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

Mehrotra, Rahul, Gutierrez, Manuel Caballer, Scott, Chad M., Arnold, Spencer, Monchanin, Coline & Chavanich, Suchana, 2020, On the Plakobranchidae (Gastropoda, Sacoglossa) from soft sediment habitats of Koh Tao, Gulf of Thailand, with descriptions of two new species, ZooKeys 969, pp. 85-121 : 85

Plakobranchus noctisstellatus sp. nov. Figures 6 , 7 , 10D-F

Plakobranchus sp.: Coleman 2008: 89 (Thailand; Gorontalo, Indonesia)

Plakobranchus sp. 2: Gosliner et al. 2008: 94 (Vanuatu; Bali, Indonesia); Gosliner et al. 2015: 98 (Papua New Guinea).

Plakobranchus ocellatus var. I: Meyers-Muñoz et al. 2016: 91, Table 2

Plakobranchus cf. ocellatus : Yonow and Jensen 2018: 30 (top of 2nd column), fig. 6E (Bali, Indonesia)

Type material.

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).

Supplementary observations.

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.

Description.

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 ).

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 ).

Biology.

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.

Derivatio nominis.

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.

Distribution.

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).

Remarks.

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.

Figures

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.

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.

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.