerata. Cerata are mostly ovoid to cylindrical in shape, terminating in numerous large pointed white papillae. In smaller individuals, two papillae may dominate the apex of each ceras giving them a bifurcate appearance. Most intriguingly and substantially different from the morphology of M. engeli are the rhinophore sheaths which lack the ‘sail-like’ appendage but instead have a single long and thin almost ‘whip-like’ extension. While the overall body shape of M. engeli has been shown to go through numerous changes during development (Burghardt and Wägele 2014), the specimens from Koh Tao do not entirely match M. engeli.

Superfamily Fionoidea Gray, 1857

Family Eubranchidae Odhner, 1934

Genus Eubranchus Forbes, 1838

Eubranchus ocellatus (Alder & Hancock, 1864)

Figure 20F

Material examined

One specimen 22 mm, SN.

Ecology

On its prey hydroid Idiellana pristis Lamouroux, 1816 rare in soft sediment habitats and absent from the coral reefs of Koh Tao. Depth 12–24 m.

Distribution

Eubranchus ocellatus is known from the Red Sea (Yonow 2008), Australia (Nimbs and Smith 2016), Tanzania, Philippines, Indonesia, and New Caledonia (Gosliner et al. 2008). Here representing a first record for Thai waters and a first record for the genus in the Gulf of Thailand.

Remarks

It necessary here to clarify the brief historical records of Eubranchidae in Thai waters. Chavanich et al. (2013) recorded Baeolidia japonica Baba, 1933 from the Gulf of Thailand as a member of the Eubranchidae, which in fact belongs to the Aeolidiidae, as a representation of the first record of the family from Thai waters. Not mentioned in the same review, however, was the observation of Eubranchus rubropunctatus Edmunds, 1969 from the Andaman coast of Thailand (Neal 2010) which is believed to represent the first record of the family from Thai waters.

Eubranchus sp.

Figure 20G

Material examined

One specimen 7 mm, HF.

Ecology

From an artificial reef structure at a reef restoration site in coral reef habitats. Depth 8–11 m.

Distribution

Eubranchus sp. 2 (Gosliner et al. 2008) is known from Indonesia only. A similar species was also documented in India (Bhave and Apte 2011: fig. 13). Here representing a first record for Thai waters.

Remarks

The present species bears numerous dark brown to black spots throughout its body with bulbous transparent cerata with the digestive gland clearly visible. The dorsal colour and that of ceratal tips is a pale yellow-brown. The rhinophores, oral tentacles, and head are colourless with numerous small white patches spread throughout.

Family Flabellinidae Bergh, 1889

Genus Coryphellina O’Donoghue, 1929

Coryphellina exoptata (Gosliner & Willan, 1991)

Figure 20H

Material examined

One specimen 15 mm, GR.

Ecology

Among rocks and corals in coral reef habitats. Depth 5–15 m.

Distribution

Widespread across the Indo-Pacific including Mozambique (Tibiriçá et al. 2017), India (Ramakrishna et al. 2010), South Africa, Réunion Island, Malaysia, the Philippines, Indonesia, Japan, Australia, Papua New Guinea, and Hawaii (Gosliner et al. 2008). Recorded from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Remarks

This species was recently transferred to the genus Coryphellina in an extensive revision of the family Flabellinidae (Korshunova et al. 2017a).

Coryphellina cf. lotos Korshunova et al., 2017

Figure 20I, J

Material examined

Two specimens 35–40 mm, HWP; two specimens 35 mm, KKR.

Ecology

Often feeding on hydroids growing among rocks and corals in deeper coral reef habitats and on stable substrates such as discarded nets and the remains of large terrestrial plant matter in soft sediment habitats. It is likely that the currently unknown prey hydroid of this species is able to grow in greater abundance away from shallow coral reef habitats. Depth 10–35 m.

Distribution

Coryphellina lotos is currently known only from Japan (Korshunova et al. 2017a). A similar species is here recorded for the first time from Thai waters.

Remarks

Specimens from Koh Tao strongly resemble Coryphellina lotos ; however, a few differences may indicate a possible cryptic species. While C. lotos is described as light violet with parts of the rhinophores, oral tentacles, and cerata apices as lilac to reddish lilac (Korshunova et al. 2017a: fig. 38A–E), specimens from Koh Tao appear have a background colour ranging from almost colourless to pale blue-violet, with rhinophore apices and subterminal bands on oral tentacles and cerata being a much deeper purple than the reddish violet of C. lotos. Most distinctive, however, is the presence of a mid-dorsal deep purple line which remains continuous in some specimens, entirely broken or limited to the oral surface in others, and completely absent in yet other specimens. The same pattern seen (if present) in the dorsal line is often mimicked laterally on both sides of some specimens. While no mention of dorsal or lateral linear pigmentation was made in the description of the species, these lines are visible in images provided supplementing the description (Korshunova et al. 2017a: fig. 38A–C). Erroneously identified as Flabellina rubrolineata by Mehrotra and Scott (2016), the true identity of which has recently been shown to be restricted to its type locality in the Red Sea (Ekimova et al. 2020; Yonow 2020).

Family Samlidae Korshunova et al., 2017

Genus Samla Bergh, 1900

Samla bicolor (Kelaart, 1858)

Figure 20K

Material examined

Two specimens 9–12 mm, CB; one specimen 18 mm, SI; one specimen 16 mm, SN.

Ecology

On rocks and under rubble, including skeletons of dead Fungiidae corals, in coral reef habitats. Depth 2–14 m.

Distribution

Widespread across the Indo-Pacific including the Red Sea (Yonow 2000), Mozambique (Tibiriçá et al. 2017), India (Apte 2009), Chagos Islands (Yonow et al. 2002), Myanmar (Sanpanich and Duangdee 2019), Indonesia (Eisenbarth et al. 2018), South Africa, Madagascar, Tanzania, Seychelles, Malaysia, Philippines, Hong Kong, Japan, Korea, Papua New Guinea, Australia and Hawaii (Gosliner et al. 2008). Recorded from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Family Trinchesiidae F. Nordsieck, 1972

Genus Phestilla Bergh, 1874

Phestilla fuscostriata Hu, Zhang, Xie & Qiu, 2020

Figure 20L

Material examined

Two specimens 11–15 mm, SRB; one specimen 12 mm, HF; one specimen 18 mm, LT.

Ecology

Exclusively on its prey, the scleractinian coral Pavona decussata Dana, 1846, which is abundant throughout the depth range of coral reefs around Koh Tao. Depth 1–19 m.

Distribution

Phestilla fuscostriata was previously known only from Hong Kong (Hu et al. 2020). Here we record it for the first time from Thai waters.

Phestilla lugubris (Bergh, 1870)

Figure 21A

Material examined

One specimen 30 mm, HWB; two specimens 25–30 mm, SO.

Ecology

Exclusively on or in the immediate vicinity of its prey, the scleractinian coral Porites in coral reef habitats. Locally observed to be predating colonies of Porites lobata Dana, 1846, Porites lutea Milne Edwards, 1860, and Porites sp., all of which have been observed hosting the distinctive egg ribbons of the species. Depth 1–16 m.

Distribution

Widespread across the Indo-Pacific including the Red Sea (Yonow 2000), Mozambique (Tibiriçá et al. 2017), India (Apte 2009), Vietnam (Risbec 1956), Indonesia (Burghardt et al. 2006), Tanzania, Madagascar, Seychelles, the Philippines, Japan, Papua New Guinea, Australia, New Caledonia, Hawaii, and the Pacific coast of North America (Gosliner et al. 2008). Recorded from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Figure 21. 

A Phestilla lugubris 30 mm B Phestilla melanobrachia 30 mm (photograph by Tom Jang) C Phestilla cf. minor 18 mm D Phestilla viei 20 mm E Phestilla subodiosa 2 mm F Phestilla sp. 1 8 mm G Phestilla sp. 2 4 mm H Phestilla sp. 3 3 mm I Trinchesia sp. 1 10 mm (photograph by Wanraya Kraikruan) J Trinchesia sp. 2 40 mm K Trinchesia sp. 3 45 mm (photograph by Pau Urgell Plaza) L Trinchesiidae sp. 6 mm.

Phestilla melanobrachia Bergh, 1874

Figure 21B

Material examined

Two specimens 15–30 mm, SI.

Ecology

Exclusively on or in the immediate vicinity of its prey, the scleractinian corals Tubastraea spp., in coral reef habitats. Depth 6–32 m.

Distribution

Widespread across the Indo-Pacific including Red Sea (Yonow 2000), Mozambique (Tibiriçá et al. 2017), Chagos Islands (Yonow et al. 2002), Maldives (Yonow 1994), Myanmar (Sanpanich and Duangdee 2019), Hong Kong (Scott 1984), South Africa, Réunion Island, Malaysia, the Philippines, Indonesia, Japan, Papua New Guinea, Australia, Hawaii, and Mexico (Gosliner et al. 2008). Recorded from both Andaman and Gulf waters of Thailand (Chavanich et al. 2013).

Phestilla cf. minor Rudman, 1981

Figure 21C

Material examined

Four specimens 15–25 mm, CA.

Ecology

Exclusively on or in the immediate vicinity of its prey, the scleractinian coral Porites in coral reef habitats. Locally observed to be predating colonies of Porites lobata and Porites lutea. Depth 6–14 m.

Distribution

Across the Indo-Pacific including Tanzania, Australia, Hawaii (Rudman 1981), Indonesia (Burghardt et al. 2006), Madagascar, Seychelles, Philippines, Japan, Papua New Guinea, and New Caledonia (Gosliner et al. 2008). Recorded from the Gulf waters of Thailand (Chavanich et al. 2013).

Remarks

Recent molecular analyses have revealed Phestilla minor to be a complex of up to six distinct species (Mehrotra et al. 2020a).

Phestilla viei Mehrotra, Caballer & Chavanich, 2020

Figure 21D

Material examined

Two specimens 20–25 mm, AMN; one specimen 25 mm, TT; one specimen 33 mm, SB.

Ecology

Exclusively on its prey, the scleractinian coral Pavona in coral reef habitats. Locally observed to be predating colonies of Pavona explanulata Lamarck, 1816. Depth 4–22 m.

Distribution

Phestilla viei is known from Madagascar, Thailand, Philippines, Indonesia, and Papua New Guinea (Mehrotra et al. 2020a).

Phestilla subodiosa Wang, Conti-Jerpe, Richards & Baker, 2020

Figure 21E

Material examined

One specimen 2 mm, SB.

Ecology

Exclusively on it its prey coral Montipora, in coral reef habitats. Depth 2–8 m.

Distribution

Phestilla subodiosa is currently known from Thailand (type locality Koh Tao) and South Korea, and possibly Singapore (Wang et al. 2020).

Phestilla sp. 1

Figure 21F

Material examined

Two specimens 8–14 mm, CB.

Ecology

Exclusively on its prey, the scleractinian coral Goniopora in coral reef habitats. Locally observed to be predating colonies of Goniopora fruticosa Saville-Kent, 1891. Depth 2–8 m.

Distribution

Phestilla sp. 3 (Gosliner et al. 2008) is recorded from Tanzania, the Philippines, Indonesia, Japan, Hong Kong, Papua New Guinea, Australia, and the Marshall Islands. Here representing a first record for Thai waters.

Phestilla sp. 2

Figure 21G

Material examined

Two specimens 4–7 mm, SB.

Ecology

Exclusively on its prey, the scleractinian coral Acropora sp. in coral reef habitats. Depth 2–8 m.

Distribution

Unknown. Here representing a first record for Thai waters.

Phestilla sp. 3

Figure 21H

Material examined

One specimen 3 mm, HWB.

Ecology

Recorded from a single individual found during sampling of the rare octocoral Nanipora (Urgell Plaza et al. 2018). The individual was recorded from the skeleton, among polyps of the coral itself. Depth 7 m.

Distribution

Unknown.

Remarks

Very similar to Phestilla subodiosa, which is considered an obligate feeder of Montipora sp. corals. While Montipora corals were observed in the vicinity, the present sample was observed upon Nanipora, although no feeding or egg masses were observed. The specimen has smooth rhinophores and oral tentacles with clear indications of a darker band approximately halfway on both. Cerata with clearly visible digestive glands and a distinct bulge followed by a subterminal black band (broken up into black spots in some cerata), terminating in translucent rounded apices.

Genus Trinchesia Ihering, 1879

Trinchesia sp. 1

Figure 21I

Material examined

One specimen 10 mm, TW.

Ecology

Among rocks and corals in coral reef habitats. Depth 5–10 m.

Distribution

Cuthona sp. 2 (Gosliner et al. 2008) is known from Tanzania, the Philippines, Papua New Guinea, Japan, Australia, and Guam. Known from the Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Incorrectly identified as Cuthona ornata Baba, 1937 by Mehrotra and Scott (2016), the present species is similar to Cuthona sp. 2 (Gosliner et al. 2008). The status of numerous taxa historically described under multiple families and genera such as Cuthona, Trinchesia, Tenellia, etc. is currently in dire need of clarification with recent attempts being made at extensive lumping of groups (Cella et al. 2016). The most recent evidence provided involved the family Trinchesiidae and genus Trinchesia being re-instated by Korshunova et al. (2017b). We therefore follow this (likely temporary) state of affairs until this group of taxa is stabilised.

Trinchesia sp. 2

Figure 21J

Material examined

Two specimens 35–40 mm, TT; one specimen 45 mm, TW.

Ecology

Cryptic on its host hydroid Macrorhynchia sp., colonies of which are found exclusively in soft sediment habitats outside the coral reef. Depth 12–24 m.

Distribution

Tenellia sp. 17 is known only from the United Arab Emirates (Gosliner et al. 2018) and Tenellia sp. (Tibiriçá et al. 2017) is known from Mozambique. Known from the Gulf of Thailand (Mehrotra and Scott 2016).

Remarks

Mistakenly identified as Cuthona yamasui Hamatani, 1993 by Mehrotra and Scott (2016). Very similar also to Trinchesia sp. 3, with which it shares a prey species (alongside Lomanotus vermiformis).

Trinchesia sp. 3

Figure 21K

Material examined

Three specimens 12–45 mm, SN.

Ecology

Cryptic on its host hydroid Macrorhynchia sp., colonies of which are found exclusively in soft sediment habitats outside the coral reef. Depth 12–24 m.

Distribution

Cuthona yamasui’ (Gosliner et al. 2008) is known from Tanzania, Oman, Malaysia, the Philippines, Indonesia, Japan, and Australia. Here representing a first record for Thai waters.

Remarks

Very similar to Trinchesia sp. 2, with which it shares a prey species (alongside Lomanotus vermiformis). Distinguished by the presence of a white body with brown rhinophores and oral tentacles, unlike the brown body with white markings around the rhinophores as seen in Trinchesia sp. 2. Cerata in the present species are basally transparent (with the blue digestive glands clearly visible) turning blue with a thin black band, a thick yellow band, and another thin black subapical band below translucent apices. The cerata of Trinchesia sp. 2 on the other hand are basally opaque white followed by a distinct large blue band and yellow apices with colourless tips. The present species is similar to Tenellia sp. 15 and Tenellia sp. 16 of Gosliner et al. (2018). See above comments regarding genus and family instability.

Trinchesiidae sp.

Figure 21L

Material examined

One specimen 6 mm, HF.

Ecology

On artificial reef structures feeding on the same orange Corydendrium sp. hydroid as Unidentia aliciae, on which it is cryptic. Depth 8–12 m.

Distribution

Cuthona sp. 19 (Gosliner et al. 2008) is known from the Philippines and Papua New Guinea. Known from the Gulf of Thailand (Mehrotra and Scott 2016: fig. 2A).

Family Unidentiidae Millen & Hermosillo, 2012

Genus Unidentia Millen & Hermosillo, 2012

Unidentia aliciae Korshunova, Mehrotra, Arnold, Lundin, Picton & Martynov, 2019

Figure 22A

Material examined

Three specimens 19–24 mm, HF.

Ecology

On artificial reef structures feeding on the same orange hydroid Corydendrium sp. as Trinchesiidae sp. This hydroid is predominantly known from artificial reef structures at Koh Tao (see Korshunova et al. 2019). Depth 5–18 m.

Distribution

Currently known only from the Gulf of Thailand (Korshunova et al. 2019).

Figure 22. 

A Unidentia aliciae 24 mm B Janolus savinkini 55 mm (photograph by Pau Urgell Plaza) C Marianina rosea 10 mm D Tritonia sp. 35 mm (photograph by Khumron Waipaka).

Superfamily Proctonotoidea Gray, 1853

Family Janolidae Pruvot-Fol, 1933

Genus Janolus Bergh, 1884

Janolus savinkini Martynov & Korshunova, 2012

Figure 22B

Material examined

Two specimens 55–70 mm, LB; one specimen 38 mm, SW.

Ecology

In soft sediment habitats outside the coral reef. Depth 18–28 m.

Distribution

Across the Indo-Pacific including the Red Sea (Yonow 2015), Australia (Nimbs and Smith 2016), Vietnam (Martynov and Korshunova 2012), the Philippines, Indonesia, and Japan (Gosliner et al. 2008). Known from the Gulf of Thailand (Mehrotra and Scott 2016).

Superfamily Tritonioidea Lamarck, 1809

Family Tritoniidae Lamarck, 1809

Genus Marianina Pruvot-Fol, 1931

Marianina rosea (Pruvot-Fol, 1930)

Figure 22C

Material examined

One specimen 10 mm, CB.

Ecology

Under rubble, among small hydroids, in coral reef habitats. Depth 5–8 m.

Distribution

Across the Indo-Pacific including India (Apte 2009), Australia (Burn 1978), South Africa, Madagascar, the Philippines, Papua New Guinea, and New Caledonia (Gosliner et al. 2008). Here representing a first record for Thai waters.

Remarks

The single specimen observed from Koh Tao appears paler than is typical for the species, though such colour differences are not unheard of (see Fraser 2000). It is clearly recognised by its cerata and rhinophoral morphology, unique among the Tritoniidae.

Genus Tritonia Cuvier, 1798

Tritonia sp.

Figure 22D

Material examined

One specimen 35 mm, GR.

Ecology

Known from a single specimen observed on the octocoral Echinogorgia sp. Depth 14 m.

Distribution

Tritonia sp. 7 (Gosliner et al. 2008) is known only from Indonesia. Known from the Gulf of Thailand (Mehrotra and Scott 2016).

Discussion

Prior to studies from Koh Tao, the documented diversity of sea slug taxa from the Gulf of Thailand numbered 111 species, with 204 species in total recorded for Thailand (Table 2). Surveys by Mehrotra and Scott (2016) increased these numbers to 154 for the Gulf specifically and 235 for Thai waters overall. The present findings further increase the documented diversity of sea slug taxa from the Gulf and Thai waters to 256 and 336 respectively. In general, species were recorded from coral reef habitats or soft sediment habitats exclusively, with only 28 species being found across both (Fig. 3). Of those found in coral-dominated habitats, numerous species were documented to have one of two further specialist habitat types that have thus far remained unexplained.

The first of these are those species that are documented preferentially from artificial substrates such as artificial reefs and debris. Nudibranch species such as Phidiana militaris, Trinchesiidae sp., and Unidentia aliciae were found in abundance over the years and were mostly or exclusively documented associated with prey growing on such substrates, with U. aliciae being described associated from these substrates (Korshunova et al. 2019). The role of artificial substrates in the benthic ecology of reef environments requires further exploration, in particular the association of sea slugs with regards to the role of synthetic materials as substrates for colonial organisms and vectors for their dispersal (Hoeksema et al. 2012; McCuller et al. 2018). Such trends may reveal unexpected ecological impacts from habitat manipulation such as coral restoration or marine debris, if they are found to promote certain organisms over others (i.e., hydroids or sponges as prey to nudibranch taxa).

The second specialist habitat type within the hard substrate habitats was the ventral surface of the skeletons of dead Fungiidae corals. These corals are known to be in high abundance around Koh Tao (Hoeksema et al. 2013; Scott et al. 2017). Benthic surveys among rubble and reef substrate environments revealed that taxa that were rarer or more cryptic due to size or camouflage were recorded in noticeably greater abundances adhering to the underside of the skeletons of dead mushroom corals than other rubble or living coral substrates in reef habitats. In particular, the species Elysia obtusa, Philine orca, Goniobranchus cf. albonares, Mexichromis trilineata, and Dermatobranchus fortunatus were all recorded almost exclusively from under these skeletons. The rugosity and proportionally larger surface area of these skeletons, in combination with the shelter from light and wave action likely provide attractive conditions for the high diversity of poriferan, algal, cnidarian, and bryozoan taxa observed under these corals. A closer investigation into the dynamics of species composition across different benthic substrate types in more coral reef areas may yield greater information on species traditionally considered ecologically cryptic.

Mehrotra and Scott (2016) found that 37% of species documented were not observed in hard substrate habitats such as fringing reefs and offshore pinnacles but were instead exclusive to deeper soft sediment habitats beyond the reef slope. Remarkably, the present findings document only 36% of all species being exclusive to these habitats, despite a more than doubling of documented taxa. These habitats support a diverse community of organisms not observed in hard substrate areas such as the fringing reef and offshore pinnacles. These include cnidarians such as pennatulacean and Dendronephthya sp. octocorals (Mehrotra et al. 2017), Heteropsammia spp. and Heterocyathus spp. scleractinian corals (Hoeksema and Matthews 2015; Mehrotra et al. 2016, 2019), and diverse assemblages of cerianthids and benthic hydrozoans (Martynov et al. 2019). Additionally, algae such as Avrainvillea longicaulis and Vaucheria sp. are often abundant in these areas (Mehrotra et al. 2019) as well as bryozoans of the family Bugulidae and other known prey items of sea slugs (McDonald and Nybakken 1997), most of which are either rare or entirely absent from hard substrate habitats around Koh Tao. Indeed, such ecological aspects have already been suggested as key drivers in the description of four new species from the soft sediments of Koh Tao (Mehrotra et al. 2017, 2020b).

It is possible that many of these specialised organisms can colonise much of the benthic environment in the Gulf due to its particular characteristics. Apart from being among the western-most ecological regions attributed to the Pacific, the Gulf of Thailand differs greatly from the Andaman coast of Thailand, being a region inundated by heavy sedimentation due to the many rivers that flow into it and remained entirely frozen until the glacial retreat into the Holocene (Sathiamurthy and Voris 2006). The Gulf of Thailand today has a maximum depth of 84 m (Cheevaporn and Menasveta 2003) although the vast majority of the Gulf is shallower than 75 m with only the central region exhibiting a depth of greater than this (Voris 2000; Sojisuporn et al. 2010). Therefore, the Gulf of Thailand is a 320,000 km2 sediment-rich basin entirely in the photic zone. It is at present challenging to draw sweeping conclusions on the comparative sea slug diversity between the Gulf of Thailand and other West-Pacific regions, as it is extremely unlikely that the majority of taxa present within either the Gulf or the surrounding seas have been documented. Nonetheless, recent years have seen attempts being made to quantitatively compare diversity estimates between West-Pacific regions (i.e., Eisenbarth et al. 2018; Undap et al. 2019). However, despite an increase in the numbers of sea slug biodiversity inventories in the past decade, there remains a paucity of in-depth ecological information for most species.

Beyond the habitat preferences, the present study further expands on the trophic dynamics of different sea slugs allowing insights into their place in the food webs of Koh Tao.

For example, predation upon sea slugs were documented from both habitat types, with predation upon Haminoeidae spp. in particular observed numerous times in the present study. In soft sediment habitats, predation by crustaceans (both decapod and stomatopod) appeared prominent (Fig. 6G), and in coral reef habitats, predation by Labriidae fish was abundant, in both cases agreeing with growing evidence of sea slugs as viable prey items to non-heterobranchs in the Indo-Pacific tropics (Mehrotra et al. 2019; Anker and Ivanov 2020). Furthermore, habitat-specifics of various prey items were distinctly visible, with certain groups (i.e., hydroids) having a much more complex habitat distribution across coral reef and soft sediment habitats than others such as major anthozoan groups. A deeper exploration of these observations is needed to investigate ecological drivers for such habitat distribution.

Conclusions

The findings presented here highlight the need for a greater documentation and understanding of sea slug ecology in the Indo-Pacific as many questions remain regarding the habitat and prey preferences of the majority of species documented from the Gulf of Thailand and elsewhere. It is apparent that the diversity of sea slug taxa in the Gulf of Thailand has been greatly under-reported and that the marine habitats in the region support a high diversity of benthic species. With the increasing availability of SCUBA infrastructure around the Gulf, it is likely that a greater area within the region will be made accessible for the study of marine benthic environments. In conjunction, the expansion of surveys into deeper soft sediment habitats will likely further expand on the known diversity of a great number of species in these areas. Very little has been documented on the distinct biological and ecological characteristics of marine soft sediment habitats (Wilson 1991), yet it has been shown that they are able to support a USD$150 million tourism industry in South-East Asia alone (De Brauwer et al. 2017). A combination of increased efforts on surveying unexplored benthic habitats and the growing utility of citizen science efforts will yield much needed advancements in the understanding of benthic ecology in the Gulf of Thailand.

Acknowledgements

We would like to thank the survey team for their assistance, in particular Pau Urgell Plaza, Alyssa Allchurch, Elouise Haskin, Kirsty Magson, Genevieve Goulet, Kaitlyn Harris, Joel Rohrer, Raphael Danieau, Jan Koschorrek, Alexander Duseljee, Matthew Muncaster, Sriploy Chaisri, Rebecca Danielli, and Lena Schenke. We are grateful for comments by Vie Panyarachun, editor Nathalie Yonow and input from the reviewers which helped improve the manuscript. The first author received support via the Ratchadaphisek Somphot Endowment Fund for Postdoctoral Fellowship, Chulalongkorn University. This work was also supported by NRCT-JSPS Core to Core, UNESCO-IOC/WESTPAC, and UNESCO Japanese Funds-in-Trust and by Mubadala Petroleum (Thailand). Surveys and logistics were supported by Conservation Diver (Registered US Charity #20183007707; http://conservationdiver.com).

References

  • Angas GF (1864) Description d’espèces nouvelles appartenant à plusieurs genres de mollusques nudibranches des environs de Port-Jackson (Nouvelle-Galles du Sud): accompagnée de dessins faits d’après nature. Journal de Conchyliologie 12: 43–70.
  • Anker A, Ivanov Y (2020) First record of the predation upon sea slugs (Cephalaspidea and Nudibranchia) by the peculiar elbow crab Lambrachaeus ramifer Alcock, 1895 (Decapoda: Parthenopidae). Marine Biodiversity 50: 1–6. https://doi.org/10.1007/s12526-020-01047-x
  • Appleton D, Sewell M, Berridge M, Copp B (2002) A new biologically active malyngamide from a New Zealand collection of the sea hare Bursatella leachii. Journal of natural products 65: 630–631. https://doi.org/10.1021/np010511e
  • Apte D (2009) Opisthobranch fauna of Lakshadweep Islands, India, with 52 new record to Lakshadweep and 40 new records to India: Part 1. Journal of the Bombay Natural History Society 106: 162–175.
  • Apte D, Bhave V, Parasharya D (2010) An annotated and illustrated checklist of the opisthobranch fauna of gulf of Kutch, Gujarat, India with 21 new records for Gujarat and 13 new records for India: part 1. Journal of Bombay Natural History Society 107: 14–23.
  • Baba K (1937) Opisthobranchia of Japan (I). Journal of the Department of Agriculture, Kyushu Imperial University 5: 195–236. https://doi.org/10.5109/22583
  • Baba K (1949) Opisthobranchia of Sagami bay: collected by His Majesty the Emperor of Japan. Hattori H (Ed). Iwanami Shoten, Tokyo, 194 pp.
  • Baba K (1957) A revised list of the species of Opisthobranchia from the northern part of Japan, with some additional descriptions (with 6 text-figures). Zoology 13: 8–14.
  • Baba K (1960) The genera Gymnodoris and Nembrotha from Japan (Nudibranchia-Polyceridae). Publication of Seto Marine Biological Laboratory 8: 71–74. https://doi.org/10.5134/174700
  • Baba K (1987) Two new green-coloured species of Tambja from Japan (Nudibranchia: Polyceridae). Venus 46: 13–18.
  • Baba K (1990) Two small species of Philine new of Japan (Cephalaspidea: Philinidae). Venus 49: 181–197.
  • Baba K, Hamatani I (1961) On two species of Doriopsis (syn. Ctenodoris) from Japan (Nudibranchia-Dorididae). Publications of the Seto Marine Biological Laboratory 9: 63–65. https://doi.org/10.5134/174662
  • Baine M, Harasti D (2007) The marine life of Bootless Bay, Papua New Guinea. Motupore Island Research Centre (MIRC), School of Natural and Physical Sciences, University of Papua New Guinea, 146 pp.
  • Bazzicalupo E, Crocetta F, Gosliner TM, Berteaux-Lecellier V, Camacho-García YE, Chandran BKS, Valdés Á (2020) Molecular and morphological systematics of Bursatella leachii de Blainville, 1817 and Stylocheilus striatus Quoy & Gaimard, 1832 reveal cryptic diversity in pantropically distributed taxa (Mollusca: Gastropoda: Heterobranchia). Invertebrate Systematics 34(5): 535–568. https://doi.org/10.1071/IS19056
  • Bebbington A (1969) Bursatella leachi guineensis new subspecies Gastropoda Opisthobranchia from Ghana. Proceedings of the Malacological Society of London 38: 323–340.
  • Bergh R (1888) Die Pleuroleuriden, eine Familie der Nudibranchiaten Gasträopoden. Zoologische Jahrbücher. Abteilung für Systematik Geographie und Biologie der Tiere 3: 348–364.
  • Bergh R (1896) Eolidiens d’Amboine. Voyage de MM. M. Bedot et C. Pictet dans l’Archipel Malais. Revue Suisse de Zoologie et Annales de Musée d’Histoire Naturelle de Geneve 4: 385–394. https://doi.org/10.5962/bhl.part.35506
  • Bergh R (1902) I. Gasteropoda Opisthobranchiata. In: Mortensen T (Ed.) The Danish Expedition to Siam 1899–1900. Det Kongelige Danske Videnskabernes Selskabs Skrifter. 6 Raekke Naturvidenskabelig og Matematisk Afdeling, 153–218.
  • Bertsch H (1972) Two additions to the opisthobranch fauna of the southern Gulf of California. The Veliger 15: 103–106.
  • Bhave V, Apte D (2011) Illustrated checklist of opisthobranch fauna of Ratnagiri, Maharashtra, India, with eight new records to India. Journal of Bombay Natural History Society 108: 172–182.
  • Bonomo L, Gosliner TM (2020) Adding stars to the Chromodoris (Nudibranchia, Chromodorididae) galaxy with the description of four new species. Zootaxa 4819: 401–435. https://doi.org/10.11646/zootaxa.4819.3.1
  • Bourjon P, Ducarme F, Quod J-P, Sweet M (2018) Involving recreational snorkelers in inventory improvement or creation: a case study in the Indian Ocean. Cahiers de Biologie Marine 59: 451–460. https://doi.org/10.21411/CBM.A.B05FC714
  • Brodie G, Willan RC, Collins J (1997) Taxonomy and occurrence of Dendrodoris nigra and Dendrodoris fumata (Nudibranchia: Dendrodorididae) in the Indo-West Pacific region. Journal of Molluscan Studies 63: 407–423. https://doi.org/10.1093/mollus/63.3.407
  • Burghardt I, Wägele H (2014) The symbiosis between the “solar-powered” nudibranch Melibe engeli Risbec, 1937 (Dendronotoidea) and Symbiodinium sp. (Dinophyceae). Journal of Molluscan Studies 80: 508–517. https://doi.org/10.1093/mollus/eyu043
  • Burghardt I, Carvalho R, Eheberg D, Gerung G, Kaligis F, Mamangkey NGF, Schroedl M, Schwabe E, Vonnemann V, Wägele H (2006) Molluscan diversity at Bunaken National Park, Sulawesi. Journal of the Zoological Society Wallacea 2: 29–43.
  • Burn R (2006) A checklist and bibliography of the Opisthobranchia (Mollusca: Gastropoda) of Victoria and the Bass Strait area, south-eastern Australia. Museum Victoria Science Reports 10: 7–13. https://doi.org/10.24199/j.mvsr.2006.10
  • Camacho-García Y, Gosliner TM (2008a) Systematic revision of Jorunna Bergh, 1876 (Nudibranchia: Discodorididae) with a morphological phylogenetic analysis. Journal of Molluscan Studies 74: 143–181. https://doi.org/10.1093/mollus/eyn002
  • Camacho-García Y, Gosliner TM (2008b) Nudibranch dorids from the Pacific Coast of Costa Rica with the description of a new species. Bulletin of Marine Science 83: 367–389.
  • Camacho-García Y, Gosliner TM, Valdés A (2005) Field guide to the sea slugs of the tropical eastern Pacific. California Academy of Sciences, San Francisco, 129 pp.
  • Camacho-García Y, Ornelas-Gatdula E, Gosliner T, Valdés Á (2013) Phylogeny of the family Aglajidae (Pilsbry, 1895) (Heterobranchia: Cephalaspidea) inferred from mtDNA and nDNA. Molecular phylogenetics and evolution 71: 113–126. https://doi.org/10.1016/j.ympev.2013.11.010
  • Carlson C, Hoff P (2003) The opisthobranchs of the Mariana Islands. Micronesica 35: 271–293.
  • Carmona L, Pola M, Gosliner T, Cervera JL (2014a) Review of Baeolidia, the largest genus of Aeolidiidae (Mollusca: Nudibranchia), with the description of five new species. Zootaxa 3802: 477–514. https://doi.org/10.11646/zootaxa.3802.4.5
  • Carmona L, Bhave V, Salunkhe R, Pola M, Gosliner TM, Cervera JL (2014b) Systematic review of Anteaeolidiella (Mollusca, Nudibranchia, Aeolidiidae) based on morphological and molecular data, with a description of three new species. Zoological Journal of the Linnean Society 171: 108–132. https://doi.org/10.1111/zoj.12129
  • Chang YW, Willan RC (2015) Molecular phylogeny of phyllidiid nudibranchs (Porostomata: Phyllidiidae) based on the mitochondrial genes (COI and 16S). In: Molluscs 2015 Program and Abstract Handbook. Coffs Harbour, 27.
  • Chanmethakul T, Chansang H, Watanasit S (2010) Soft coral (Cnidaria: Alcyonacea) distribution patterns in Thai waters. Zoological Studies 49: 72–84.
  • Chinnadurai S, Bhave V, Apte D, Mohamed K (2014) First record of long-tailed pelagic sea slug Stylocheilus longicauda (Gastropoda: Opisthobranchia) from southwest coast of India Original Article. Journal of the Marine Biological Association of India 56: 81–84. https://doi.org/10.6024/jmbai.2014.56.2.01794-12
  • Christa G, Wescott L, Schäberle T, König G, Wägele H (2012) What remains after 2 months of starvation? Analysis of sequestered algae in a photosynthetic slug, Plakobranchus ocellatus (Sacoglossa, Opisthobranchia), by barcoding. Planta 237: 559–572. https://doi.org/10.1007/s00425-012-1788-6
  • Coleman N (2001) 1001 Nudibranchs: catalogue of Indo-Pacific sea slugs. Neville Coleman’s Underwater Geographic Pty Ltd., Springwood, 144 pp.
  • Coleman N (2008) Nudibranchs Encyclopedia: catalogue of Asia/Indo-Pacific sea slugs. Neville Coleman’s Underwater Geographic Pty Ltd., Springwood, 416 pp.
  • Collin R, Diaz M, Norenburg J, Rocha R, Sanchez JA, Schulz A, Schwartz M, Valdés Á (2005) Photographic identification guide to some common marine invertebrates of Bocas Del Toro, Panama. Caribbean Journal of Science 41: 638–707.
  • Cowie RH, Regnier C, Fontaine B, Bouchet P (2017) Measuring the Sixth Extinction: what do mollusks tell us? The Nautilus 131: 3–41.
  • Dayrat B (2010) A monographic revision of basal discodorid sea slugs (Mollusca: Gastropoda: Nudibranchia: Doridina). Proceedings of the California Academy of Sciences 61: 1–403.
  • De Brauwer M, Harvey E, Mcilwain J, Hobbs J-P, Jompa J, Burton M (2017) The economic contribution of the muck dive industry to tourism in Southeast Asia. Marine Policy 83: 92–99. https://doi.org/10.1016/j.marpol.2017.05.033
  • Debelius H (1996) Nudibranchs and sea snails: Indo-Pacific field guide. IKAN-Unterwasserarchiv, Frankfurt, 321 pp.
  • Dixit S, Raghunathan C, Chandra K (2017) New records of sea slugs (Heterobranchia: Opisthobranchia) from India. Proceedings of the International Academy of Ecology and Environmental Sciences 7: 47.
  • Dorgan K, Valdés Á, Gosliner TM (2002) Phylogenetic systematics of the genus Platydoris (Mollusca, Nudibranchia, Doridoidea) with descriptions of six new species. Zoologica Scripta 31: 271–319. https://doi.org/10.1046/j.1463-6409.2002.00105.x
  • Edmunds M (1972) Opisthobranchiate Mollusca from the Seychelles, Tanzania, and the Congo, now in the Tervuren Museum. Revue Zoologie Botanie Africaines 85: 67–92.
  • Eisenbarth J-H, Undap N, Papu A, Schillo D, Dialao J, Reumschüssel S, Kaligis F, Bara R, Schäberle T, König G, Yonow N, Wägele H (2018) Marine Heterobranchia (Gastropoda, Mollusca) in Bunaken National Park, North Sulawesi, Indonesia – Follow-up diversity Study. Diversity 10(4): e127. https://doi.org/10.3390/d10040127
  • Ekimova I, Antokhina TI, Schepetov D (2019) “Invasion” in the Russian Arctic: is global climate change a real driver? A remarkable case of two nudibranch species. Ruthenica 29: 103–113.
  • Ekimova I, Antokhina TI, Schepetov D (2020) Molecular data and updated morphological description of Flabellina rubrolineata (Nudibranchia: Flabellinidae) from the Red and Arabian seas. Ruthenica 30: 183–194.
  • Epstein HE, Hallas JM, Johnson RF, Lopez A, Gosliner TM (2018) Reading between the lines: revealing cryptic species diversity and colour patterns in Hypselodoris nudibranchs (Mollusca: Heterobranchia: Chromodorididae). Zoological Journal of the Linnean Society 1: 116–189. https://doi.org/10.1093/zoolinnean/zly048
  • Espinosa J, Ortea Rato J (2004) Nuevas especies de moluscos gastéropodes marinos Mollusca: Gastropoda de las Bahamas, Cuba y el Mar Caribe de Costa Rica. Revista de la Academia Canaria de Ciencias 15: 207–216.
  • Fahey SJ, Gosliner TM (1999) Preliminary phylogeny of Halgerda (Nudibranchia: Halgerdidae) from the tropical Indo-Pacific, with descriptions of three new species. Proceedings of the California Academy of Sciences, 4th series 51: 425–448.
  • Fatemi Y, Attaran G (2015) Checklist of the opisthobranchs (Heterobranchia: Gastropoda) along the Iranian Coasts of the Gulf of Oman. Journal of Biodiversity Environmental Science 6: 1–7.
  • Fernández-Vilert R, Giménez J, Mas G, Figueroa I, Moles J (2018) First records of the Red Sea alien mollusc Haminoea cyanomarginata (Gastropoda: Heterobranchia: Cephalaspidea) in the Western Mediterranean. Journal of Natural History 52: 1817–1823. https://doi.org/10.1080/00222933.2018.1497212
  • Galvão Filho H, Araujo A, Silva F, Azevedo V, Meirelles C, Matthews-Cascon H (2015) Sea slugs (Gastropoda: Heterobranchia) from a poorly known area in north-east Brazil: Filling gaps in Atlantic distributions. Marine Biodiversity Records 8: 1–11. https://doi.org/10.1017/S1755267215000494
  • García-Méndez K, Camacho-García Y (2016) New records of heterobranch sea slugs (Mollusca: Gastropoda) from Isla del Coco National Park, Costa Rica. Revista de Biología Tropical 64: 205–219. https://doi.org/10.15517/rbt.v64i1.23449
  • Goddard J, Hermosillo A (2008) Developmental mode in opisthobranch molluscs from the tropical Eastern Pacific Ocean. The Veliger 50: 83–96.
  • Goddard JHR, Treneman N, Prestholdt T, Hoover C, Green B, Pence WE, Mason DE, Dobry P, Sones JL, Sanford E, Agarwal R, McDonald GR, Johnson RF, Gosliner TM (2018) Heterobranch sea slug range shifts in the Northeast Pacific Ocean associated with the 2015–16 El Niño. Proceedings of the California Academy of Sciences 65: 107–131.
  • Golestani H, Crocetta F, Padula V, Camacho-García Y, Langeneck J, Poursanidis D, Pola M, Yokes MB, Cervera JL, Jung D-W, Gosliner T, Araya J-F, Hooker Y, Schrödl M, Valdés Á (2019) The little Aplysia coming of age: from one species to a complex of species complexes in Aplysia parvula (Mollusca: Gastropoda: Heterobranchia). Zoological Journal of the Linnean Society 187: 279–330. https://doi.org/10.1093/zoolinnean/zlz028
  • González-Wangüemert M, Domínguez-Godino JA, Casalduero F, Serrao E (2014) Genetic signature of a recent invasion: The ragged sea hare Bursatella leachii in Mar Menor (SE Spain). Biochemical Systematics and Ecology 54: 123–129. https://doi.org/10.1016/j.bse.2014.01.008
  • Gosliner TM (1987) Nudibranchs of Southern Africa: A guide to opisthobranch molluscs of Southern Africa. Sea Challengers, California Academy of Sciences, San Francisco, 136 pp.
  • Gosliner TM (1994) New species of Chromodoris and Noumea (Nudibranchia: Chromodorididae) from the western Indian Ocean and southern Africa. Proceedings of the California Academy of Sciences 48: 239–252.
  • Gosliner TM (1995) The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of the phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences, 4th series 49: 1–54.
  • Gosliner TM, Johnson S (1994) Review of the genus Hallaxa (Nudibranchia: Actinocyclidae) with descriptions of nine new species. The Veliger 37: 155–191.
  • Gosliner TM, Vallès Y (2006) Shedding light onto the genera (Mollusca: Nudibranchia) Kaloplocamus and Plocamopherus with description of new species belonging to these unique bioluminescent dorids. Veliger 48: 178–205.
  • Gosliner TM, Fahey S (2008) Systematics of Trapania (Mollusca: Nudibranchia: Goniodorididae) with descriptions of 16 new species. Systematics and Biodiversity 6: 53–98. https://doi.org/10.1017/S1477200007002587
  • Gosliner TM, Fahey S (2011) Previously undocumented diversity and abundance of cryptic species: A phylogenetic analysis of Indo-Pacific Arminidae Rafinesque, 1814 (Mollusca: Nudibranchia) with descriptions of 20 new species of Dermatobranchus. Zoological journal of the Linnean Society 161: 245–356. https://doi.org/10.1111/j.1096-3642.2010.00649.x
  • Gosliner TM, Behrens D, Valdés Á (2008) Indo-Pacific nudibranchs and sea slugs: a field guide to the world’s most diverse fauna. Sea Challengers Natural History Books, Gig Harbor, 425 pp.
  • Gosliner TM, Valdés Á, Behrens D (2015) Nudibranch & Sea Slug Identification Indo-Pacific. New World Publications Inc., Jacksonville, 408 pp.
  • Gosliner TM, Valdés Á, Behrens D (2018) Nudibranch & Sea Slug Identification – Indo-Pacific. New World Publications Inc., Jacksonville, 451 pp.
  • Guang-Yu L, Tchang S (1965) Opisthobranchia from the inter-tidal zone of Hainan Island, China. Oceanologia Et Limnologia Sinica 7: 1–20.
  • Händeler K, Wägele H (2007) Preliminary study on molecular phylogeny of Sacoglossa and a compilation of their food organisms. Bonner Zoologische Beiträge 55: 231–254.
  • Ho SL (1989) Guide to common nudibranchs of Malaysia. Nature Malaysiana 14: 4–17.
  • Hoeksema B, Matthews J (2015) Partial bleaching in an assemblage of small apozooxanthellate corals of the genera Heteropsammia and Heterocyathus. Coral Reefs 34: e1227. https://doi.org/10.1007/s00338-015-1314-y
  • Hoeksema B, Roos P, Cadée G (2012) Trans-Atlantic rafting by the brooding reef coral Favia fragum on man-made flotsam. Marine Ecology Progress Series 445: 209–218. https://doi.org/10.3354/meps09460
  • Hoeksema BW, Scott C, True JD (2013) Dietary shift in corallivorous Drupella snails following a major bleaching event at Koh Tao, Gulf of Thailand. Coral Reefs 32: 423–428. https://doi.org/10.1007/s00338-012-1005-x
  • Hori S (2017) Family Haminoeidae. In: Okutani T (Ed.) Marine Mollusks in Japan, 2nd edn. Tokai University Press, Hiratsuka, 1093–1095.
  • Horwitz R, Jackson MD, Mills SC (2017) The embryonic life history of the tropical sea hare Stylocheilus striatus (Gastropoda: Opisthobranchia) under ambient and elevated ocean temperatures. PeerJ 5: p.e2956. https://doi.org/10.7717/peerj.2956
  • Hu J, Zhang Y, Xie J, Qiu J (2020) A new species of predatory nudibranch (Gastropoda: Trinchesiidae) of the coral Pavona decussata. Zoological Studies 59: 1–11. https://doi.org/10.6620/ZS.2020.59-30
  • Huang H-D, Tsai Y, Chen C-K, Hung H-T, Lin H, Lee K-S (2016) Diverse opisthobranchs and polyclad flatworms in Houwan, Kenting National Park, Southern Taiwan. Collection and Research 28: 55–65.
  • Huang R-Y, Chen W-T, Kurtán T, Mándi A, Ding J, Li J, Li X-W, Guo Y-W (2015) Bioactive isoquinolinequinone alkaloids from the South China Sea nudibranch Jorunna funebris and its sponge-prey Xestospongia sp. Future Medicinal Chemistry 8: 17–27. https://doi.org/10.4155/fmc.15.169
  • Ichikawa M (1993) Sacoglossa (Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory 36: 119–139. https://doi.org/10.5134/176229
  • Jensen KR (1989) A new species of Cylindrobulla from Phuket, Thailand, with a discussion of the systematic affiliation of the genus. Phuket Marine Biological Center Research Bulletin 52: 1–11.
  • Jensen KR (1992) Anatomy of some Indo-Pacific Elysiidae (Opisthobranchia: Sacoglossa (= Ascoglossa)), with a discussion of the generic division and phylogeny. Journal of Molluscan Studies 58: 257–296. https://doi.org/10.1093/mollus/58.3.257
  • Jensen KR (1994) Sublittoral Notaspidea and Nudibranchia (Opisthobranchia) from Hong Kong, with a description of a new species. In: Morton B (Ed.) The malacofauna of Hong Kong and southern China, III. Proceedings of the Third International Workshop on the Malacofauna of Hong Kong and Southern China, Hong Kong, 13 April–1 May 1992. Hong Kong University Press, Hong Kong, 160–187.
  • Jensen KR (1997) Sacoglossa (Mollusca, Opisthobranchia) from the Houtman Abrolhos Islands and central Western Australia. In: Wells FE (Ed.) The Marine Flora and Fauna of the Houtman Abrolhos Islands, Western Australia. Western Australian Museum, Perth, 307–333.
  • Jensen KR (1998) Anatomy of some opisthobranch molluscs from Phuket, Thailand, with a list of Opisthobranchia recorded from Thai Waters. Phuket Marine Biological Center, Special Publication 18: 243–262.
  • Jensen KR (2000) Anatomy of some subtidal Opisthobranchia (Mollusca: Gastropoda) from Hong Kong waters. In: The Marine Flora and Fauna of Hong Kong and Southern China. Hong Kong, 157–181.
  • Jensen KR (2003) Distributions, diets and reproduction of Hong Kong Sacoglossa (Mollusca: Opisthobranchia): a summary of data, 1980–2001. In: Morton B (Ed.) Perspectives on Marine Environmental Change in Hong Kong and Southern China, 1977–2001. Proceedings of an International Workshops Reunion Conference, Hong Kong 2001. Hong Kong University Press, Hong Kong, 347–365.
  • Jensen KR (2007) The opisthobranch molluscs collected during the Thai-Danish BIOSHELF project in the Andaman Sea, Thailand. Phuket Marine Biological Center Research Bulletin 68: 21–30.
  • Jensen KR (2009) Sacoglossa (Mollusca: Gastropoda: Opisthobranchia) from Singapore. The Raffles Bulletin of Zoology, Supplement 22: 207–223.
  • Jensen KR (2015) Sacoglossa (Mollusca: Gastropoda: Heterobranchia) from northern coasts of Singapore. The Raffles Bulletin of Zoology, Supplement 31: 226–249.
  • Jensen KR, Kohnert P, Bendell B, Schrödl M (2014a) Life on a leaf: 3D-reconstruction and description of a new limapontiid sacoglossan (Gastropoda: Heterobranchia: ‘Opisthobranchia’) living on the seagrass Halophila ovalis. Journal of Molluscan Studies 80: 624–641. https://doi.org/10.1093/mollus/eyu071
  • Jensen KR, Krug PJ, Dupont A, Nishina M (2014b) A review of taxonomy and phylogenetic relationships in the genus Costasiella (Heterobranchia: Sacoglossa), with a description of a new species. Journal of Molluscan Studies 80: 562–574. https://doi.org/10.1093/mollus/eyu048
  • Jochum A, Favre A (2017) First record of the sea slug Stylocheilus striatus (Quoy & Gaimard, 1825) (Anaspidea, Aplysiidae) and swarming behavior for Bazaruto Archipelago, Mozambique with the first record of Pleurobranchus forskalii Rüppell & Leuckart, 1828 (Nudipleura, Pleurobranchidae) for Bazaruto Island (Gastropoda, Heterobranchia). Check List 13: 435–441. https://doi.org/10.15560/13.5.435
  • Kay EA (1979) Hawaiian marine shells. Reef and Shore Fauna of Hawaii. Section 4. Mollusca. Bishop Museum Press, Honolulu, 653 pp.
  • Kay EA, Young DK (1969) The Doridacea (Opisthobranchia; Mollusca) of the Hawaiian Islands. Pacific Science 23: 172–231.
  • Kelaart EF (1858) Description of new and little known species of Ceylon nudibranchiate molluscs, and zoophytes. Journal of the Ceylon Branch of the Royal Asiatic Society 3: 84–139.
  • Kelaart EF (1859) Descriptions of new and little-known species of Ceylonese nudibranchiate mollusks. In: Annals and Magazine of Natural History. Taylor and Francis, Ltd., London, 291–304. https://www.biodiversitylibrary.org/item/71836
  • Koh DB (2006) Sea slugs of Korea. Pungdeung Publishing, Korea, 250 pp.
  • Korshunova T, Martynov A, Picton B (2017a) Ontogeny as an important part of integrative taxonomy in Tergipedid aeolidaceans (Gastropoda: Nudibranchia) with a description of a new genus and species from the Barents Sea. Zootaxa 4324: 1–22. https://doi.org/10.11646/zootaxa.4324.1.1
  • Korshunova T, Mehrotra R, Arnold S, Lundin K, Picton B, Martynov A (2019) The formerly enigmatic Unidentiidae in the limelight again: A new species of the genus Unidentia from Thailand (Gastropoda: Nudibranchia). Zootaxa 4551: 556–570. https://doi.org/10.11646/zootaxa.4551.5.4
  • Korshunova T, Martynov A, Bakken T, Evertsen J, Fletcher K, Mudianta W, Saito H, Lundin K, Schrödl M, Picton B (2017b) Polyphyly of the traditional family Flabellinidae affects a major group of Nudibranchia: aeolidacean taxonomic reassessment with descriptions of several new families, genera, and species (Mollusca, Gastropoda). ZooKeys 717: 1–139. https://doi.org/10.3897/zookeys.717.21885
  • Krug P, Vendetti J, Valdés Á (2016) Molecular and morphological systematics of Elysia Risso, 1818 (Heterobranchia: Sacoglossa) from the Caribbean region. Zootaxa 4148: 1–137. https://doi.org/10.11646/zootaxa.4148.1.1
  • Krug P, Vendetti J, Rodriguez A, Retana J, Hirano Y, Trowbridge C (2013) Integrative species delimitation in photosynthetic sea slugs reveals twenty candidate species in three nominal taxa studied for drug discovery, plastid symbiosis or biological control. Molecular phylogenetics and evolution 69: 1101–1119. https://doi.org/10.1016/j.ympev.2013.07.009
  • Kumar JS, Sreeraj CR, Sornaraj R (2011) Opisthobranchs of the Gulf of Mannar Biosphere Reserve, Tamil Nadu, India. Indian journal of Fisheries 58: 105–114.
  • Kuroda T, Habe T (1952) Checklist and bibliography of the recent marine Mollusca of Japan. Stach LW (Ed). Hosokawa printing company, Tokyo, 210 pp.
  • Layton K, Gosliner T, Wilson N (2018) Flexible colour patterns obscure identification and mimicry in Indo-Pacific Chromodoris nudibranchs (Gastropoda: Chromodorididae). Molecular Phylogenetics and Evolution 124: 27–36. https://doi.org/10.1016/j.ympev.2018.02.008
  • Lee C-H, Kaang B-K, Lee Y-D (2014) Spawning behavior and egg development of Aplysia kurodai inhabiting the coastal waters of Jeju Island, Korea. Development & Reproduction 18: 25–31. https://doi.org/10.12717/DR.2014.18.1.025
  • Lim CF, Chou LM (1970) The dendrodorid and doridid nudibranchs of Singapore. Malayan Nature Journal 23: 92–117.
  • Lin G (1990) Opisthobranchia Fauna of the Hainan Island, China. Bulletin of Marine Science 47: 134–138.
  • Macnae W, Kalk M (1958) The Natural History of Inhaca Island, Mozambique. 2nd edn. Witwatersrand University Press, Johannesburg, 120 pp.
  • Maeda T, Hirose E, Chikaraishi Y, Kawato M, Takishita K, Yoshida T, Verbruggen H, Tanaka J, Shimamura S, Takaki Y, Tsuchiya M, Iwai K, Maruyama T (2012) Algivore or phototroph? Plakobranchus ocellatus (Gastropoda) continuously acquires kleptoplasts and nutrition from multiple algal species in nature. PLoS ONE 7: e42024. https://doi.org/10.1371/journal.pone.0042024
  • Malaquias M, Calado G, Padula V, Villani G, Cervera JL (2009) Molluscan diversity in the North Atlantic Ocean: New records of opisthobranchs gastropods from the Archipelago of the Azores. Marine Biodiversity Records 2: e38. https://doi.org/10.1017/S175526720800016X
  • Mangubhai S (2007) Chelidonura punctata (Eliot, 1903) preys on acoel flatworms recruiting onto Platygyra daedalea (Ellis and Solander, 1786) in Kenya. Coral Reefs 26: 1057–1057. https://doi.org/10.1007/s00338-007-0299-6
  • Marcus ER, Burch JB (1965) Marine euthyneuran Gastropoda from Eniwetok atoll, western Pacific. Malacologia 3: 235–262.
  • Marshall JG, Willan RC (1999) Nudibranchs of Heron Island, Great Barrier Reef: A survey of the Opisthobranchia (Sea Slugs) of Heron and Wistari reefs. Backhuys Publishers, Leiden, 268 pp.
  • Martynov AV, Korshunova TA (2012) Opisthobranch molluscs of Vietnam (Gastropoda: Opisthobranchia). In: Britayev TA, Pavlov DS (Eds) Benthic fauna of the Bay of Nhatrang, Southern Vietnam. KMK Scientific Press Ltd., Moscow, 142–257.
  • Martynov A, Mehrotra R, Chavanich S, Nakano R, Kashio S, Lundin K, Picton B, Korshunova T (2019) The extraordinary genus Myja is not a tergipedid, but related to the Facelinidae s. str. with the addition of two new species from Japan (Mollusca, Nudibranchia). ZooKeys 818: 89–116. https://doi.org/10.3897/zookeys.818.30477
  • Mastrototaro F, Panetta P, D’Onghia G (2004) Further records of Melibe viridis (Mollusca, Nudibranchia) in the Mediterranean Sea, with observations on the spawning. Vie et Milieu 54: 251–253.
  • Matsuda SB, Gosliner TM (2018) Glossing over cryptic species: Descriptions of four new species of Glossodoris and three new species of Doriprismatica (Nudibranchia: Chromodorididae). Zootaxa 4444: 501–529. https://doi.org/10.11646/zootaxa.4444.5.1
  • McCuller M, Carlton J, Geller J (2018) Bugula tsunamiensis n. sp. (Bryozoa, Cheilostomata, Bugulidae) from Japanese tsunami marine debris landed in the Hawaiian Archipelago and the Pacific Coast of the USA. Aquatic Invasions 13: 163–171. https://doi.org/10.3391/ai.2018.13.1.12
  • Mehrotra R, Scott CM (2016) Species inventory of sea slugs (Gastropoda: Heterobranchia) for Koh Tao, Thailand, with 25 first records for Thai waters. Marine Biodiversity 46: 761–771. https://doi.org/10.1007/s12526-015-0424-7
  • Mehrotra R, Caballer Gutierrez M, Chavanich S (2017) On the genus Armina (Gastropoda: Heterobranchia: Nudibranchia) in Thailand. Marine Biodiversity 47: 549–559. https://doi.org/10.1007/s12526-017-0691-6
  • Mehrotra R, Monchanin C, Scott CM, Phongsuwan N, Caballer Gutierrez M, Chavanich S, Hoeksema BW (2019) Selective consumption of sacoglossan sea slugs (Mollusca: Gastropoda) by scleractinian corals (Cnidaria: Anthozoa). PLoS ONE 14: e0215063. https://doi.org/10.1371/journal.pone.0215063
  • Mehrotra R, Arnold S, Wang A, Chavanich S, Hoeksema BW, Caballer M (2020a) A new species of coral-feeding nudibranch (Mollusca: Gastropoda) from the Gulf of Thailand. Marine Biodiversity 50: e36. https://doi.org/10.1007/s12526-020-01050-2
  • Mehrotra R, Caballer Gutierrez M, Scott CM, Arnold S, Monchanin C, Chavanich S (2020b) On the Plakobranchidae (Gastropoda, Sacoglossa) from soft sediment habitats of Koh Tao, Gulf of Thailand, with descriptions of two new species. ZooKeys 969: 85–121. https://doi.org/10.3897/zookeys.969.52941
  • Mercier A, Hamel J-F (2005) Note on the association between Plakobranchus ocellatus (Mollusca: Gastropoda: Opisthobranchia) and Holothuria atra (Echinodermata: Holothuroidea). Cahiers de biologie marine 46: 399–402.
  • Meyers-Muñoz M, Van der Velde G, van der Meij S, Stoffels B, Alen T, Tuti Y, Hoeksema B (2016) The phylogenetic position of a new species of Plakobranchus from West Papua, Indonesia (Mollusca, Opisthobranchia, Sacoglossa). ZooKeys 594: 73–98. https://doi.org/10.3897/zookeys.594.5954
  • Nabhitabhata J (2009) Checklist of Mollusca Fauna in Thailand. Office of Natural Resources and Environmental Policy and Planning, Ministry of Natural Resources and Environment, Bangkok, 576 pp.
  • Nakano R, Hirose E (2011) Field experiments on the feeding of the nudibranch Gymnodoris spp. (Nudibranchia: Doridina: Gymnodorididae) in Japan. The Veliger 51: 66–75.
  • Nakano R, Fujii T (2014) The soft-coral associated pistol shrimp Synalpheus neomeris (De Man) (Decapoda: Alpheidae) defends its host against nudibranchs in Okinawa, Japan. The Raffles Bulletin of Zoology 62: 759–763.
  • Nimbs M, Smith S (2016) An illustrated inventory of the sea slugs of New South Wales, Australia (Gastropoda: Heterobranchia). Proceedings of the Royal Society of Victoria 128: 44–113. https://doi.org/10.1071/RS16011
  • Nimbs M, Smith S (2018) Beyond Capricornia: tropical sea slugs (Gastropoda, Heterobranchia) extend their distributions into the Tasman Sea. Diversity 10: e99. https://doi.org/10.3390/d10030099
  • Nimbs MJ, Larkin M, Davis TR, Harasti D, Willan RC, Smith SDA (2016) Southern range extensions for twelve heterobranch sea slugs (Gastropoda: Heterobranchia) on the eastern coast of Australia. Marine Biodiversity Records 9: e27. https://doi.org/10.1186/s41200-016-0027-4
  • Nimbs MJ, Willan RC, Smith SD (2017) Is Port Stephens, eastern Australia, a global hotspot for biodiversity of Aplysiidae (Gastropoda: Heterobranchia)? Molluscan Research 37: 45–65. https://doi.org/10.1080/13235818.2016.1207280
  • Okutani T (2000) Marine Mollusks in Japan. Dai 1-han edition. Tōkai Daigaku Shuppankai, Tokyo, 1221 pp.
  • Oladi M, Nasrolahi A, Ahmadzadeh F, Alizadeh O, Krug P (2018) Molecular confirmation of the occurrence of Elysia cf. tomentosa (Mollusca: Heterobranchia) in the Persian Gulf. Zoology in the Middle East 64: 124–130. https://doi.org/10.1080/09397140.2018.1444571
  • Ong E, Hallas J, Gosliner T (2017) Like a bat out of heaven: the phylogeny and diversity of the bat-winged slugs (Heterobranchia: Gastropteridae). Zoological Journal of the Linnean Society 180: 755–789. https://doi.org/10.1093/zoolinnean/zlw018
  • Ono A (2004) Opisthobranchs of Ryukyu Islands. Rutles Inc., Tokyo, 304 pp.
  • Orr J (1981) Hong Kong Nudibranchs. Urban Council, Hong Kong, 82 pp.
  • Ortea Rato J, Buske Y (2018) Lista inicial ilustrada de las babosas marinas (Heterobranquios) de la expedición Madibenthos, realizada en 2016 en Martinica (Antillas Menores, mar Caribe). Revista de la Academia Canaria de Ciencias 30: 67–102.
  • Ortea Rato J, Espinosa J, Caballer Gutiérrez M, Buske Y (2012) Initial inventory of the sea slugs (Opisthobranchia and Sacoglossa) from the expedition KARUBENTHOS, held in May 2012 in Guadeloupe (Lesser Antilles, Caribbean Sea). Revista de la Academia Canaria de Ciencias. Biología 24: 153–182.
  • Ortea Rato J, Caballer Gutiérrez M, Moro L, Espinosa J (2014) What the shell tells in Aglajidae: A new genus for Aglaja felis (Opisthobranchia: Cephalaspidea). Revista de la Academia Canaria de Ciencias 26: 83–119.
  • Ortigosa D, Lemus E, Simoes N (2015) New records of “opisthobranchs” (Gastropoda: Heterobranchia) from Arrecife Alacranes National Park, Yucatan, Mexico. Marine Biodiversity Records 8: 1–18. https://doi.org/10.1017/S1755267215000925
  • Oskars TR, Malaquias MAE (2020) Systematic revision of the Indo-West Pacific colourful bubble-snails of the genus Lamprohaminoea Habe, 1952 (Cephalaspidea : Haminoeidae). Invertebrate Systematics 34(7): 727–756. https://doi.org/10.1071/IS20026
  • Oskars TR, Bouchet P, Malaquias MAE (2015) A new phylogeny of the Cephalaspidea (Gastropoda: Heterobranchia) based on expanded taxon sampling and gene markers. Molecular Phylogenetics and Evolution 89: 130–150. https://doi.org/10.1016/j.ympev.2015.04.011
  • Osumi D, Yamasu T (1994) The nudibranch, Gymnodoris nigricolor Baba, parasitic with marine gobies. Zoological Science [Tokyo] 11: 54.
  • Papu A, Undap N, Martinez NA, Segre MR, Datang IG, Kuada RR, Perin M, Yonow N, Wägele H (2020) First study on marine Heterobranchia (Gastropoda, Mollusca) in Bangka Archipelago, North Sulawesi, Indonesia. Diversity 12(2): e52. https://doi.org/10.3390/d12020052
  • Parasharya D, Patel B (2014) Spawning aggregation of Melibe viridis Kelaart (1858) from Gulf of Kachchh – Western India. International Journal of Scientific and Research Publications 4: 1–5.
  • Patel B, Apte D (2014) Recent record of Mexichromis mariei (Crosse, 1872) from Gulf of Kachchhh, Gujarat, India. Scholars Research Library 3: 9–12.
  • Pola M, Rudman W, Gosliner TM (2009) Systematics and preliminary phylogeny of Bornellidae (Mollusca: Nudibranchia: Dendronotina) based on morphological characters with description of four new species. Zootaxa 1975: 1–57. https://doi.org/10.11646/zootaxa.1975.1.1
  • Pola M, Camacho-García Y, Gosliner TM (2012) Molecular data illuminate cryptic nudibranch species: the evolution of the Scyllaeidae (Nudibranchia: Dendronotina) with a revision of Notobryon. Zoological Journal of the Linnean Society 165: 311–336. https://doi.org/10.1111/j.1096-3642.2012.00816.x
  • Pruvot-Fol A (1956) Un Aeolidien nouveau des mers tropicales: Aeolidiopsis ransoni n. g., n. sp. Bulletin du Muséum National d’Histoire Naturelle 28: 228–231.
  • Ramakrishna CR, Sreeraj CR, Sivaperuman C, Kumar JS, Raghuraman R, Immanuel T, Raghunathan R, Rajan PT (2010) Guide to opisthobranchs of Andaman and Nicobar Islands. Zoological Survey of India, Kolkata, 196 pp.
  • Rao KV (1960) On two opisthobranchiate molluscs, Placobranchus ocellatus Hasselt and Discodoris boholiensis Bergh, from Indian waters not hitherto been recorded. Journal of the Marine Biological Association of India 3: 253–255.
  • Rezai H, Mohtarami S, Dehghani H, Tavakoli-kolour P, Bargahi H, Kabiri K (2016) Nudibranchs from the Northern Persian Gulf. Journal of the Persian Gulf 7: 71–78.
  • Risbec J (1928) Contribution à l’étude des nudibranches Néo-Calédoniens. Faune des Colonies Françaises, 2: 1–328. Ph.D. thesis, Université de Paris, Paris, 328 pp.
  • Risbec J (1953) Mollusques nudibranches de la Nouvelle Calédonie. Faune de l’Union Française 15: 1–189.
  • Risbec J (1956) Nudibranches du Viet-Nam. Archives du Múseum National d’Histoire Naturelle, Paris, 7éme Série 4: 1–34.
  • Robba E, Di Geronimo I, Chaimanee N, Negri MP, Sanfilippo R (2007) Holocene and recent shallow soft-bottom mollusks from the Western Gulf of Thailand: Pak Phanang Bay and additions to Phetchaburi fauna. Società italiana di malacologia, Bollettino Malacologico 43: 1–98.
  • Romani L, Bartolini S, Raveggi A (2015) Colinatys Ortea, Moro et Espinosa, 2013 from Eastern Mediterranean Sea (Opisthobranchia Haminoeidae). Biodiversity Journal 6: 83–86.
  • Rothman S, Mienis H, Galil B (2017) Alien facelinid nudibranchs in the Eastern Mediterranean: First report of Phidiana militaris (Alder and Hancock, 1864) and report of Caloria indica (Bergh, 1896) 30 years after its previous sighting. BioInvasions Records 6: 125–128. https://doi.org/10.3391/bir.2017.6.2.06
  • Rudman WB (1985) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris aureomarginata, C. verrieri and C. fidelis colour groups. Zoological Journal of the Linnean Society 83: 241–299. https://doi.org/10.1111/j.1096-3642.1985.tb00875.x
  • Rudman WB (1986) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: the genus Glossodoris Ehrenberg (= Casella, H. and A. Adams). Zoological Journal of the Linnean Society 86: 101–184. https://doi.org/10.1111/j.1096-3642.1986.tb01814.x
  • Rudman WB (1990) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: further species of Glossodoris, Thorunna and the Chromodoris aureomarginata colour group. Zoological Journal of the Linnean Society 100: 263–326. https://doi.org/10.1111/j.1096-3642.1990.tb01864.x
  • Rudman WB (1991) Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies 57: 167–203. https://doi.org/10.1093/mollus/57.2.167
  • Rudman WB, Darvell BW (1990) Opisthobranch molluscs of Hong Kong: Part 1. Goniodorididae, Onchidorididae, Triophidae, Gymnodorididae, Chromodorididae (Nudibranchia). Asian Marine Biology 7: 31–79.
  • Sanpanich K, Duangdee T (2019) A survey of marine mollusc diversity in the Southern Mergui archipelago, Myanmar. Phuket Marine Biological Center Research Bulletin 75: 45–60.
  • Sathiamurthy E, Voris HK (2006) Maps of Holocene sea level transgression and submerged lakes on the Sunda Shelf. Tropical Natural History 2: 1–44.
  • Scott C (2012) The Koh Tao Ecological Monitoring Program. Save Koh Tao Community Group, Koh Tao, 109 pp.
  • Scott CM, Mehrotra R, Cabral M, Arunrugstichai S (2017) Changes in hard coral abundance and composition on Koh Tao, Thailand, 2006–2014. Coastal Ecosystems 4: 26–38.
  • Scott P (1984) The Corals of Hong Kong. Hong Kong University Press, Hong Kong, 120 pp.
  • Selfati M, El Ouamari N, Crocetta F, Mesfioui A, Boissery P, Bazairi H (2017) Closing the circle in the Mediterranean Sea: Bursatella leachii Blainville, 1817 (Mollusca: Gastropoda: Anaspidea) has reached Morocco. BioInvasions Records 6: 129–134. https://doi.org/10.3391/bir.2017.6.2.07
  • Sethi S, Kokane MR, Otta SK, Sethi G (2015) First record of ragged sea hare Bursatella leachii Blainville, 1817 (Opisthobranchia: Euopisthobranchia: Aplysiidae) in Pulicat Lake, east coast of India. Marine Biodiversity Records 8: e34. https://doi.org/10.1017/S1755267215000147
  • Sheeja G, Padma Kumar K (2014) New record of Plakobranchus ocellatus Van Hasselt, 1824 from Kerala Coast, India. Indian Journal of Life Sciences 4: 1–5.
  • Sojisuporn P, Morimoto A, Yanagi T (2010) Seasonal variation of sea surface current in the Gulf of Thailand. Coastal Marine Science 34: 91–102.
  • Song J, Han Y, Lee W, Kim I, Paik S, Lee J, Soh H, Lee W, Jung J, Kim S, Lee J, Kim J, Park T, Yoo J, Kil H, Nam E, Min G (2017) Unrecorded species of Korean metazoans discovered through the project of “Discovery of Korean Indigenous Species” (2006–2010). Journal of Species Research 6: 164–171. https://doi.org/10.12651/JSR.2017.6(S).164
  • Soong GY, Wilson NG, Reimer JD (2020) A species complex within the red-reticulate Goniobranchus Pease, 1866 (Nudibranchia: Doridina: Chromodorididae). Marine Biodiversity 50: e25. https://doi.org/10.1007/s12526-020-01048-w
  • Sreeraj CR, Chandrakasan S, Raghunathan C (2012) An annotated checklist of opisthobranch fauna (Gastropoda: Opisthobranchia) of the Nicobar Islands, India. Journal of Threatened Taxa 4: 2499–2509. https://doi.org/10.11609/JoTT.o2783.2499-509
  • Stimpson W (1855) Descriptions of some of the new marine Invertebrata from the Chinese and Japanese Seas. Academy of Natural Sciences, Philadelphia, 22 pp. https://doi.org/10.5962/bhl.title.51444
  • Stoffels B, van der Meij S, Hoeksema B, Alphen J, Alen T, Meyers-Muñoz M, de Voogd N, Tuti Y, Van der Velde G (2016) Phylogenetic relationships within the Phyllidiidae (Opisthobranchia, Nudibranchia). ZooKeys 605: 1–35. https://doi.org/10.3897/zookeys.605.7136
  • Su Y, Huang L-J, Chang Y-W, Mok H-K (2009) Temporal changes in nudibranch composition at a coastal site off Penghu (the Pescadores) in the Taiwan Strait. Zoological Studies 48: 2249–2509.
  • Swennen C (1997) Two new gastropods, Elysia bangtawaensis and E. siamensis from Southern Thailand (Opisthobranchia, Sacoglossa, Elysiidae). Bulletin Zoölogisch Museum, University of Amsterdam 16: 33–39.
  • Swennen C (2001) Two new sacoglossans (Gastropoda: Opisthobranchia) from Thailand. Beaufortia 51: 75–81.
  • Swennen C (2007) Costasiella coronata, new species, and a revised diagnosis for the family Costasiellidae (Mollusca: Opisthobranchia: Sacoglossa). The Raffles Bulletin of Zoology 55: 355–362.
  • Swennen C (2011) Large mangrove-dwelling Elysia species in Asia, with descriptions of two new species (Gastropoda: Opisthobranchia: Sacoglossa). The Raffles Bulletin of Zoology 59: 29–37.
  • Swennen C, Buatip S (2009) Aiteng ater, new genus, new species, an amphibious and insectivorous sea slug that is difficult to classify [Mollusca: Gastropoda: Opisthobranchia: Sacoglossa (?): Aitengidae, new family]. The Raffles Bulletin of Zoology 57: 495–500.
  • Swennen C, Buatip S (2012) Murphydoris singaporensis (Gastropoda: Nudibranchia: Goniodorididae) found in Thailand; with external and first internal description of the specimens. Strombus 19: 9–14.
  • Swennen C, Moolenbeek R, Ruttanadakul N, Hobbelink H, Dekker H, Hajisamae S (2001) 4 The molluscs of the southern Gulf of Thailand. Biodiversity Research and Training Program, Bangkok, 210 pp.
  • Thaman B, Thaman R, Balawa A, Veitayaki J (2017) The recovery of a tropical marine mollusk fishery: a transdisciplinary community-based approach in Navakavu, Fiji. Journal of Ethnobiology 37: 494–513. https://doi.org/10.2993/0278-0771-37.3.494
  • Tibiriçá Y, Malaquias M (2016) The bubble snails (Gastropoda, Heterobranchia) of Mozambique: an overlooked biodiversity hotspot. Marine Biodiversity 47: 791–811. https://doi.org/10.1007/s12526-016-0500-7
  • Tibiriçá Y, Pola M, Cervera JL (2017) Astonishing diversity revealed: an annotated and illustrated inventory of Nudipleura (Gastropoda: Heterobranchia) from Mozambique. Zootaxa 4359: 1–133. https://doi.org/10.11646/zootaxa.4359.1.1
  • Toh CH (2016) New Singapore record of nudibranch Hypselodoris tryoni. Singapore Biodiversity Records 2016: 61.
  • Too CC, Carlson C, Hoff PJ, Malaquias MAE (2014) Diversity and systematics of Haminoeidae gastropods (Heterobranchia: Cephalaspidea) in the tropical West Pacific Ocean: new data on the genera Aliculastrum, Atys, Diniatys and Liloa. Zootaxa 3794: 355–392. https://doi.org/10.11646/zootaxa.3794.3.3
  • Travaglini A, Crocetta F (2019) Natural history collections and alien species: an overlooked sample of Bursatella leachii Blainville, 1817 (Mollusca: Gastropoda: Aplysiida) backdates its confirmed presence in Italy. Thalassas: An International Journal of Marine Sciences 35: 137–141. https://doi.org/10.1007/s41208-018-0101-2
  • Trowbridge C, Hirano YM, Hirano YJ (2011) Inventory of Japanese sacoglossan opisthobranchs: historical review, current records, and unresolved issues. American Malacological Bulletin 29: 1–23. https://doi.org/10.4003/006.029.0201
  • Turner LM, Wilson NG (2012) The Chelidonura tsurugensis–sandrana (Gastropoda: Cephalaspidea) species complex: do reproductive decisions maintain colour polymorphism? Journal of Molluscan Studies 78: 166–172. https://doi.org/10.1093/mollus/eyr057
  • Undap N, Papu A, Schillo D, Ijong FG, Kaligis F, Lepar M, Hertzer C, Böhringer N, König GM, Schäberle TF, Wägele H (2019) First survey of heterobranch sea slugs (Mollusca, Gastropoda) from the island Sangihe, North Sulawesi, Indonesia. Diversity 11: e170. https://doi.org/10.3390/d11090170
  • Urgell Plaza P, Mehrotra R, Scott CM, Reimer JD (2018) Rare zooxanthellate Nanipora octocoral (Helioporacea) in the Gulf of Thailand. Marine Biodiversity 48: 1961–1967. https://doi.org/10.1007/s12526-017-0705-4
  • Uyeno D, Nagasawa K (2012) Four new species of splanchnotrophid copepods (Poecilostomatoida) parasitic on doridacean nudibranchs (Gastropoda, Opisthobranchia) from Japan, with proposition of one new genus. ZooKeys 247: 1–29. https://doi.org/10.3897/zookeys.247.3698
  • Vadher P, Kardani H (2018) First record of Dendrodoris atromaculata (Alder & Hancock, 1864) from West coast of India. Journal of Entomology and Zoology Studies 6: 2663–2666.
  • Valdés Á (2002) A phylogenetic analysis and systematic revision of the cryptobranch dorids (Mollusca, Nudibranchia, Anthobranchia). Zoological Journal of the Linnean Society 136: 535–636. https://doi.org/10.1046/j.1096-3642.2002.00039.x
  • Valdés Á, Mollo E, Ortea Rato J (1999) Two new species of Chromodoris (Mollusca, Nudibranchia, Chromodorididae) from southern India, with a redescription of Chromodoris trimarginata (Winckworth, 1946). Proceedings of the California Academy of Sciences 51: 461–472.
  • Valdés Á, Hamann J, Behrens D, Dupont A (2006) Caribbean sea slugs: a field guide to the opisthobranch mollusks from the tropical northwestern Atlantic. Sea Challengers Natural History Books, Etc. Gig Harbor, Washington, 289 pp.
  • Vayssière A (1912) Recherches zoologiques et anatomiques sur les Opisthobranches de la Mer Rouge et du Golfe d’Aden. Partie 2. Annals de la Faculté des Sciences, Univ. Marseille 20: 5–157.
  • Vendetti JE, Trowbridge CD, Krug PJ (2012) Poecilogony and population genetic structure in Elysia pusilla (Heterobranchia: Sacoglossa), and reproductive data for five sacoglossans that express dimorphisms in larval development. Integrative and Comparative Biology 52: 138–150. https://doi.org/10.1093/icb/ics077
  • Wade R, Sherwood A (2016) Molecular determination of kleptoplast origins from the sea slug Plakobranchus ocellatus (Sacoglossa, Gastropoda) reveals cryptic bryopsidalean (Chlorophyta) diversity in the Hawaiian Islands. Journal of phycology 53: 467–475. https://doi.org/10.1111/jpy.12503
  • Wägele H, Stemmer K, Burghardt I, Händeler K (2010) Two new sacoglossan sea slug species (Opisthobranchia, Gastropoda): Ercolania annelyleorum sp. nov. (Limapontioidea) and Elysia asbecki sp. nov. (Plakobranchoidea), with notes on anatomy, histology and biology. Zootaxa 2676: 1–28. https://doi.org/10.11646/zootaxa.2676.1.1
  • Wägele H, Deusch O, Händeler K, Martin M, Schmitt V, Christa G, Pinzger B, Gould S, Dagan T, Klussmann-Kolb A, Martin W (2011) Transcriptomic evidence that longevity of acquired plastids in the photosynthetic slugs Elysia timida and Plakobranchus ocellatus does not entail lateral transfer of algal nuclear genes. Molecular biology and evolution 28: 699–706. https://doi.org/10.1093/molbev/msq239
  • Wallace CC, Phongsuwan N, Muir PR (2012) A new species of staghorn coral, Acropora sirikitiae sp. nov. (Scleractinia: Astrocoeniina: Acroporidae) from western Thailand. Phuket Marine Biological Center Research Bulletin 71: 117–124.
  • Wang A, Conti-Jerpe I, Richards J, Baker D (2020) Phestilla subodiosus sp. nov. (Nudibranchia, Trinchesiidae), a corallivorous pest species in the aquarium trade. ZooKeys 909: 1–24. https://doi.org/10.3897/zookeys.909.35278
  • Willan RC, Chang YW (2017) Description of three new species of Tambja (Gastropoda, Nudibranchia, Polyceridae) from the western Pacific Ocean reveals morphological characters with taxonomic and phylogenetic significance for traditional Polyceridae and related “phaneorobranch” nudibranchs. Basteria 81: 1–23.
  • Yokes MB, Daylan C, Karhan SU, Demir V, Tural U, Kalkan E (2012) Alien opisthobranchs from Turkish coasts: first record of Plocamopherus tilesii Bergh, 1877 from the Mediterranean. Triton 5: 1–9.
  • Yonow N (1984) Doridacean nudibranchs from Sri Lanka, with descriptions of four new species. The Veliger 26: 214–228.
  • Yonow N (1989) Red Sea Opisthobranchia. 2. The family Chromodorididae (Mollusca, Nudibranchia). Fauna of Saudi Arabia 10: 290–309.
  • Yonow N (1990) Red Sea Opisthobranchia. The orders Sacoglossa, Cephalaspidea, and Nudibranchia: Doridacea (Mollusca, Opisthobranchia). Fauna of Saudi Arabia 11: 286–299.
  • Yonow N (1994) Opisthobranchs from the Maldive Islands, including descriptions of seven new species (Mollusca: Gastropoda). Revue française d’aquariologie (1993) 20: 97–129.
  • Yonow N (2000) Red Sea Opisthobranchia 4: the orders Cephalaspidea, Anaspidea, Notaspidea and Nudibranchia: Dendronotacea and Aeolidacea. Fauna of Arabia 18: 87–132.
  • Yonow N (2001) Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 11. Doridacea of the families Chromodorididae and Hexabranchidae (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia), including additional Moluccan material. Zoologische Mededelingen 75: 1–50. http://www.repository.naturalis.nl/document/45054
  • Yonow N (2008) Sea Slugs of the Red Sea. Pensoft Publishers, Sofia, 304 pp.
  • Yonow N (2012) Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda). ZooKeys 197: 1–130. https://doi.org/10.3897/zookeys.197.1728
  • Yonow N (2017) Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 16. The NudibranchiaDendronotina, Arminina, Aeolidina, and Doridina (Mollusca: Gastropoda: Heterobranchia). Archiv für Molluskenkunde International Journal of Malacology 146: 135–172. https://doi.org/10.1127/arch.moll/146/135-172
  • Yonow N, Hayward PJ (1991) Opisthobranches de l’île Maurice, avec la description de deux espèces nouvelles (Mollusca: Opisthobranchia). Revue française d’aquariologie herpétologie 18: 1–30.
  • Yonow N, Jensen KR (2018) Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 17. The Cephalaspidea, Anaspidea, Pleurobranchida, and Sacoglossa (Mollusca: Gastropoda: Heterobranchia). Archiv für Molluskenkunde International Journal of Malacology 147: 1–48. https://doi.org/10.1127/arch.moll/147/001-048
  • Zakhama-Sraieb R (2009) On the occurrence of Bursatella leachii De Blainville, 1817 and Pinctada radiata (Leach, 1814) in the Ghar El Melh lagoon (NE Tunisia). Aquatic Invasions 4: 381–383. https://doi.org/10.3391/ai.2009.4.2.14
  • Zamora-Silva A, Malaquias M (2018) Molecular phylogeny of the Aglajidae head-shield sea slugs (Heterobranchia: Cephalaspidea): new evolutionary lineages revealed and proposal of a new classification. Zoological Journal of the Linnean Society 183: 1–51. https://doi.org/10.1093/zoolinnean/zlx064
  • Zenetos A, Gofas S, Verlaque M, Çinar M, García Raso JE, Bianchi CN, Morri C, Azzurro E, Bilecenoglu M, Froglia C, Siokou-Frangou I, Violanti D, Sfriso A, San Martin G, Giangrande A, Katagan T, Ballesteros E, Ramos-Espla AA, Mastrototaro F, Ocana O, Zingone A, Gambi MC, Streftaris N (2010) Alien species in the Mediterranean Sea by 2010. A contribution to the application of European Union’s Marine Strategy Framework Directive (MSFD). Part I. Spatial distribution. Mediterranean Marine Science 11: 381–493. https://doi.org/10.12681/mms.87