Opisthobranch of the Week Data
The following information concerning the genus Rostanga is taken from Rudman & Avern (1989):
The genus Rostanga consists of a group of relatively small sponge-feeding dorid nudibranchs, usually coloured orange-red. The mantle is characterized by a dense covering of caryophyllidia which are small tubercles with a subterminal ring of protruding spicules.
In those species for which biological information is available it is clear that shape of egg-ribbon, larval development, and specific food sponge are also important characters for distinguishing species.
As of the current date (mid-January, 2006) I've collected a series of not-yet identified Rostanga and I'll add some of these in future features here on the site (see Rostanga sp. 2). Rostanga specimens are not commonly seen here in the waters of Okinawa's main island. This perhaps is explained in part by their cryptic coloration which is very similar to that of their sponge food, as well as their not usually being found off of their sponge host, or not very far from it.
Rostanga sp. 1 was photographed by Atsushi Ono during May of 2005 in the waters off Gahi Island, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa capitol, Naha. The animal was found on the underside of a rock. The above image is used with Atsushi's kind permission and he reports (pers. comm.) seeing only the above single individual, and of course considers the species to be very rare in the Keramas.
Of the fifteen described species of Rostanga currently on Bill Rudman's Sea Slug Forum, as well as a series of unidentified Rostanga species, only three are found with a white or partially white color (R. australis, R. lutescens, and R. phepha). The majority of the described species of Rostanga apparently feed on orange or reddish-orange sponges from which the dorids retain the color of the host. The majority of described species are orange or a reddish orange color. Rostanga risbeci described by Baba (1991) from Japan however is black and is known to feed on a black sponge (Baba, 1991).