Ceratosoma trilobatum
(J. E. Gray, 1827)

Order: NUDIBRANCHIA
Suborder: DORIDINA
Superfamily: EUDORIDOIDEA
Family: Chromodorididae

DISTRIBUTION

Tropical Indo-West Pacific.

PHOTO

A, Île aux Canards, Nouméa, New Caledonia, 19m, 20 October 1988. B, Chenal de la Passe de Koumac, New Caledonia, 66m, in lagoon, mud, shell-sand, October 1993, 47mm long alive.
C, Pointe de Pandop, Koumac, New Caledonia, 2 m, hard substrate, 18 October 1993, 120mm long alive.
PHOTOS: Bill Rudman.

Widespread Indo-West Pacific species often confused with Ceratosoma tenue. It is characterised by having only one large lateral lobe on each side of the body, just anterior to the gills. There is usually a ridge joining each lateral lobe to the anterior head lobe. The background colour can vary from pale green to orange-red or brown, usually, but not always, mottled and spotted with lighter colours. There are usually small yellow or orange spots and the edge of the mantle, and sometimes the foot, is bordered with a broken or continuous purple line.
See separate page for colour variants in northwestern Australia.

Most species of Ceratosoma have a long recurved dorsal 'horn' which acts as a defensive lure attracting potential predators to the part of the animal which contains most of the distasteful chemicals stored from their food. See Mark Levin's photo. See also Ceratosoma brevicaudatum for photos of that species' conspicuously marked 'tail', especially in juveniles. This colour 'target' probably also attracts potential predators to the most distasteful part of the animal (Rudman, 1991). See photo of Ceratosoma tenue from Dar es Salaam, in which whole dorsal horn has been cut off, probably bitten off.

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. (1988b) 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. (1991) Purpose in Pattern: The evolution of colour in chromodorid nudibranchs. Journal of Molluscan Studies, 57 (T.E.Thompson Memorial Issue): 5-21.

Authorship details
Rudman, W.B., 1999 (September 29) Ceratosoma trilobatum (J. E. Gray, 1827). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/ceratril

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