Known from Hong Kong
Cape d'Aguilar, Hong Kong, South China Sea. Depth: less than 1 m during low tide. 12 April 1995. Very exposed rocky shore. Photographer: Kathe Jensen. The lack of black pigmentation on the dorsum of this specimen and the mottled black on the cerata suggests this is an intermediate colour form between intertidal and subtidal specimens.
This species was described with 2 distinct colour forms. Subtidal animals are almost completely black with bright orange tips to the cerata. The tips and ventral parts of the rhinophores are colourless, and a colourless band extends down each side of the head. There is a black medio-dorsal band running back between the cerata [which is absent in accompanying photos]. Intertidal specimens differ considerably, especially in the absence of black and orange pigmentation on the cerata, which appear green - from digestive gland contents - wit a white tip. The pigmentation on the rest of the body is similar to subtidal specimens but the intensity of the black pigmentation is lower. The photographed animal has an intermediate colour pattern. It grows to approximately 20 mm in length. Subtidal animals found feeding on a coarse dark green filamentous green algae, Cladophora sp.; and intertidal animals fed on a coarse species of Chaetomorpha.
Jensen, K.R. (1985) Annotated checklist of Hong Kong Ascoglossa (Mollusca: Opisthobranchia), with descriptions of four new species. In: Proceedings of the 2nd International Workshop on the Malacofauna of Hong Kong and Southern China, Hong Kong, 1983. (Eds: Morton, B; Dudgeon, D) Hong Kong University Press, Hong Kong, 77-107.
Rudman, W.B., 2005 (February 28) Ercolania emarginata Jensen, 1985. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/ercoemar