Plakobranchus ocellatus van Hasselt, 1924

Yonow, Nathalie, 2012, Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda), ZooKeys 197, pp. 1-130 : 15-16

Plakobranchus ocellatus van Hasselt, 1924 Plate 17

Plakobranchus ocellatus . - Yonow 1990: 288, pl. 2 (Red Sea); Apte 2009: 169, fig. 2a (Laccadive Islands); Richmond 2011: 276 (East Africa).

Placobranchus ocellatus . - Rao 1962: 1, figs. 1, 2e, f (India); Jensen 1992: 283, figs. 23, 24d, e (Red Sea and SW Thailand + Guam, Hawaii).

Material.

Kenya: 25 × 11 mm preserved, Vipingo, 25 miles N of Mombasa, rock pool ELW, 23 September 1984, leg. J Hognerud. - Maldives: 9 mm in length (MDV/AB/96/7, specimen disintegrated, no radula located), Fulidhoo Lagoon, Felidhoo Atoll, 10 m depth on sand, 04 May 1996, leg. RC Anderson & SG Buttress. - Zanzibar: 14 × 8 mm preserved, Matemwe Lagoon, in Xenia sp. and sand near encrusted, partly submerged rock with Didemnum molle and Halimeda sp., 01 March 1995, leg. MD Richmond. - Seychelles: 30 × 10 mm alive (PK-A, one of four individuals preserved), Source d’Argent, La Digue, 1 m depth on broken Acropora sp. behind reef in algal growth, 26 January 1992, leg. P Kemp. - La Réunion and Mayotte: photographs of several individuals http://seaslugs.free.fr/nudibranche/a_intro.htm.

Description/Distribution.

Plakobranchus ocellatus has been recorded in excess of 35 mm in the Indian Ocean ( Rao 1962). The western Indian Ocean specimens e xamined here conform to the description of Red Sea specimens ( Yonow 1990, 2008). Jensen (1992) listed numerous synonyms from the Pacific Ocean, stating that colour pattern and distribution of ocelli are variable in just a single species; however, in 2006 she suggested, "It is possible that a complex of sibling species is involved."

Remarks.

In the Marshall Islands, an interesting commensal association was observed between Plakobranchus ocellatus and the sea cucumber Holothuria atra ( Mercier and Hamel 2005); this is the first report of an opisthobranch occurring on an echinoderm species. The authors provide evidence of a real association between the two animals through a set of well-designed experiments and natural observations. In the Indian Ocean Holothuria atra is also a common species, with a similar habitat to Plakobranchus ocellatus , but there are no observations of a similar association.