Rostanga risbeci Baba, 1991

Description

A medium-sized dorid, body length around 30 mm. The overall dorsum is purplish black to black, with scattered opaque white spots. The rhinophores are white on the tip, black on the club and colourless on the stalk. The gills are black with white spots on the outside; the foot sole is uniformly black. The dorsum is densely covered with the spiculate caryophyllidia characteristic of Rostanga, giving a finely velvety texture. About seven bipinnate branchial plumes form a circle around the anus. The jaw plates are reduced and rudimentary.

Distribution

Type locality: Aitsu, Amakusa, Kumamoto Prefecture, Japan. Also recorded from Miyata (Amakusa), Nuka (Echizen coast, Fukui), and Hayama (Sagami Bay), always on a black sponge.

Etymology

The specific epithet honours the French malacologist Jean Risbec, who worked extensively on a black Rostanga from New Caledonia.

Remarks

Japanese black Rostanga had earlier been treated as a colour variant of the Australian Rostanga arbutus; a later revision restricted R. arbutus to New South Wales and the Japanese orange specimens were reassigned to R. orientalis, leaving the black form to be separated as this distinct species. The diet is the black sponge Halichondria okadai, and the matching black coloration is presumed to be cryptic. The spawn is white — in contrast to the orange spawn of R. orientalis. Sometimes sympatric with R. orientalis, but rarer.

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References

• Rostanga risbeci n. sp., 馬場菊太郎. (1991). 日本産イソウミウシ属の再検討及び1新種の記載 (Review of the Genus Rostanga of Japan with the Description of a New Species, Nudibranchia: Dorididae). Venus (Jap. Jour. Malac.). 50(1): 43-54.
• 高岡生物研究会. (2002). 日本海のウミウシ. 第2版.
• クロイソウミウシ, 小野篤司. (2004). 沖縄のウミウシ. ラトルズ.
• クロイソウミウシ, 中野理枝. (2004). 本州のウミウシ. ラトルズ.