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

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.

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).

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.

Three specimens 2–6 mm, LB.

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.

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).

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

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

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

Three specimens 25–32 mm, CB.

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.

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).

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

Three specimens 19–30 mm, SN.

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.

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).

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

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

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

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

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

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

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).

Three specimens 65–82 mm, CB.

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.

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.

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.

One specimen 12 mm, SO.

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

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.

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

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

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.

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.

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.

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

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.

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.

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.

One specimen 37 mm, CB.

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.

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.

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.

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

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

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).

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).

One specimen 3 mm, CB.

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

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.

Two specimens 25–30 mm, LB.

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.

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).

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

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.

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).

Two specimens 5–9 mm, LB.

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.

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.

Two specimens 8–12 mm, LB.

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.

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.

One specimen 11 mm, AMB.

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

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

Three individuals 2–5 mm, LB.

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.

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.

Three individuals 3–6 mm, LB.

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

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.

Three individuals 2–5 mm, LB.

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 I ‘Philinopsis’ coronata 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).

Unknown.

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

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.

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.

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.

Two specimens 62–74 mm, TT.

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

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).

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.

Four specimens 32–39 mm, TT.

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

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.

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.

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

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

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

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.

One specimen 35 mm, SRB.

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

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

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.

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

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

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).

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

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.

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).

One specimen 30 mm, SN.

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

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.

One specimen 3 mm, LB.

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

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

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).

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

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

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).

One specimen 9 mm, SB.

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

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.

One specimen 65 mm, SB.

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

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).

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.

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

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.

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).

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).

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

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

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).

Three specimens 10–15 mm, TW.

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

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).

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.

One specimen 60 mm, LB.

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

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).

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.

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

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

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).

One specimen 10 mm, SO.

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

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.

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

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.

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.

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

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

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).

One specimen 3 mm, SI.

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

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.

One specimen 5 mm, CB.

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

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

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.

Three specimens 8–25 mm, LB.

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

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

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.

One specimen 30 mm, LT.

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

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.

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

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

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).

Three specimens 29–42 mm, CB.

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

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.

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

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

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).

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

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.

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).

Two specimens 24–88 mm, TT.

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.

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).