Plakobranchus ocellatus van Hasselt, 1824

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 ocellatus van Hasselt, 1824 Figure 4 , Suppl. material 2

Plakobranchus ocellatus : Christa et al. 2013: 560, fig. 1A, D (Luminau, Guam; Australia)

Plakobranchus ocellatus (white): Krug et al. 2013: (Andaman Sea, Thailand; Japan; Australia; Guam)

Plakobranchus ocellatus (white): Takano et al. 2013: fig. 3K (Japan)

? Plakobranchus ocellatus s. s.: Meyers-Muñoz et al. 2016: 91, Table 2

Plakobranchus ocellatus : Tanamura and Hirose 2016: 5, fig. 3A (Ryuku Archipelago, Japan)

Plakobranchus sp. 6: Gosliner et al. 2018: 434 (Philippines)

Plakobranchus ocellatus : Yonow and Jensen 2018: 20, fig. 5I (Bohol, Philippines)

Material examined.

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

Supplementary observations.

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.

Description.

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.

Ecology.

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.

Distribution.

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).

Remarks.

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.

Figures

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.