Chromodoris alternata
(Burn, l957)

Order: NUDIBRANCHIA
Suborder: DORIDINA
Family: Chromodorididae

DISTRIBUTION

This species is known only from southern Australia. It is found from the intertidal to 13 metres.

PHOTO

Rocky Cape, NW Tasmania, 6m, October 1984, 13mm long.
PHOTOS: Bill Rudman.

Chromodoris alternata is easily distinguishable externally by the orange-brown patch on the mantle and the white patched translucent purple mantle edge.

Similarly coloured species
Digidentis perplexa and Chromodoris ambiguus both have a similar distribution and a similar colour pattern to C. alternata but D. perplexa can be separated by the opaque white background colour, opaque orange spots and submarginal, sometimes broken, purple band. In both C. alternata and C. ambiguus the colouration is more translucent and on first sight the two species appear to be colour forms of one species. The major difference is that in C. alternata the central region of the mantle has a large orange to orange-brown patch while in C. ambiguus there are a series of orange spots. Around the mantle edge in C. alternata there is a purple tinge over all the epithelium except where there are opaque white epithelial patches. In C. ambiguus the white markings at the mantle edge are the subepithelial mantle glands showing through and the alternating purple spots are epithelial pigmentation.

There are many red and orange-spotted species of chromodorid in New South Wales and southeastern Australia. I have discussed this example of mimicry on a separate page.

Reference:
• 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 (June 1) Chromodoris alternata (Burn, l957). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/chroalte

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