An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters

Rahul Mehrotra; Manuel A Caballer Gutiérrez; Chad M Scott; Spencer Arnold; Coline Monchanin; Voranop Viyakarn; Suchana Chavanich

doi:10.3897/zookeys.1042.64474

. 2021 Jun 9;1042:73–188. doi: 10.3897/zookeys.1042.64474

An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters

Rahul Mehrotra ^1,², Manuel A Caballer Gutiérrez ^3,⁴, Chad M Scott ⁵, Spencer Arnold ⁵, Coline Monchanin ^2,⁶, Voranop Viyakarn ¹, Suchana Chavanich ^1,^7,^✉

PMCID: PMC8208966 PMID: 34163291

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.

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.

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

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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

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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

2EA91BF2-9233-5FFD-B9F5-EFD728D71AD5

Figure 4A

Material examined.

Superfamily Plakobranchoidea Gray, 1840

Family Plakobranchidae Rang, 1829

Genus Plakobranchus van Hasselt, 1824

Plakobranchus noctisstellatus

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

5AA8A540-454C-5DC5-AE72-AC8079CE7EC3

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

Genus Elysia Risso, 1818

Elysia asbecki

Wägele, Stemmer, Burghardt & Händeler, 2010

4EC8315D-3409-5FA3-A18F-97134E11E0BB

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)

217CD247-786C-53A3-807D-0524288C191C

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

3C35222C-ABE2-5935-B24E-DF962D7417B1

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)

A2AD99A3-22DD-5AD9-AF01-9EBED097B161

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.

994D3877-8295-5897-A10A-AF3CBB17B9DC

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.

Superfamily Haminoeoidea Pilsbry, 1895

Family Haminoeidae Pilsbry, 1895

Genus Aliculastrum Pilsbry, 1896

. Aliculastrum* cylindricum

(Helbling, 1779)

08BE1560-7153-5585-B1E2-4DEE2CFB3FDE

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.

. Atys*

sp.

8488E0EC-A36E-5F7F-86CF-EB504503AEBE

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)

03B7BE58-2874-582C-876D-D0DEC7DF5DAA

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)

DCDB4554-B0C3-5A52-B099-88D761B2B498

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.

4492F331-656F-50F4-B108-03CD1362FFED

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.

Distribution.

Unknown.

Lamprohaminoea Habe, 1952

. Lamprohaminoea* ovalis

(Pease, 1868)

A7807007-6702-5D2E-AD6B-B6501A184BB4

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

8DA68F14-9290-5428-BC36-81B87685A82A

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

14A34B7C-2018-5E23-88C4-0B9932179A1D

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.

0C316ADE-22D5-5A51-953A-39FD8B611720

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

09089962-11E7-5F81-8BD0-B48E0581A9DE

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)

3B76337F-4984-5930-9642-4F536FF39738

Figure 7I

Material examined.

One specimen 35 mm, SRB.

Ecology.

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

Distribution.

‘Philinopsis’ coronata 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)

8C499320-956B-5E57-A309-89E37F31AB69

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.

FA250F23-BCE7-5E43-872E-FE0ABC3F2D1E

Figure 8A

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

5D26A752-5D4F-5561-94C1-75558CC4C43F

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

A689B93C-5DE1-5E34-B981-3235DBC95917

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)

55ABA1A3-2CFA-5E5E-85A9-D6D7C1E91A9C

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)

7C6286D4-29EB-52F6-9D8D-E1624CD81F39

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)

FFD12CE9-D03A-5847-9170-256D568E60EB

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)

E00BEE42-6E98-5AC4-BBEF-C83E1EF427A7

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)

4B08B9EA-987A-5975-92C5-79FA9B564A8B

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

1830410A-0149-5726-A4BB-34EAD1B7FA6E

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

D30E6DBD-C4B7-58DD-83BD-DFAA4EDFC71B

Figure 9A

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

237F78C0-A302-5732-9656-43D5D619CF6C

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

A1963B5A-0DD0-549B-AE85-9DFCBBADCD7C

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

F6891A72-7CD4-5359-8F3D-1058521946FB

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)

3560B722-1B6A-5C6D-847F-491555560D10

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)

AAD58787-ECBF-5579-AB40-7405C74400B5

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)

998A031E-1648-595B-A8A0-50D1E053F21C

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)

3391151A-61B4-5941-BC57-E6827BA82841

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)

BBAB1F18-EE36-560D-BD5A-1D34CF43EE2C

Figure 10A

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)

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

2F7B532F-438B-53FB-8615-F54C3CA138A9

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

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)

18D1B157-F1D7-58D7-82D7-F97B12E9638C

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)

697BECC4-95F5-585E-A629-27716C1B7EC1

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)

4DCDDF2C-5948-5D32-B96D-5ED2315029EE

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)

B5E7BD1D-E7FE-5FCD-A345-593BDDF74591

Figure 12A

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

E24AD215-25D0-51A7-A16F-FE43C4C2C518

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

C6FE4668-2FA7-52F0-806F-1E60E65276CF

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

A1F530E7-CCD8-5F0F-B87D-4F2796FFEB8D

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

8BB7CA96-0804-5D70-9C68-04B20C3CBE12

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)

E7E727B1-6E3D-5837-9A86-1E160D981B52

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.

6047B869-2EBC-5021-95E1-E81AE5C37378

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)

F18583DF-F8A2-5CC2-B892-5503BE90AE76

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)

FBFF0F68-0C4C-569B-9616-BF6A7AFDC7F9

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 cinereobranchiataDorgan 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.

2E634E51-98A4-5FCB-AB8D-C1787E75BCB2

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.

Genus Sebadoris Er. Marcus & Ev. Marcus, 1960

. Sebadoris* fragilis

(Alder & Hancock, 1864)

42C7B48F-C920-5958-B50E-48DDB60A435E

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.

6FF3ECE8-8851-553F-82E0-6C4F2715EE01

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

CFCAFE4C-E2BA-56A2-9064-6138FE147E86

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.

C65D9343-DAAA-5768-A864-C81C3183B264

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

14BE2ACC-AA61-571B-B4EF-3AB5624EC40E

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

E87A636C-081F-5AFF-BC03-85982667E286

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

CBF38768-82B6-5C22-8598-FF1A1A0C2D88

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 gibberaGosliner 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

92757309-F1A2-532C-BBFA-8BDC445B0678

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.

Superfamily Phyllidioidea Rafinesque, 1814

Family Dendrodorididae O’Donoghue, 1924 (1864)

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

28E67061-63FF-5C28-AD1C-56A4ABAFFBEF

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.

Phyllidia varicosa

Lamarck, 1801

A63966A5-572D-522C-8AC9-1994945185A3

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)

142B9804-D892-5BB9-8CEC-EE682B1634B3

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.

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

DE558BC2-4C75-5815-B22E-DEFC2000E535

Figure 16A

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.

AC26C7B8-F22B-5720-B3A3-C74669A70EC1

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.

F17ECF75-7305-5447-A584-12FB090A849B

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

3FB9112C-8FAF-5628-BF71-5C03CC64159C

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

DBF00491-B433-5558-8BE2-185FC020B980

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.

Genus Baeolidia Bergh, 1888

. Baeolidia* salaamica

(Rudman, 1982)

99904634-9DA1-56B0-A746-6051E84CB3A7

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

7AD4E17D-2152-5AA3-A413-79703BE1B8D2

. Limenandra* confusa

Carmona, Pola, Gosliner & Cervera, 2014

4ED30D86-4A6D-5B88-852C-AEA832719819

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)

637B2730-91B0-52EA-9656-7204F7C9ABEE

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

816EFE20-A510-5D3B-9380-9F5ADB40AE7E

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

1490924A-DC4B-50B1-A6C4-EC7BD08EF566

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

85C358ED-E8E5-5DCE-AD7C-E68E6F559520

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.

Genus Noumeaella Risbec, 1937

. Noumeaella*

sp. 1

9DB3FBF6-7EF4-56BF-8823-8DC26883DA84

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

D157ACE8-86FB-544E-BE1A-2F29A84DBC0D

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)

6C8467A7-AF37-50EE-AB32-197F0BAFF057

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.

2712885E-C7B2-5830-A3CE-9223C9F8FBBA

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

89412753-EEE1-5725-8A17-B51F1766F344

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

4A67AF78-C0F7-5F20-94FB-B55BCD92C115

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).

Genus Dermatobranchus van Hasselt, 1824

. Dermatobranchus* caeruleomaculatus

Gosliner & Fahey, 2011

59C00D86-261E-53F8-AA09-BE31BB62288E

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

EC94CC40-F130-5BF4-8C4A-2275ABBC8139

Bornella hermanni

Angas, 1864

052DCEAB-36A9-5EE4-83A2-42E71DBE612C

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

CE6C4865-9398-5C69-8B5F-B819AAEBDE7F

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)

02E184C5-E19B-5639-AFA0-775EC7BCA8D5

Figure 20A

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)

BCB0AF87-F569-5864-AF6C-8E68FCC46375

(Bergh, 1870)

23465E55-8C25-5B5E-BA31-8B3F44DE76E7

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).

Phestilla melanobrachia

Bergh, 1874

A76B8107-4104-573B-9554-1853C1EE2A57

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

0BF9536C-C845-5C96-B7FD-888716B76ED3

Figure 21C

Material examined.

Four specimens 15–25 mm, CA.

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

E5C714BB-7677-50D7-90E6-E069D64327C0

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

sp.

9F25472B-F1F5-5980-8F97-6CCE5EE68175

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.

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.

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).

Citation

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. https://doi.org/10.3897/zookeys.1042.64474

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PERMALINK

An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters

Rahul Mehrotra

Manuel A Caballer Gutiérrez

Chad M Scott

Spencer Arnold

Coline Monchanin

Voranop Viyakarn

Suchana Chavanich

Abstract

Introduction

Materials and methods

Surveys and sampling

Figure 1.

Table 1.

Figure 2.

Table 2.

Results

Systematics

Class Gastropoda Cuvier, 1795

Genus Costasiella Pruvot-Fol, 1951

Costasiella cf. kuroshimae

Figure 4.

Material examined.

Ecology.

Distribution.

Remarks.

Costasiella usagi

Material examined.

Ecology.

Distribution.

Family Limapontiidae Gray, 1847

*. Limapontiidae sp.

Material examined.

Ecology.

Distribution.

Remarks.

Suborder Plakobranchacea Gray, 1840

Genus Plakobranchus van Hasselt, 1824

Plakobranchus noctisstellatus

Material examined.

Ecology.

Distribution.

Plakobranchus ocellatus

Material examined.

Ecology.

Distribution.

Remarks.

Plakobranchus papua

Material examined.

Ecology.

Distribution.

Remarks.

Elysia aowthai

Material examined.

Ecology.

Distribution.

Genus Elysia Risso, 1818

Elysia asbecki

Material examined.

Ecology.

Distribution.

*. Elysia cf. marginata

Material examined.

Ecology.

Distribution.

Remarks.

*. Elysia mercieri

Material examined.

Ecology.

Distribution.

Remarks.

*. Elysia obtusa

Material examined.

Ecology.

Distribution.

Remarks.

*. Elysia pusilla

Figure 6.

Material examined.

. Limapontiidae* sp.

. Elysia* cf. marginata

. Elysia* mercieri

. Elysia* obtusa

. Elysia* pusilla

. Elysia* cf. tomentosa

. Colinatys* sp.

. Aliculastrum* cylindricum

. Roxaniella* multistriata

. Atys* semistriatus

. Atys*

. Diniatys* dentifer

. Diniatys* dubius

. Haloa*

. Lamprohaminoea* ovalis

. Chelidonura* cf. castanea