Corresponding author: Suchana Chavanich (
Academic editor: Nathalie Yonow
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.
Mehrotra R, Caballer Gutiérrez MA, Scott CM, Arnold S, Monchanin C, Viyakarn V, Chavanich S (2021) An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters. ZooKeys 1042: 73–188.
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 (
Among the earliest records of sea slugs from Thailand were provided by
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 (
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
Surveyed sites with location codes given in each species after specimen size.
Location | Code | Coordinates |
---|---|---|
Leuk Bay | LB |
|
Suan Olan Artificial Reef | SO |
|
Coral-Aid Artificial Reef | CA |
|
Shark Island | SI |
|
Sai Daeng | SD |
|
Shark Bay | SB |
|
Chalok Bay | CB |
|
Tao Tong | TT |
|
Sai Nuan and Three Rocks | SN |
|
Mae Haad | MH |
|
Sairee Beach | SRB |
|
Hin Pee Wee | HPW |
|
Sattakut Wreck | SW |
|
White Rock | WR |
|
Hin Fai Artificial Reef | HF |
|
Twins | TW |
|
Green Rock | GR |
|
Red Rock | RR |
|
Mango Bay | MB |
|
Hin Wong Pinnacle | HWP |
|
Hin Wong Bay | HWB |
|
Mao Bay North | AMN |
|
Mao Bay | AM |
|
Laem Thien | LT |
|
Tanote Bay | TB |
|
King Kong Rocks | KKR |
|
Chumphon Pinnacle | CP |
|
Southwest Pinnacle | SWP |
|
Sail Rock | SR |
|
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.
Schematic outline of benthic zones classified in the surveys conducted at Koh Tao
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
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 |
---|---|---|
|
19 | 20 |
|
19 | 24 |
|
19 | 38 |
|
20 | 39 |
28 | 63 | |
46 | 81 | |
49 | 88 | |
|
49 | 88 |
|
50 | 100 |
|
51 | 101 |
|
52 | 102 |
|
111 | 203 |
|
111 | 204 |
|
154 | 239 |
156 | 241 | |
160 | 245 | |
Present study | 256 | 336 |
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.
In total, 191 species of heterobranch sea slugs have been documented from Koh Tao to date across the orders
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
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 (
Three specimens 2–6 mm, LB.
In soft sediment habitats beyond the coral reef where it feeds predominantly on
Across the Indo-Pacific including India (
One specimen 3 mm, location unknown.
Local ecology is unknown.
Unknown.
Similar to
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 (
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.
Specimens from Koh Tao were recently reviewed by
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
Known only from the Philippines, Malaysia, Indonesia, and Papua New Guinea (
Specimens from Koh Tao were recently reviewed (
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 (
One specimen 15 mm, HF; one specimen 23 mm, TW.
In coral reef habitats throughout the island. Depth 3–18 m.
Australia, Samoa (
Three specimens 65–82 mm, CB.
Mostly recorded from specimens inhabiting a shallow, isolated patch of
Recent molecular investigations (
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 (
One specimen 9 mm, CB; one specimen 12 mm, TW.
Among rubble, particularly found underneath the skeletons of dead
Across the Indo-Pacific including India (
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
Widespread across the Indo-Pacific including the Red Sea (
The status of
One specimen 37 mm, CB.
In soft sediment habitats outside the coral reef. Observed feeding on a pinnate form of
At present
Recent molecular investigations have found that specimens recognised as
1 specimen 18 mm, HWB; 2 specimens 8–15 mm, LB.
Found in coral reef habitats throughout the island. Depth 2–25 m.
The taxonomic status of
Variation in the heads of
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
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
Widespread across the Indo-Pacific including Mozambique (
Two specimens 30 mm, LB; one specimen 28 mm, TB.
Extremely similar to
Across the western Pacific including the Philippines, Guam, Tahiti, Fiji, and Hawaii (
Two specimens 5–9 mm, LB.
Very similar to those of
Known across the Indo-Pacific including Tanzania, the Philippines, Fiji (
Two specimens 8–12 mm, LB.
Very similar to
Across the Indo-Pacific including Japan (
One specimen 11 mm, AMB.
In soft sediment habitats outside the coral reef. Depth 23 m.
Three individuals 2–5 mm, LB.
Very similar to the other soft-sediment associated
Three individuals 3–6 mm, LB.
Very similar to
Three individuals 2–5 mm, LB.
While rarer than most other soft sediment associated
Unknown.
Six individuals 9–35 mm, SB; eight individuals 6–21 mm; LB.
White morphs (Fig.
With the recent comprehensive review of the genus
Two specimens 62–74 mm, TT.
In soft sediment habitats outside the coral reef. Depth 22–26 m.
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 (
Four specimens 32–39 mm, TT.
In soft sediment habitats outside the coral reef. Depth 22–26 m.
Two specimens 4 mm, CB; one specimen 3 mm, TT.
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 (
One specimen 35 mm, SRB.
In soft sediment habitats outside the coral reef. Depth 10 m.
‘
The taxonomic validity of this species name remains unresolved after it was designated the type species for the recently erected genus
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 (
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
Widespread across the Indo-Pacific including Mozambique (
One specimen 30 mm, SN.
In soft sediment habitats outside the coral reef. Depth 14–16 m.
Widespread across the Indo-Pacific including Mozambique (
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.
One individual 3 mm, SO.
In soft sediment habitats outside the coral reef. Depth 12–16 m.
Currently known only from the Philippines, Indonesia, and Australia (
One individual 3 mm, RR.
In soft sediment habitats outside the coral reef. Depth 20 m.
Unknown
Two individuals 2–4 mm, CB; one individual 3 mm, SB.
Observed exclusively under dead
Widespread across the Indo-Pacific including Japan (
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 (
One specimen 9 mm, SB.
Under a dead
Across the Indo-Pacific including Mozambique, Mauritius, the Philippines, Indonesia, Papua New Guinea, Vanuatu (
One specimen 65 mm, SB.
In soft sediment habitats where it grazes on cyanobacterial mats on the benthos. Depth 18–25 m.
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
Circumtropical including Brazil (
Recent work (
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 (
Three specimens 10–15 mm, TW.
Under coral rubble in shallow coral reef habitats. Depth 6–8 m.
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
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 (
Individuals from Koh Tao have a dark, almost black mantle with numerous inconspicuous black spots.
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
Widespread throughout the Indo-Pacific including Mozambique (
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 (
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 (
Two specimens 12–30 mm, TT.
On rocks and among corals at offshore pinnacles. Depth 8–25 m.
Widespread in the Indo-Pacific including New Caledonia (
One specimen 85 mm, AM.
Soft sediment habitat. Depth 26 m.
Widespread throughout the Indo-Pacific including Japan (
One specimen 50 mm, CP.
Among corals and coral rubble at offshore pinnacle sites. 9–20 m.
Widespread throughout the Indo-Pacific including Mozambique (
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 (
Three specimens 5–10 mm, CB.
Among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Depth 2–8 m.
Very similar to
One specimen 25 mm, CB.
Among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Depth 4–8 m
Similar to
One specimen 30 mm, SI.
Found upon its pink host sponge,
Red Sea (
This species was mistakenly identified as
One specimen 8 mm, TT.
One specimen 60 mm, CP.
On rocks and among corals at offshore pinnacles. Depth 8–25 m.
Widespread throughout the Indo-Pacific including South Africa, Red Sea, French Polynesia, Solomon Islands, China, Philippines (
One specimen 40 mm, HWB.
Coral reefs throughout the island. Depth 5–15 m.
Papua New Guinea, the Philippines and Madagascar (
Previously recorded as
Three specimens 4–6 mm, CB.
Observed under dead fungiid coral skeletons and occasionally among rubble in shallow coral reef. Depth 3–8 m
Externally resembling both
One specimen 45 mm, SN.
Locally rare, known only from soft sediment habitats outside the coral reef. Depth 12–16 m.
Across the Indo-Pacific including Myanmar (
Two specimens 6–18 mm CB; one specimen 15 mm, SB; one specimen 12 mm, SD.
Coral reef habitats throughout the region. Depth 2–25 m.
Widespread throughout the Indo-Pacific including Mozambique (
One specimen 15 mm, KKR.
Locally rare, known only from the reef edge and soft sediment habitats outside the coral reef. Depth 12–25 m.
Widespread throughout the Indo-Pacific including Myanmar (
None found presently.
Soft sediment habitats outside the coral reef. Depth 18–22 m.
Across the Indo-Pacific including Indonesia (
The species has historically been recorded from Koh Tao (
Three specimens 20–55 mm, CP.
Among rocks and corals at offshore rocky pinnacles. Depth 9–22 m.
Across the Indo-Pacific including the Gulf of Oman (
Incorrectly identified as
Two specimens 10–15 mm, CP.
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.
Across the western Pacific including Vietnam (
One specimen 10 mm, GR.
Coral reefs. Depth 5–10 m.
Widespread throughout the Indo-Pacific including Mozambique (
Locally known only from a single individual.
One specimen 55 mm, CP.
Among rocks and corals at offshore rocky pinnacles. Depth 18 m.
Red reticulated specimens of
One specimen 4 mm, CB.
Among reef rubble in shallow coral reef. Depth 4 m.
Japan, Malaysia, and Taiwan (
Though a small individual, it is identified as
Two specimens 12–24 mm, SB.
Locally found exclusively from deeper soft sediment habitats of the island. Depth 14–25 m.
Philippines, Papua New Guinea, probably Indonesia and Hong Kong (
Two specimens 8–18 mm, CB.
Abundant among reef rubble, in particular under dead fungiid coral skeletons, in shallow coral reef areas. Rare in other habitats. Depth 2–12 m.
Widespread throughout the Indo-Pacific including Malaysia, Philippines, Indonesia, Papua New Guinea, New Caledonia, Vanuatu, and the Marshall Islands (
One specimen 25 mm, SRB; one specimen 12 mm, CB; one specimen 8 mm, SB.
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.
Widespread and abundant in the Indo-Pacific including Mozambique (
One specimen 6 mm, CB.
Observed under a dead fungiid coral skeleton in shallow coral reef. Depth 6 m.
Externally similar to both
One specimen 40 mm, TW; two specimens 10–25 mm, SB.
Locally found exclusively from deeper soft sediment habitats of the island. Depth 14–25 m.
Misidentified by
Two specimens 5–15 mm, CB.
Observed under dead fungiid coral skeletons and on rocks in shallow coral reef. Depth 3–15 m.
Indonesia, Eastern Malaysia, and the Philippines (
Recorded as undescribed from Koh Tao (
One specimen 10 mm, CB.
Observed under dead fungiid coral skeletons and on rocks in shallow coral reef. Depth 5 m.
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
Three specimens 14–30 mm, TT; two specimens 15–25 mm, SWP; one specimen 20 mm, SR.
On rocks and rubble within coral reef. Depth 5–25 m.
Initially recorded as
Two specimens 30–35 mm, CB.
On rocks and rubble within coral reef. Depth 1–30 m.
Across the Indo-Pacific including Singapore (
One specimen 15 mm, SB.
In deep soft sediment habitats. Depth 20 m. Feeding on
Widespread throughout the Indo-Pacific including India (
Locally rare with only a single individual observed in the present surveys. Sharing the same habitat and prey preference as
One specimen 30 mm, SB, one specimen 8 mm, SN.
Observed in deep soft sediment habitats throughout the island, though uncommon. Often found associated with or actively feeding on
Widespread throughout the Indo-Pacific including, India (
Three specimens 5–8 mm, CB; two specimens 5 mm, TW.
Usually found immersed in prey sponge
Across the western Pacific including Indonesia (
One specimen 6 mm, CB; one specimen 4 mm, TW.
Observed under dead fungiid coral skeleton and rubble in shallow coral reef. Depth 3–8 m.
Initially recorded as
One specimen 120 mm, CB.
Locally found exclusively in soft sediment habitats. Depth 1–16 m.
Widespread Indo-Pacific including Australia, Indonesia, Mauritius (
One specimen 25 mm, CB.
Coral reefs. Depth 5–10 m.
Widespread Indo-Pacific including the Red Sea (
One specimen 95 mm, CB.
Observed upon prey sponge
Gulf of Oman (
One specimen 32 mm, CB.
Observed among rubble in shallow coral reef habitats at night with no observed association with prey. Depth 4–6 m.
Externally distinct from
Two specimens 9–17 mm, HF.
Among rubble in shallow coral reef habitats. Depth 3–8 m.
Specimens from Koh Tao resemble both
One specimen 30 mm, SN.
Locally found exclusively in soft sediment habitats. Depth 14–18 m.
Across the Indo-Pacific including the Red Sea (
One specimen 55 mm, SD.
Found exclusively in soft sediment habitats. Depth 12–24 m.
Widespread across the Indo-Pacific including India (
No specimen collected.
Shallow coral reef habitats. Depth 5–10 m.
The range of this species appears to be very limited thus far including only Myanmar (
Recorded at Koh Tao from a single individual in 2011 and not recorded since. The included figure (Fig.
Two specimens 15–25 mm, LB; one specimen 30 mm, SI; one specimen 87 mm, CB.
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
Widespread in the Indo-Pacific including the Red Sea (
Two specimens 10–15 mm, SB.
Found exclusively in deeper soft sediment habitats. Depth 16–25 m.
An undescribed species covered in numerous long caryophyllidia, similar to
One specimen 30 mm, SR.
Documented here from a single record found upon submerged concrete artificial substrate at an offshore pinnacle site. Depth 30 m.
Widespread in the Indo-Pacific including the Red Sea (
One specimen 20 mm, CA; two specimens 35–40 mm, SI.
Among rubble at the coral reef edge. Depth 8–16 m.
Externally, the specimens bear similarity to
Two specimens 6–18 mm, CB.
Exclusively found under coral rubble and the skeletons of dead
Unknown.
The present species is only identified based on external morphology and as such has not been identified to species level.
One specimen 70 mm, SB.
Found under coral rubble and the skeletons of dead
Across the Indo-Pacific including Madagascar (
One specimen 12 mm, TT.
Found in soft sediment habitats at 16 m depth.
Unknown.
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
One specimen 7 mm, HF.
Under reef rubble and on rocks in coral reef habitats. Cryptic on its yellow prey sponge. Depth 6–12 m.
While clearly resembling
Two specimens 6 mm, CB.
Among rubble in coral reef habitats. Depth 3–6 m.
Across the Indo-Pacific including Taiwan (
Two specimens 4–6 mm, CB.
Among rubble in coral reef habitats. Depth: 3–6 m.
Known from China (
One specimen 5 mm, HF.
Under reef rubble and on rocks in coral reef habitats. Cryptic on its orange prey sponge. Depth 6–12 m.
Broadly matches the description and variations highlighted by
Three specimens 6–15 mm, SN.
In soft sediment habitats beyond the coral reef.
Unknown. Currently only documented from Koh Tao.
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.
One specimen 5 mm, TT; two specimens 6–8 mm, CA.
Rare and cryptic within coral reef habitats. Depth 8–24 m.
Similar to
One specimen 3 mm, SI.
Rare and cryptic within coral reef habitats. Depth 14 m.
Similar to
One specimen 5 mm, CB.
Under rubble in shallow coral reefs. Depth 4–6 m.
Specimens from Koh Tao differ slightly from
Three specimens 8–25 mm, LB.
Among colonies of Didemnid tunicates in soft sediment habitats. Depth 12–15 m.
Known only from Indonesia (
While known to be predators of
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 (
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 (
Three specimens 29–42 mm, CB.
Found under coral rubble and the skeletons of dead
Across the Indo-Pacific including the Red Sea (
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 (
Two specimens 8–16 mm, LT; one specimen 25 mm, MH.
Found under coral rubble and the skeletons of dead
Widespread in the Indo-Pacific including Mozambique (
Two specimens 24–88 mm, TT.
Found in soft sediment habitats outside the coral reef alongside
Widespread in the Indo-Pacific including the Red Sea (
Three specimens 25–35 mm, GR.
Abundant in coral reef habitats. Depth 3–30 m
Widespread in the Indo-Pacific including Mozambique (
One specimen 45 mm, LT; one specimen 30 mm, RR; one specimen 45 mm, SP.
Abundant in coral reef habitats. Depth 3–30 m
Widespread in the western Pacific including), Indonesia, Taiwan, Australia, Guam, Solomon Islands (
One specimen 30 mm, SI.
Rare, found in coral reef habitats, Depth 15 m.
Known from the Maldives (
One specimen 30 mm, AMN; one specimen 65 mm, SP; three specimens 45–60 mm, HF.
Abundant in coral reef, reef edge, and soft sediment habitats. Depth 3–30 m.
Widespread in the Indo-Pacific including Mozambique (
Two specimens 28 mm, CB; one specimen 33 mm, TB.
Abundant in coral reef habitats. Depth 3–30 m
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 (
External identification of some specimens yielded the incorrect inclusion of
One specimen 32 mm, HWB; one specimen 55 mm, SR.
Abundant in coral reef habitats. Depth 3–30 m.
Widespread in the Indo-Pacific including the Red Sea (
One specimen 50 mm, AMN; one specimen 50 mm, TB.
Abundant in coral reef habitats. Depth 3–30 m.
Widespread in the Indo-Pacific including the Philippines, Australia, Papua New Guinea (
Two specimens 35–40 mm, TT; one specimen 35 mm, CB.
Abundant in coral reef habitats. Depth 3–30 m.
‘
Morphological and molecular work (
One specimen 28 mm, SB; one specimen 15 mm, CP.
Under and among rocks and coral rubble. Depth 3–30 m.
Across the Indo-Pacific including Australia, Guam, Marshall Islands, Tahiti (
One specimen 25 mm, SI.
Among coral and rubble in coral reef habitats. Depth 4–18 m.
Similar to
Two specimen 35 mm, TB.
Exclusively recorded from soft sediment habitats beyond the fringing coral reef. Depth 14 m.
Widespread in the Indo-Pacific including Mozambique (
One specimen 45 mm, TT; two specimens 65 mm, HF; one specimen 60 mm, TW.
In coral reef, rubble and soft sediment habitats throughout the island and nearby offshore pinnacles. Depth 5–25 m.
Widespread in the Indo-Pacific including the Red Sea (
One specimen 28 mm, HF.
Among coral and rubble in coral reef habitats. Depth 6–12 m.
Widespread in the Indo-Pacific including Mozambique (
One specimen 25 mm, TT.
On rocks and among coral and rubble in coral reef habitats. Depth 4–18 m.
First documented as
One specimen 14 mm, CB.
Among rubble in shallow coral reef habitats, observed feeding on
Unknown.
Resembling multiple species considered undescribed according to
One specimen 12 mm, HF.
Among rubble in coral reef habitats. Depth 6–12 m.
Smooth, translucent white dorsum with club shaped gill leaves. Observed feeding on an unknown nudibranch species (Fig.
Two specimens 5–7 mm, SN.
Recorded exclusively from deeper soft sediment habitats, crawling on the substrate. Depth 19–27 m.
The present species closely resembles externally the description by
One specimen 6 mm, SB; one specimen 7 mm, LB.
Recorded exclusively from deeper soft sediment habitats, crawling on the substrate. Depth 19–27 m.
Unknown
Strikingly similar to
One specimen 9 mm, TW.
In soft sediment habitats beyond fringing coral reefs. Depth 12–16 m.
So far only recorded in the Gulf of Thailand.
Incorrectly identified as
Two specimens 102 mm, TT.
Exclusively recorded from soft sediment habitats beyond the fringing coral reef. Depth 18–25 m.
One specimen 12 mm, SN.
Exclusively recorded from soft sediment habitats beyond the fringing coral reef where it feeds on
One specimen 6 mm, CP.
Among rocks and corals on offshore pinnacle sites.
Widespread in the Indo-Pacific including Mozambique (
One specimen 40 mm, SWP.
Locally recorded exclusively in deep soft sediment habitats at an offshore pinnacle. Depth 25–30 m.
Known from South Korea, Japan, Taiwan, Malaysia, Papua New Guinea Australia, and New Zealand (
Externally, the present species matches the species recently described by
One specimen 4 mm, TT.
Recorded from soft sediment habitats beyond the fringing coral reef where it was found on
Two specimens 6–16 mm, SB; one specimen 12 mm, KKR.
Exclusively recorded from soft sediment habitats beyond the fringing coral reef where it feeds on
Similar species are known from Mozambique (
While similar to
One specimen 15 mm, CA.
Among corals and rubble in coral reef habitats. Depth 6–10 m.
Two specimens 21–29 mm, LB; one specimen 17 mm, SRB.
Cryptic on their prey,
Known from Japan (
Specimens from Koh Tao have papillate rhinophores (similar to
Two specimens 25–30 mm, SB; one specimen 23 mm, CB.
Exclusively recorded on its prey species, the zoanthid
Across the Pacific including Japan (
Separated from
One specimen 10 mm, CA.
Among corals and rubble in coral reef habitats. Depth 6–10 m.
Across the Indo-Pacific including Tanzania (
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.
One specimen 10 mm, TT; one specimen 20 mm, SN.
Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.
Across the Indo-Pacific including Mozambique (
One specimen 8 mm, TT.
Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.
Across the Indo-Pacific including Australia (
Two specimens 7–12 mm, TT.
Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.
One specimen 22 mm, SN.
Exclusively found in deeper soft sediment habitats outside coral reef habitats, where it exhibits an endo-benthic substrate preference. Depth 14–20 m.
One specimen 4 mm, LT; one specimen 7 mm, LB; two specimens 6–12 mm, CB.
On rocks and under rubble, including skeletons of dead fungiid corals, in coral reef habitats. Depth 2–14 m.
Until recently, known only from the Pacific including Costa Rica (
One specimen 26 mm, TW; two specimens 31–39 mm, SWP; two specimens 18–25 mm, HF.
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.
Widespread across the Indo-Pacific including Mozambique (
One specimen 5 mm, CB; one specimen 5 mm, MH.
Documented in habitats ranging from shallow reef rubble and soft sediments to deeper soft sediment habitats. Depth 5–18 m.
Similar to
Two specimens SB, 5 mm.
Documented from deeper soft sediment habitats only. Depth 18–25 m.
One specimen GR, 9 mm.
Found exclusively on its prey hydroid
This genus was recently reviewed and expanded based on specimens from Japan and the Gulf of Thailand (
One specimen 5 mm, CB.
Found under rocks and coral rubble in shallow coral reef habitats. Depth 4–8 m.
One specimen 5 mm, SN.
Recorded from soft sediment habitats beyond the fringing coral reef. Depth 15–20 m.
Unknown. Here documented as a first record for Thai waters.
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.
Two specimens 30–35 mm, SO; two specimens 30–40 mm, HF.
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.
Across the Indo-Pacific including the Gulf of Oman (
One specimen 7 mm, HWP.
Among rocks and coral rubble. Depth 5–16 m.
Unknown.
Bearing some similarities to
Two specimens 30–40 mm, TT.
In coral reef habitats where it feeds on the octocoral
Widespread across the Indo-Pacific including Mozambique (
One specimen 15 mm, TT; one specimen 35 mm, SB.
In soft sediment habitats outside coral reef habitats, found regularly on or in the immediate vicinity of
Unknown
The present species externally appears to match
One specimen 35 mm, CP.
Among coral, rocks, and discarded fishing equipment at an offshore pinnacle site. Depth 16 m.
Known from the Philippines, Hong Kong, Korea, and Japan (
Two specimens 40–55 mm, HWB.
In shallow coral reef habitats where it observed feeding on a different species of
Similar to
Two specimens 65–72 mm, SN.
Exclusively known from soft sediment habitats outside the coral reef where it feeds on the sea pen
Known from the western Pacific including Indonesia (
Two specimens 35–49 mm, SB; one specimen 41 mm, TT.
Exclusively known from soft sediment habitats outside the coral reef where it feeds on the sea pen
Known from the western Pacific including Japan (
Specimens identified as
Two specimens 55–67 mm, SN.
Exclusive to the soft sediment habitats outside the coral reef. Depth 14–24 m.
Known from the western Pacific including Malaysia, Indonesia, and the Philippines (
Two specimens 40–50 mm, SB.
Exclusive to the soft sediment habitats outside the coral reef where it may be found feeding on
Known from the West Pacific including Japan and Australia (
Recorded as
Two specimens 7–11 mm, CB; one specimen 9 mm, SB.
On rocks and under rubble, in particular under skeletons of dead
Across the Indo-Pacific including the Indian Ocean of Java (
One specimen 33 mm, TB.
Exclusive to the soft sediment habitats outside the coral reef. Observed on
Known from the West Pacific including Malaysia, the Philippines, Indonesia, and Papua New Guinea (
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
Two specimens 9–12 mm, SI; two specimens 11 mm, HPW; one specimen 14 mm, SW.
On rocks in coral reef habitats where it feeds on the octocoral
This species was recorded as
Three specimens 28–40 mm, SB.
Exclusive to the soft sediment habitats outside the coral reef where it feeds on colonies of the octocoral
Unknown.
A species that vaguely resembles but is distinct from
One specimen 27 mm, TT.
Exclusive to the soft sediment habitats outside the coral reef. Depth 21 m.
Unknown.
Two specimens 12–18 mm, HWB.
Among corals, rocks and under rubble in shallow coral reef habitats. Depth 2–12 m.
Across the Indo-Pacific including Christmas Island in the Indian Ocean, Malaysia, the Marshall Islands (
One specimen 35 mm, CB.
On rocks and under rubble, in particular under skeletons of dead fungiid corals, in shallow coral reef habitats. Depth 2–8 m.
The present species matches the external description of the species (
One specimen 31 mm, CB.
On rocks and under rubble, in particular under skeletons of dead
Widespread across the Indo-Pacific including the Red Sea (
Two specimens 15–25 mm, SN; one specimen 17 mm, TW; one specimen 30 mm, SI.
Cryptic on its host hydroid
Circumtropical, recorded from Florida, the Bahamas (
One specimen 45 mm, CB.
Recorded from a single individual on floating algae
Across the Indo-Pacific including Mozambique (
One specimen 12 mm, TT; one specimen 122 mm, SB; one specimen 97 mm, SN.
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.
Across the Indo-Pacific including Mozambique (
Two specimens 15–22 mm, TT.
In soft sediment habitats beyond the coral reef, grazing upon the substrate. Depth 17–22 m.
Externally similar to
One specimen 22 mm, SN.
On its prey hydroid
It necessary here to clarify the brief historical records of
One specimen 7 mm, HF.
From an artificial reef structure at a reef restoration site in coral reef habitats. Depth 8–11 m.
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.
One specimen 15 mm, GR.
Among rocks and corals in coral reef habitats. Depth 5–15 m.
Widespread across the Indo-Pacific including Mozambique (
This species was recently transferred to the genus
Two specimens 35–40 mm, HWP; two specimens 35 mm, KKR.
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.
Specimens from Koh Tao strongly resemble
Two specimens 9–12 mm, CB; one specimen 18 mm, SI; one specimen 16 mm, SN.
On rocks and under rubble, including skeletons of dead
Widespread across the Indo-Pacific including the Red Sea (
Two specimens 11–15 mm, SRB; one specimen 12 mm, HF; one specimen 18 mm, LT.
Exclusively on its prey, the scleractinian coral
One specimen 30 mm, HWB; two specimens 25–30 mm, SO.
Exclusively on or in the immediate vicinity of its prey, the scleractinian coral
Widespread across the Indo-Pacific including the Red Sea (
Two specimens 15–30 mm, SI.
Exclusively on or in the immediate vicinity of its prey, the scleractinian corals
Widespread across the Indo-Pacific including Red Sea (
Four specimens 15–25 mm, CA.
Exclusively on or in the immediate vicinity of its prey, the scleractinian coral
Across the Indo-Pacific including Tanzania, Australia, Hawaii (
Recent molecular analyses have revealed
Two specimens 20–25 mm, AMN; one specimen 25 mm, TT; one specimen 33 mm, SB.
Exclusively on its prey, the scleractinian coral
One specimen 2 mm, SB.
Exclusively on it its prey coral
Two specimens 8–14 mm, CB.
Exclusively on its prey, the scleractinian coral
Two specimens 4–7 mm, SB.
Exclusively on its prey, the scleractinian coral
Unknown. Here representing a first record for Thai waters.
One specimen 3 mm, HWB.
Recorded from a single individual found during sampling of the rare octocoral
Unknown.
Very similar to
One specimen 10 mm, TW.
Among rocks and corals in coral reef habitats. Depth 5–10 m.
Incorrectly identified as
Two specimens 35–40 mm, TT; one specimen 45 mm, TW.
Cryptic on its host hydroid
Mistakenly identified as
Three specimens 12–45 mm, SN.
Cryptic on its host hydroid
‘
Very similar to
One specimen 6 mm, HF.
On artificial reef structures feeding on the same orange
Three specimens 19–24 mm, HF.
On artificial reef structures feeding on the same orange hydroid
Currently known only from the Gulf of Thailand (
Two specimens 55–70 mm, LB; one specimen 38 mm, SW.
In soft sediment habitats outside the coral reef. Depth 18–28 m.
Across the Indo-Pacific including the Red Sea (
One specimen 10 mm, CB.
Under rubble, among small hydroids, in coral reef habitats. Depth 5–8 m.
Across the Indo-Pacific including India (
The single specimen observed from Koh Tao appears paler than is typical for the species, though such colour differences are not unheard of (see
One specimen 35 mm, GR.
Known from a single specimen observed on the octocoral
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
The first of these are those species that are documented preferentially from artificial substrates such as artificial reefs and debris. Nudibranch species such as
The second specialist habitat type within the hard substrate habitats was the ventral surface of the skeletons of dead
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 (
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
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 (
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;