Yaene, Hachijo Island, Japan. Depth: 9m. 1 July 2001. Length: 50mm. Photo: Nishina Masayoshi.
Background colour ranges from white to violet-purple, with an irregular median orange band running from the gills to the from tof the head. The gills are orange. There is a reticulate pattern of orange on the posterior part of the foot behind the gills. In lighter coloured animals there is a distinct purplish submarginal band around the mantle edge, with a thin band of white at the extreme edge. In animals in which the whole body is purple, the submarginal purple border is obscured and only the thin white edge is noticeable. Baba (1949) described this species from a lighter coloured animal, and later (1989) reported on an animal which was a darker purple colour. In a revision of the genus (Rudman, 1988), I considered this to be just a colour form of Ceratosoma trilobatum as I could find no morphological or anatomical differences. There still seem to be no differences other than the distinctive colour pattern. As Baba (1989) comments, more specimens need to be studied before we can decide whether it should be considered a distinct species.
• Baba, K. (1949) Opisthobranchia of Sagami Bay collected by His Majesty The Emperor of Japan. Iwanami Shoyen, Tokyo. 194pp., 50 Pls pages.
• Baba, K. (1989) Review of the genus Ceratosoma from Japan (Nudibranchia: Chromodorididae). Venus, The Japanese Journal of Malacology, 48(3): 141-149.
• Rudman, W.B. (1984) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society, 81: 115-273.
• Rudman, W.B. (1988) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: the genus Ceratosoma J.E. Gray. Zoological Journal of the Linnean Society, 93(2): 133-185
Rudman, W.B., 2001 (August 20) Ceratosoma bicolor Baba, 1949. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/cerabico