Plakobranchus papua Meyers-Munoz & van der Velde, 2016

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 papua Meyers-Munoz & van der Velde, 2016 Figure 5 , Suppl. material 3

Plakobranchus ocellatus : Coleman 2008: 88 (Milne Bay, Papua New Guinea; Uepi Island, Solomon Islands)

Plakobranchus sp. 1: Gosliner et al. 2008: 94 (Philippines; Indonesia; Papua New Guinea)

Plakobranchus papua Meyers-Muñoz & van der Velde in Meyers-Muñoz et al. 2016: 80-88, figs 2-7a (West Papua, Indonesia).

Plakobranchus papua : Gosliner et al. 2018: 434 (Indonesia)

Plakobranchus cf. papua : Yonow and Jensen 2018: 21, 27-30, figs 6C, D, 12, 13C, D, 14C, D (Ambon, Indonesia).

Material examined.

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

Supplementary observations.

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.

Description.

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.

Ecology.

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).

Distribution.

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).

Remarks.

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.

Figures

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.