Body wide, dorsoventrally flattened with broad parapodial flaps folding along dorsal midline, tail truncate. Head broad and flattened with a pair of small, raised, median eyes and rolled, laterally originating, rhinophores. Parapodial lamellae with digestive gland branches and dorsal haemolymph sinuses. Anus anterodorsal and penis armed with curved stylet. Radular teeth denticulate.

Three specimens 25–32 mm Chalok Bay, 10°3'44.77"N, 99°49'30.35"E, Koh Tao, Thailand.

Figure 4. 

Living specimens of Plakobranchus ocellatus from Koh Tao. A, B close-up of head with retracted rhinophores and dorsolateral view, 25 mm C sequenced specimen, 32 mm.

More than 100 individuals, ranging in size 5 mm–45 mm, observed in regular diving surveys between 2012 and 2019, Chalok Bay, 10°3'44.77"N, 99°49'30.35"E, Shark Bay 10°3'39.75"N, 99°50'4.43"E; Tao Tong 10°3'58.13"N, 99°49'4.76"E; Sai Nuan 10°4'43.24"N, 99°48'48.51"E; Twins 10°7'1.93"N, 99°48'44.26"E; Hin Wong Bay 10°6'12.30"N, 99°50'58.63"E, Koh Tao, Thailand; not collected.

Length alive up to 45 mm. Background colour pale yellowish white to pale brown, covered in ocelli, increasing in size laterally from parapodial margins. Dorsal ocelli small, brown or brown with yellow centres, surrounded by a diffuse ring of white. Dorso-laterally, ocelli that have a yellow centre and a brown ring followed by a white diffuse ring are also found on the head between rhinophores. Lateral ocelli large with a grey centre, thick black ring followed by thin diffuse white ring; 3–7 of these are also found on the anteriormost part of the head. Tips of rhinophores translucent bluish grey, not easily visible upon retraction, followed by white diffusing to the same pale colour as the dorsum. Rhinophores rolled, long, extending laterally from the head, curved like bull horns.

Parapodial margins translucent when opened, with yellowish white spots visible along the edge beneath the tissue surface. Internally parapodial ridges thick, bright green, with no visible spots. Eyes black, very close together, placed centrally on the head, held raised above the rest of the head when crawling. Oral prominences globose with a very fine black line on the edge of the upper lip. Anterior foot corners and tail edged in the same translucent bluish grey as rhinophore tips. Male genital opening located behind the right rhinophore, above the foot corner, in front of the anterior part of the parapodia. Penis translucent white when extended in living specimens. Foot sole white with numerous black spots throughout.

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

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

The genus Plakobranchus has undergone dramatic changes over the past two centuries with more than a dozen species being described in the 1800’s and all being synonymised with the type taxon Plakobranchus ocellatus by numerous authors in later years (e.g., Bergh 1887; Jensen 1992). Plakobranchus ocellatus was described based on blue spots with yellow centres seen dorsally and laterally on a pale ground colour and some information on the pericardial and reproductive anatomy. The species has regularly been recognised/identified by numerous authors based on many of these external characteristics (Rao 1960; Mercier and Hamel 2005; Wägele et al. 2010; Maeda et al. 2012; Sheeja and Padma Kumar 2014; Mehrotra and Scott 2016; Wade and Sherwood 2016). Recent research suggests that the dramatic synonymisation of species under the name P. ocellatus may have been premature, with molecular evidence suggesting at least ten independent clades within the complex of P. ocellatus (Krug et al. 2013, 2016). This supports the findings of previous authors who have observed different morphs of P. ocellatus which appeared to be externally distinguishable based on the general colouration and the distribution of the ocelli or the spots on the dorsal or ventral surface (Ono 2005; Trowbridge et al. 2011; Krug et al. 2013; Yonow and Jensen 2018). While Krug et al. (2013) were able to provide evidence that multiple species historically identified as P. ocellatus are likely different, no images nor detailed morphological descriptions or comparisons were provided. It is assumed, however, that all morphotypes identified therein bear some external resemblance to P. ocellatus sensu stricto, in particular a white or pale ground dorsal colour.

Three specimens 19–30 mm Sai Nuan, 10°4'43.24"N, 99°48'48.51"E, Koh Tao, Thailand.

Figure 5. 

Living specimens of Plakobranchus papua from Koh Tao A close-up of head, 28 mm specimen B dorsal view, 30 mm (photograph by Pau Urgell Plaza) C dorsolateral view of sequenced specimen, 27 mm.

27 individuals, ranging in size 12 mm–35 mm, observed in regular diving surveys between 2012 and 2019, Chalok Bay, 10°3'44.77"N, 99°49'30.35"E, Shark Bay 10°3'39.75"N, 99°50'4.43"E; Tao Tong 10°3'58.13"N, 99°49'4.76"E; Sai Nuan 10°4'43.24"N, 99°48'48.51"E, Koh Tao, Thailand; not collected.

Length alive up to 35 mm. Background colour varies from pale yellow to an almost translucent greyish white, lacking the brownish/ochre background in the original description (Meyers-Muñoz et al. 2016: fig. 3a–c) and other Indonesian material (Yonow and Jensen 2018). Dorsally covered in white dots of varying sizes from the anterior-most part of the head to posterior edge of parapodia. These are also visible dorsally on the blunt anterior foot corners. There are prominent spots on the dorsum where no white dots are present and only the background colour is present. Rhinophores are translucent white with a diffuse band of deep blue to dark purple before the translucent tips. Rhinophores rolled, long, extending laterally from the head, curved like bull horns.

Parapodial margin with distinct white rod-like spots, almost identical to but more continuous than those in the Indonesian specimens (Meyers-Muñoz et al. 2016: fig. 3a; Yonow and Jensen 2018). Internal parapodial ridges bright to dark green, slightly thinner than in P. ocellatus, with no visible spots. Eyes black, very close together, centrally on the head, held slightly raised above the rest of the head when crawling. Tail tip dark blueish purple, almost black, diffusing to white anteriorly. Male genital opening located behind the right rhinophore, above the foot corner, in front of the anterior part of the parapodia. Penis transparent to translucent with a bluish white tip when extended in living specimens, penile bulb and ducts clearly visible inside. Foot sole completely white with no spots, posteriorly tinged in deep purple visible dorsally on the tail tip.

Abundant in shallow soft sediment habitats and among the corals and soft sediments of the reef edge. Uncommon, but present in dense coral reef habitats; rare in deeper soft sediment habitats outside the coral reef. Has been observed being ingested naturally by the scleractinian coral Pleuractis paumotensis but is mostly considered unpalatable by such corals (Mehrotra et al. 2015, 2019).

Known only from the Philippines, Malaysia, Indonesia, and Papua New Guinea (Meyers-Muñoz et al. 2016; Yonow and Jensen 2018). Known from Gulf waters of Thailand (Mehrotra et al. 2015).

This species has previously been referred to, erroneously, as Plakobranchus ianthobaptus Gould, 1852 (Mehrotra and Scott 2016) and Plakobranchus cf. papua (Mehrotra et al. 2019). Externally, specimens of P. papua from Koh Tao differ from the original description of the species. Specifically, the background pigmentation of the parapodia varies from pale yellow to an almost translucent greyish white, lacking the ochre background tinge and the yellow discontinuous line in the border of the parapodia known to date for the species. Furthermore, the specimens of P. papua in the original description show scattered white dots of varying sizes on the surface of the parapodia, whilst in specimens from Koh Tao they almost completely cover the surface. The rhinophores and tail from Koh Tao specimens are deep blue to dark purple at the tips (rarely black), rather than almost entirely covered by black pigment as in the specimens from the original description. A very similar variant to that documented from Koh Tao was documented from Ambon, Indonesia, as Plakobranchus cf. papua by Yonow and Jensen (2018: fig. 6C, D). Minor differences between the Ambon specimens and those from Koh Tao are the paler dorsal colouring and more continuous rod-structures along the parapodial margins in Koh Tao specimens. Additionally, the longitudinal white line visible behind the eyes in Ambon specimens appears to be broken up in specimens from West Papua (Meyers-Muñoz et al. 2016: fig. 3A–D) and scattered in Koh Tao specimens (Fig. 5). Molecular data presented here suggest the present specimens as conspecific with P. papua and the additional material highlights the external variation in the species.

Figure 6. 

Living specimens of Plakobranchus noctisstellatus sp. nov. from Koh Tao. A 21 mm (photograph by Pau Urgell Plaza) B 15 mm C, D (paratype) 31 mm.

Holotype : adult, 28 mm long (alive), collected in silty sand at 21 m depth, Sai Nuan, 10°4'43.24"N, 99°48'48.51"E, Koh Tao, Thailand, 06 April 2016, deposited in MNHN (IM-2000- 35324). Paratype: adult, 31 mm long (alive), collected in silty sand at 18 m depth, Tao Tong, 10°3'58.13"N, 99°49'4.76"E, Koh Tao, Thailand, 18 March 2017, deposited in MNHN (IM-2000- 35325). Paratype dissected: reproductive system studied, and jaw, radula, and penis mounted for optical microscopy. Paratype: adult, 26 mm long (alive), collected in silty sand at 24 m depth, Tao Tong, 10°3'58.13"N, 99°49'4.76"E, Koh Tao, Thailand, 17 February 2020, deposited in RBRG (PkII-NR011).

More than ten individuals, ranging from 10 mm to up to 45 mm, observed in regular diving surveys between 2016 and 2018, Tao Tong 10°3'58.13"N, 99°49'4.76"E; Sai Nuan 10°4'45.02"N, 99°48'45.23"E; Shark Bay 10°3'39.75"N, 99°50'4.43"E; Koh Tao, Thailand, not collected.

Length alive up to 45 mm. Body wide, dorsoventrally flattened with wide parapodial flaps folding along dorsal midline. Background colour bright green to dark green, with scarce black spots, and abundant opaque white spots all over. Some white spots with blue hue, others with yellowish tinge. Five or six prominent black spots similar in size and shape to the eyes found laterally on both sides. Tips of rhinophores and tail are electric blue, followed by a black band, not so evident in the tail. Rhinophores long, rolled, extended from lateral edges of the head, curved like bull horns when crawling. Internal parapodial flaps ridges bright green to dark green, with white and electric blue spots. Eyes black, very close to each other, sometimes with a blue hue between them, located in a groove between rhinophores. Oral prominences globose, with a big black patch on each side, and a very fine, undulating black line on the edge of upper lip. Anterior-upper end of the oral prominences green with white spots and the same electric blue as rhinophore tips. Anterior foot corners in preservation blunt. Foot sole the same green as the dorsal surface with several small iridescent light blue spots. A possible transverse foot groove may be present; however, this was not distinct in living specimens and equally as vague upon preservation. Tail truncated. Dorsal region dark green to black, with big opaque white to electric blue spots (Fig. 6).

Renopericardial prominence composed of two globose, oval lobes (in preservation). Posterior lobe pointed at the end, with pair of major haemolymph sinuses, both perpendicular to its lateral surface and turning at right angles once in the parapodia. Haemolymph sinuses thick, cord-like, white in preservation, longitudinal, parallel to each other (Fig. 7A), not joining together at the ends. Most external haemolymph sinuses shorter. Internal surface of parapodia smooth in preservation, except for the haemolymph sinuses that externally seem to run from the renopericardial prominence. Anal opening at the right anterior side of the pericardium (Fig. 7A).

Figure 7. 

Plakobranchus noctisstellatus sp. nov. A dorsal view of the animal B ventral view of the animal, C detail of the eye D last teeth of the ascending series and active teeth E teeth in the ascus F reproductive system. Abbreviations: am – ampulla; bc – bursa copulatrix; hd – hermaphrodite duct; mg – mucus gland; ot – ovotestis, p – penis; s – stylet; vd – vas deferens.

Uniserial radula (in one 20 mm long specimen) with eight teeth including ghost teeth in ascending row, ten in descending row, 24 in ascus. Teeth triangular, sharp, bearing striations (Fig. 10), and with two cutting edges and 12 or 13 strong and blunt denticles on each of them. First teeth in ascending series 65 μm long, 18 μm high. Active teeth 60 μm long, 18 μm high. Last teeth in descending series 50 μm long, 17 μm high. Teeth in the ascus in varied stages of degradation, 25–66 μm long, packed haphazardly (Fig. 7E).

Penial bulb approximately 1 mm long, below rhinophores, at same level as the eyes, bearing a sharp cuticular stylet. Stylet 280 µm long, hollow, kinked at the base, with the tip like the tip of a hypodermic syringe. Vas deferens connected to penial bulb, curved and strong; medial part thinner and coiled; proximal part straight, over mucus gland, at the end joins a wider conduct. This conduct connected to mucus gland, then coiled and bifurcated to ampulla and to ovotestis. Bursa copulatrix connected to the underside of the central fertilisation area near by the end of the vas deferens (Fig. 7A). Ovotestis grape-shaped, composed of spheres filling the parapodia. Spheres variable in diameter, from 350 to 500 µm. Mucus gland large, globose (Fig. 7F).

All animals were observed year-round at different locations around the island, exclusively in deeper soft sediment habitats at Koh Tao. Animals found either partially buried in or moving across the open silt and sand dominated habitats beyond the fringing coral reefs of the island. Animals found at depths from 15–25 m with no indication of seasonal variation. No observations made shallower than 15 m depth or in the vicinity of coral reef or reef edge habitats. Not observed to be in association with any particular prey algae, nor any other organism in particular, and as such, its prey remains unknown. While multiple individuals have been recorded in close proximity, mating was never observed, nor egg masses identified.

Plakobranchus noctisstellatus from the Latin words noctis (night) and stellatus (stellate), in reference to the small iridescent blue and green spots hidden under the dark parapodia that each resemble stars at night.

Plakobranchus noctisstellatus sp. nov. is known from Thailand and has been recorded under different names in Vanuatu, Indonesia, and Papua New Guinea (Gosliner et al. 2008, 2015).

Plakobranchus papua Meyers-Muñoz and van der Velde, 2016, was described based on morphological and molecular evidence, distinguishing it from P. ocellatus. Specimens of P. papua are characterised by an ochre body with white non-ocellated spots, black rhinophores and tail, and a foot sole without spots. Additionally, the radular teeth of P. papua were described as more ‘arched’ than those shown in descriptions provided of Plakobranchus ocellatus sensu lato. Molecular evidence in this study sheds some light on the external variation of the species (see Discussion). Meyers-Muñoz et al. (2016) also tackled a visual comparison of multiple Plakobranchus varieties that have historically been identified as Plakobranchus ocellatus, including a species almost identical and likely corresponding to Plakobranchus noctisstellatus sp. nov. being referred to as Plakobranchus ocellatus var. I. Meyers-Muñoz et al. (2016, Table 2) also indicate that an illustration of this animal (similar to Plakobranchus noctisstellatus sp. nov.) may have been provided by Gould (1852: pl. 26, fig. 407a–c, as Placobranchus ianthobaptus) but Yonow and Jensen (2018) refuted this and stated that there was no resemblance between the two. We concur, in that Placobranchus ianthobaptus can in fact be distinguished externally from P. noctisstellatus sp. nov. by its pale brown dorsal ground colour.

Plakobranchus noctisstellatus sp. nov. is easily distinguished both externally and internally from both P. papua and P. ocellatus. Externally P. noctisstellatus sp. nov. is most easily separated from its congeners and the variants of P. ocellatus by its vibrant green ground colour and electric blue rhinophores and tail, and notably the dense collections of white and electric blue spots under the parapodial flaps. The presence of non-ocellated black spots, prominent laterally, and iridescent blue spots on the green foot sole also make separation between species easy. The present study suggests that rhinophore colouration in P. papua may vary from black (as described) to deep purple with white tips such as those from Koh Tao; however, this variation remains distinct from the electric blue tips and black band found in P. noctisstellatus sp. nov. While little information on rhinophore colouration was provided for P. ocellatus upon description, the original illustrations of the species by van Hasselt (1824) do show rhinophores that are entirely white or cream, much like specimens of Plakobranchus ocellatus from Koh Tao. The range in rhinophore colouration of different morphs and synonyms of P. ocellatus vary from white to purple tips or bands and black lines along rhinophore edges, all of which differ from P. noctisstellatus sp. nov. While Placobranchus guttatus Stimpson, 1855 was described as bearing a very similar dorsal colour of dark olive, the species was also described as having green ocellated spots with white rings, which are absent from P. noctisstellatus sp. nov.

The reproductive system of P. ocellatus is distinguished from that of P. noctisstellatus sp. nov. by having two bursae copulatrix, a bilobed mucus gland (not rounded), and much smaller ovotestis (follicles). Compared to that of P. papua, the reproductive system of P. noctisstellatus sp. nov. shows a smaller stylet and much larger acorn-shaped penial bulb, lacking the groove observed in P. papua; a curved vas deferens that rapidly decreases in thickness to become convoluted and connects to the ovotestis, the bursa copulatrix and the ampulla, that Meyers-Muñoz et al. (2016) identify as a second bursa copulatrix; the ovotestis, which Meyers-Muñoz et al. (2016) call follicles are much larger in P. noctisstellatus sp. nov.

The number of radular teeth seen in P. noctisstellatus appears similar to those of its congeners with eight in the ascending limb (eight in P. papua and 7–11 in P. ocellatus s. l.) and ten in the descending limb (seven in P. papua and 7–9 in P. ocellatus s. l.). The number of denticles per radular tooth seen in P. noctisstellatus sp. nov. (12 or 13) fits within the range of species described thus far (10–14 in P. papua and 10–13 in P. ocellatus s. l.); however, the overall shape of the teeth appears to show some variation among the species. The teeth of P. noctisstellatus sp. nov. have striations and are more curved than those of P. papua, which in turn was described to have teeth that appeared to be more arched than those of P. ocellatus by Bergh (1873) and Jensen (1997). Additionally, the denticles seen in the SEM image of P. ocellatus by Jensen (1992) are more prominent and proportionally larger than those seen in P. noctisstellatus sp. nov., which seem to be more uniform and regular in appearance. It should be noted that van Hasselt (1824) provided no information on the radula of P. ocellatus in the original description. Additionally, there appears to be significant plasticity in radular morphology based on diet in numerous species of Sacoglossa (Jensen 1993), but conclusive evidence pertaining to the diet of P. noctisstellatus sp. nov. could not be obtained and thus requires further investigation. At Koh Tao, there are significant differences in the ecology of P. noctisstellatus when compared to P. papua and P. ocellatus. While the former exists exclusively in the deeper soft sediment habitats of the island, the latter species are mostly observed among coral reefs and reef flats closer to shore and the soft sediments therein.

Many of these points and observations were also made in the most recent documentation of P. noctisstellatus sp. nov. (as Plakobranchus sp.), where Yonow and Jensen (2018) also provided a tabulation comparing the morphology of all species thus far synonymised with P. ocellatus. Including their sightings, most observations recorded for specimens most likely being P. noctisstellatus sp. nov. are from Indonesia, with Gosliner et al. (2008) also recording the species from Vanuatu; therefore, specimens from the Gulf of Thailand represent the western-most range of the species so far.

Body smooth to papillate, with parapodia that may cover much of the dorsal surface; however, parapodia can be highly variable and may not be held close to the body. Head differentiated from the body with eyes behind dorsal rhinophores. Dorsal sinuses usually branched, anus anterodorsal, reproductive system pseudo-diaulic or triaulic, penis normally unarmed although a hollow apical stylet may be present. Radular teeth blade-shaped ranging from denticulate to smooth.

Holotype : adult, 14 mm long (alive), collected from soft sediment habitats at 12 m depth, Leuk Bay (type locality) 10°4'15.23"N, 99°50'32.86"E, Koh Tao, Thailand, 20 December 2015, deposited in MNHN (IM-2000- 35326). Paratype: adult, 16 mm long (alive), collected from soft sediment habitats at 16 m depth, Tao Tong 10°3'58.13"N, 99°49'4.76"E, Koh Tao, Thailand, 19 June 2017, deposited in MNHN (IM-2000- 35327). Paratype dissected: reproductive system studied, and jaw, radula, and penis mounted for optical microscopy.

Paratype : adult, 14 mm long (alive), collected in silty sand at 25 m depth, Tao Tong, 10°3'58.13"N, 99°49'4.76"E, Koh Tao, Thailand, 17 February 2020, deposited in RBRG (EcjIV-NR012).

More than 100 individuals, ranging in size 5 mm–16 mm, observed in regular diving surveys between 2012 and 2019, Leuk Bay 10°4'15.23"N, 99°50'32.86"E; Suan Olan Artificial Reef 10°4'6.70"N, 99°50'26.29"E; Shark Bay 10°3'39.75"N, 99°50'4.43"E; Tao Tong 10°3'58.13"N, 99°49'4.76"E; Sai Nuan 10°4'43.24"N, 99°48'48.51"E; Twins 10°7'1.93"N, 99°48'44.26"E; Hin Wong Bay 10°6'12.30"N, 99°50'58.63"E; Laem Tien 10°5'19.13"N, 99°51'17.64"E Koh Tao, Thailand; not collected.

Length alive up to 16 mm. Body translucent white, with tips of rhinophores deep blue to purple, fading to the base and lacking tubules of digestive gland. Opaque white specks all over the body, concentrated on the edge of parapodia, renopericardial prominence, and dorsal surface of head and rhinophores. Digestive gland variable from reddish brown to light green, forming a characteristic reticulated pattern of thin tributaries. Eyes black, conspicuous, comma-shaped, behind rhinophores, bearing 75 µm diameter lentil-shaped crystalline lenses. Rhinophores long, pointed at the tip, with groove along entire length. Lateral groove on right side from anterior border of right parapodium to foot. Foot’s transversal groove at same height as end of lateral groove, very subtle, almost invisible, dividing foot in two. Anterior foot corners slightly extended (in preservation), bluntly pointed. Body and parapodia in some specimens with sparse small papillae which disappear upon preservation. Tail pointed, extending beyond parapodia (Fig. 8).

Figure 8. 

Illustrations and images of specimens belong to the Elysia japonica complex A–E illustrations taken from the original descriptions of species belonging to the Elysia japonica complex A, D Elysia abei Baba, 1955 B Elysia amakusana Baba, 1955 C Elysia furvacauda Burn, 1958 E illustration of radula of Elysia japonica Eliot, 1913 by Baba (1949) F–J Elysia aowthai sp. nov. showing variation in colouration, 12 mm (F), 9 mm (G), 11 mm (H, I), 8 mm (J).

Renopericardial prominence long and narrow, oval anteriorly, slightly constricted in the middle, and straight posteriorly. Posterior end with two pairs of major dorsal sinuses: first short, perpendicular, fading at the middle of the parapodium; second long, oblique, curved, orientated towards but not reaching edge of parapodium, ending before the tail. Area between second pair of dorsal sinuses appearing translucent in preserved specimens (Fig. 9B) and probably corresponding to the ‘smooth trench’ mentioned by Eliot (1913: 47) describing E. japonica (KR Jensen, pers. comm.). Renal pore at anterior right side of pericardium. Anus in groove separating the right parapodium from the neck (Fig. 9).

Figure 9. 

Elysia aowthai sp. nov., Koh Tao Thailand A–F 16 mm alive A right side view, the black dot at the end of the parapodia represents the vaginal opening B dorsal view, the shaded area probably being the same as that mentioned by Eliot (1913: 47) C ventral view with a detail of the head D detail of the eyes E buccal bulb F last teeth of the ascending series and active teeth G, H 10 mm alive G ascus H last teeth of the ascending series and active teeth.

Buccal bulb 500 μm long (in three 10, 15, and 16 mm long specimens). Uniserial radula with seven teeth including ghost teeth in ascending row, 8–11 in descending row. Discarded teeth driven out of buccal bulb through a short tube; 12–30 in the ascus, varied in size, some broken, stacked in several groups, some loose. Teeth narrow, blade-shaped. Cutting edges smooth or finely denticulated. Denticulation difficult to observe with the light microscope and random (Fig. 10). First teeth in ascending series 72–76 μm long, 22–25 μm high. Active teeth 71–80 μm long, 27–30 μm high. Last teeth in descending series 50–76 μm long, 20–29 μm high.

Figure 10. 

SEM images of radulae of Elysia aowthai sp. nov. and Plakobranchus noctisstellatus sp. nov. A–C Elysia aowthai sp. nov., Koh Tao, Thailand, 15 mm alive A general view of the radula without the ascus B active teeth from the radula with smooth cutting edge C sixth teeth of the descending series with denticulated cutting edge D–F Plakobranchus noctisstellatus sp. nov. Koh Tao, Thailand, 21 mm alive D general view of the radula without the ascus E lateral view of the descending series F detail of the cutting edge of a tooth.

Penis unarmed, pseudo-conical with a narrow and curved end (chilli-shaped), 500 μm long. Vas deferens long and convoluted, passing through the mucus gland. Male genital opening below right rhinophore, next to right eye. Vagina fusiform, narrow at the extremes, passing through the mucus gland. Bursa copulatrix connected to the vagina. Mucus gland quasi-oval, translucent, 900–1000 μm long, located under the cardiac area. Ducts from the follicles, prostate, and albumen gland coming from the parapodia meet in the mucus gland, but the connections between them are unclear (Fig. 11).

Figure 11. 

Genitalia and reproductive system of Elysia aowthai sp. nov., Koh Tao, Thailand, 15 mm alive. Abbreviations: ag – to the albumen gland; bc – bursa copulatrix; f – to the follicles; mg – mucus gland; p – penis; pr – to the prostate; v – vagina; vd – vas deferens; vo – vaginal opening.

Biology. All animals were observed year-round at different locations around the island, exclusively in deeper soft sediment habitats at Koh Tao. Animals were always found moving along the surface of the sediments, rarely stationary and with no specific ecological relationship with any other species. Not observed to be in association with any particular prey algae, but often found in the vicinity of a currently unknown filamentous red algae, which may contribute to the reddish colouration seen in most specimens. Specimens with green, brown, purple, and unpigmented digestive glands are likely due to some degree of variability in prey; however, further investigations are required. No observations were made shallower than 10 m depth or in the vicinity of coral reef or reef edge habitats, deeper observations up to 24 m.

Elysia aowthai in reference to the type locality of the species in the Gulf of Thailand, ‘Aow Thai’ in the local vernacular language, and to honour the local populations that speak it.

Gulf of Thailand (present study), Guam (as E. cf. japonica, Bass 2006), and Australia (as E. amakusana, Wägele et al. 2010) based on our molecular analyses.

The general appearance of Elysia aowthai sp. nov. places it within the E. japonica species complex. The problem with this complex has its origins in the incomplete description of E. japonica itself, but also in the provenance of the type material and the subsequent specimens captured, sometimes associated with descriptions and illustrations and sometimes not. Elysia japonica was described based on preserved specimens, which makes the external anatomy and living colouration very difficult to untangle, collected from an unknown location in Japan. The complete description by Eliot (1913) is as follows:

“18 specimens. Locality – unknown. The largest is about 20 mm long and the wings are moderately ample. In two specimens which were dissected, the radula was found to contain 5 teeth in the ascending row, 15 in the descending and about 20 more various sizes lying in a heap. The structure and the shape of the teeth is as usual in the genus. No denticles are to be seen. I think that this form is probably a new species distinguished by the following characters:

1) Colour: In all specimens, the rhinophores and the tip of the tail are conspicuously black or dark brown. Otherwise the colour is uniform and in the best preserved specimens is yellowish brown. The wings have no coloured borders and the head and the pericardium are of the same colour as the dorsal surface.

2) The arrangement of the dorsal surface. This is similar in all specimens and I have not seen it in any other species. The pericardium is not ovate but is constricted in the middle. Its length is greater than its breadth but it is short in comparison with the length of the whole animal. The dorsal ridges which run into it are very distinct and the two hindmost, which run backwards towards the tail, are parallel to one another and enclose an area which is differentiated from the back and forms a smooth trench.”

The first illustration showing the external anatomy of a specimen assumed to be Elysia japonica was provided by Baba (1949), with a description of the species. However, this was later changed in a supplement to the previous paper wherein the same specimen (Baba 1949: plate IX, fig. 27) was reidentified and claimed as a new species (Baba 1955), Elysia amakusana Baba, 1955, based exclusively on the presence of finely denticulated teeth, which are (theoretically) not present in E. japonica. Also, in the same supplement, Baba described Elysia abei Baba, 1955, which also has finely denticulated teeth but is differentiated by being green in colour with fine orange-red spots (Baba 1949: plate VIII, fig. 26; Baba 1955). Afterwards, however, Baba determined all three species as valid in a summarised inventory of Elysia species from Japan (Baba 1957). A year later, Elysia furvacauda Burn, 1958 was described from Australia (Burn 1958), based entirely on the external anatomy (therefore lacking any mention of radular detail), as a red-brown animal with small blue spots and rhinophores, and tail tipped in black. The type material for this species was reported as lost by Jensen (1985), but it is actually deposited in Museums Victoria Collections (MVC 2020, registration number F19467) (KR Jensen, pers. comm.).

Subsequently, Marcus (1980) and later Jensen (1985) both pointed out the similarity in radular teeth between E. amakusana and E. abei, with Jensen providing a description of three different colour morphs of E. japonica from Hong Kong, a comprehensive analysis of the taxonomic confusion to date, and concluding that E. amakusana and E. abei are junior synonyms of E. japonica. Importantly, Jensen showed that the morphology of specimens of three different external colourations was remarkably similar and that radular teeth have ‘blunt tips and are finely serrulate’. Between 1999 and 2002, discussions on the Sea Slug Forum (hosted by the Australian Museum) between Rudman, Jensen, and others (Rudman 2001) generally supported the view that some or all of E. abei, E. amakusana, and E. japonica were likely synonyms, although Rudman suggested that the name Elysia japonica may need to be abandoned due to the lack of type material or information on the type locality. Burn (2006) restated that E. furvacauda was distinct from E. japonica which was supported by an external illustration of the former (Fig. 8), although no internal anatomy was discussed. Trowbridge et al. (2010, 2011) also discussed in detail the complexity and need for clarification between these species.

The first molecular evidence on any species in the complex was provided by Händeler and Wägele (2007), who sequenced a portion of the mitochondrial 16s rRNA for a species they called Elysia amakusana (from Australia) as part of a larger analysis. While this analysis provided no illustration, description, or discussion of the species in particular, an illustration of a specimen identified as E. amakusana from Lizard island (Australia) was published a year before by the second author (Wägele et al. 2006: 41, fig. 8H), with this specimen looking similar to Elysia aowthai sp. nov. A more thorough molecular assessment regarding the presently discussed species was later provided by Takano et al. (2013), where a comparison between specimens they considered representative of E. abei, E. amakusana, and E. cf. japonica was carried out. These authors provided colour photographs of E. abei and E. amakusana from Japan (Takano et al. 2013: fig. 3A, B) matching Baba´s (1949: plate VIII, fig. 26; plate IX, fig. 27) illustrations. Takano et al. (2013: fig. 3C) also provided an illustration of a specimen determined as E. cf. japonica from Japan that matches the general appearance of a species in the complex of E. japonica, and shows a character that could have gone unnoticed in the original description of the species due to the state of preservation of the specimens: big rounded blotches of orange pigment (different from the orange dots of E. abei). In their analysis, these authors concluded that E. abei and E. amakusana were likely synonyms despite their morphological variability, but E. cf. japonica appeared distinct. Takano´s et al. (2013) species identifications were based on external colouration and morphology alone. However, if the internal anatomy of their E. cf. japonica would match that of the original description, E. japonica could be re-described, which would help to clarify the taxonomic/systematic problems around the species complex in the area. Nonetheless, the fact is that there are at least two lineages in Japan corresponding to the external morphology of E. abei/E. amakusana and (potentially) E. japonica. An extensive phylogeny by Krug et al. (2015) showed specimens determined as E. abei (Japan), E. amakusana (Australia), and E. furvacauda (Australia) as distinct species, clustered with species they call Elysia sp. 5 (Hawaii) and Elysia sp. 30 (Japan), but again the external anatomy of the species is not shown nor discussed, which makes it impossible to draw conclusions on the validity of the different taxa.

After this review it seems clear that, within the complex, several groups may be present based on the external anatomy: one composed of specimens similar to E. abei /E. amakusana with a wide range of morphological variability, another with unclear characteristics belonging to E. japonica sensu stricto, and a third including E. furvacauda. However, the reliability of species delimitation using external colouration in this group is questionable when used alone.

On the other hand, the integrated analysis of the COI of numerous species/specimens within the complex conducted here reveals a clear geographical structure. There are at least two separate lineages in Japan (Fig. 2, clade D) corresponding to the true E. abei/E. amakusana group and (potentially) to the true E. japonica sensu stricto (whose type appears to be lost: Trowbridge et al. 2011). The remainder of the groups within the complex (Fig. 2, clades A–C) include E. furvacauda from Australia and at least two undescribed species within the range of morphological variability observed for E. abei/E. amakusana scattered in the Indo-West Pacific (I-WP).

In this work, to contribute to the untangling of the E. japonica complex, we describe one of these apparently widely distributed species scattered in the I-WP, E. aowthai sp. nov., by providing detailed morphological (external and internal) and molecular evidence. Future findings including precise sampling coordinates, complete diagnoses, colour images, and molecular data will resolve the identities of the members of this species complex and its distribution.

Despite the fact that the radular teeth of E. aowthai sp. nov. are indeed finely serrated, two points must be noted: these appear to be rapidly worn away with use leaving smoother edges in older teeth, and this feature is not visible under light microscopy alone and required the use of SEM observations to confirm their presence. Hence, the use of this feature alone does not have enough diagnostic significance in distinguishing the species of the complex.

Elysia aowthai sp. nov. is not conspecific to the specimens studied by Jensen (1985) from Hong Kong and determined as E. japonica: there are some differences that should be noted such as the morphology of the pericardium which is shorter, wider, and bearing four anastomosed dorsal sinuses in Jensen´s specimens, and the shape of the teeth, which are blade-shaped in E. aowthai sp. nov. instead of having the rounded blunt tips observed in Jensen´s specimens (Jensen 1985: fig. 2C) which appear significantly more rounded than those of E. aowthai sp. nov. The reproductive apparatus of Jensen´s (1985) specimens were described as ‘very similar’ to that of Elysia verrucosa Jensen 1985, but this statement is insufficient to establish any comparison.