Shimoda, Izu Peninsula, Sagami Bay, Japan, 0.3m (Intertidal). Size: about 12mm, 13 Jan 2002. Photos: Jun Imamoto
Until quite recently it was considered there was a single species of Rostanga in Japan which ranged in colour from orange red to black. Following Rudman & Avern's (1989) review of the genus, Baba (1991) showed that there are at least two species in Japan, and he named the black form as a distinct species, R. risbeci, after Jean Risbec who described a black Rostanga from New Caledonia many years ago.
The main external features of Rostanga risbeci are that it is black, or grayish black with scattered opaque white specks or patches. The rhinophores are white-tipped and the gills are speckled with white. All but the innermost radular teeth are long with a bifid tip, very similar to R. dentacus Rudman & Avern, 1989, which differs in being orange red in colour. R. risbeci is reported by Dr Baba to feed on the black sponge Halichondria okadai (Kadota, 1922).
• Rudman, W.B., Avern, G. (1989) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. Zoological Journal of the Linnean Society, 96(3): 281-338.
• Baba, K. (1991) Review of the genus Rostanga of Japan with the description of a new species (Nudibranchia: Dorididae). Venus, The Japanese Journal of Malacology 50(1): 43-54. (Figs 1-5, Pl.1)
Rudman, W.B., 2002 (January 16) Rostanga risbeci Baba, 1991. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/rostrisb