Chromodoris geminus
Rudman, 1987

Order: NUDIBRANCHIA
Suborder: DORIDINA
Family: Chromodorididae

DISTRIBUTION

Tropical Indian Ocean.

PHOTO

UPPER: Showing characteristic mantle wave where all the mantle edge is raised and lowered together, as in C. kuniei. Fungu Yasin, Dar es Salaam, Tanzania, November 1976, 35mm long alive. PHOTO: Bill Rudman.
LOWER: Phi Phi Is., Southern Thailand, 16m, December 1989. PHOTO: D.J.Brunckhorst.

This is one of a group of similarly coloured species which I have called the "Risbecia tryoni colour group". Other species include Risbecia tryoni, Chromodoris kuniei and Chromodoris leopardus. They all have large purple or purple-brown spots or marks, usually ringed with white, an often reticulate brownish background, and a purple border. Chromodoris geminus differs in having four colour bands around the mantle edge, an outermost white, then translucent greyish purple, then white, then yellow. The other three have a distinct purple line at the border, narrow in C. leopardus and R. tryoni, but broad in C. kuniei. The shape of the body of R. tryoni, high with a reduced mantle overlap, is typical of the genus Risbecia. The three species of Chromodoris have a wide mantle overlap. Of the three with the purple mantle edge, C. leopardus and C. kuniei can be distinguished from R. tryoni by the broad purple border, consisting of three slightly different coloured bands in C. kuniei, and by the leopard-like multiple spots, or marks, forming a hollow square, found in C. leopardus.

See also the recently described Chromodoris tritos, which may be a colour form of C. geminus and Chromodoris cf. geminus which has recently been reported from South Africa and has a very similar colour pattern.

References:
• 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. (1987) The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris epicuria, C. aureopurpurea, C. annulata, C. coi and Risbecia tryoni colour groups. Zoological Journal of the Linnean Society, 90: 305-407.

Authorship details
Rudman, W.B., 1999 (January 22) Chromodoris geminus Rudman, 1987. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chrogemi

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