Onchidoris depressa
(Alder & Hancock, 1842)

Suborder: DORIDINA
Family: Onchidorididae


This species is known from the British Isles and the Atlantic coast of Europe. It also recorded here from southeastern Australia (New South Wales, Tasmania) and from New Caledonia. This remarkably disjunct distribution, and presence in tropical waters would suggest that it has been artificially introduced to the southern hemisphere.


Upper: North point of Île Nou, Nouméa, New Caledonia. 11 October 1988, intertidal, on bryozoan with many egg masses, AM C156621.
Lower: near Hobart, southern Tasmania, Australia, 14 Feb 1984.
Photos: Bill Rudman

Onchidoris depressa is a flattened, ovate animal, which is probably often overlooked because of its excellent camouflage on the encrusting bryozoan colonies on which it lives and feeds. The mantle is translucent clear with scattered red-brown spots, especially in the mid-region, and covered with tapering translucent soft papillae. It can gave a glistening appearance form the crowded glassy spicules which are usually visble through the mantle skin This species seldom reaches more than 9mm in length. The rhinophores are sparsely lamellate and there can be up to 12 simple gills arranged in a circle around the anus. In the British Isles it is reported to feed on the bryozoan Schizomavella linearis. Alder & Hancock report the spawn to be 'pinkish'

• Alder, J. & Hancock, A. (1842) Descriptions of several new nudibranchous Mollusca found on the coast of Northumberland. Annals and Magazine of Natural History, 9: 31-36.
• Alder, J. & Hancock, A. (1851) Part 5. In: A monograph of the British Nudibranchiate Mollusca: with figures of all the species. The Ray Society, London.
• Just, H. & Edmunds, M. (1985) North Atlantic Nudibranchs (Mollusca) seen by Henning Lemche. With additional species from the Mediterranean and north east Pacific. Ophelia, Supplement 2: 1-149.
• Thompson, T.E. & Brown, G.H. (1984) Biology of Opisthobranch Molluscs, Vol 2. Ray Society: London

Authorship details
Rudman, W.B., 2003 (October 13) Onchidoris depressa (Alder & Hancock, 1842). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/onchdepr

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