Risbecia tryoni
(Garrett, 1873)

Order: NUDIBRANCHIA
Suborder: DORIDINA
Family: Chromodorididae

DISTRIBUTION

Tropical Western Pacific.

PHOTO

UPPER: Coffs Harbour region, northern New South Wales, Australia, December 1990. PHOTO: Bill Rudman.
MIDDLE: Gorgonia Hole, Heron Is., Qld. 15m, 20 Dec. 1980, 61mm long alive. PHOTO: G.Avern.
LOWER: juvenile, Solitary Is., northern New South Wales, October 1990. PHOTO: Bill Rudman.

This is one of a group of similarly coloured species which I have called the "Risbecia tryoni colour group". Other species include Chromodoris geminus, 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.

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) Risbecia tryoni (Garrett, 1873). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/risbtryo

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