Ceratosoma trilobatum from Mussandam, Oman

July 23, 2008
From: Yahia Mokhtar

Concerning message #21625:

I think this is another "Risbecia pulchella " despite the weird "H" shape.

Locality: Ras Dillah, 15-20 m, Mussandam, Oman, Arabian Gulf / Indian Ocean, 13 June 2008, Rocks. Length: 5-6 cm. Photographer: Yahia Mokhtar.

Yahia

yahia@adma.ae

Mokhtar, Y., 2008 (Jul 23) Ceratosoma trilobatum from Mussandam, Oman. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21726

Dear Yahia,

The yellow spots certainly suggest R. pulchella and its yellow-spotted 'friends', but its shape means it must be a species of Ceratosoma. Unlike most other chromodorids, colour is not a good character for identifying species of Ceratosoma, but fortunately, their external shape is consistent. This is Ceratosoma trilobatum.

Of great interest is that your photos show this animal feeding on the bluish sponge colony. I have included a close-up alongside your middle photo which shows the oral tube extended out into the sponge. It's not perfectly in focus but it is clear enough to confirm what is happening. The sponge colony is partly overgrown by algae and the bits that we can see looks as though they are the remains of a large colony which has been almost completely eaten by the Ceratosoma. Although I can't see detail in the sponge structure, I think I have seen enough 'remains' like this to be confident that this is a dysideid sponge, perhaps a species of Euryspongia. This is the first direct evidence we have of the food of this species. At present all we have is indirect evidence from chemists [Mollo et al, 2005] who have found molecules characteristic of dysideid sponges in the mantle glands of C. trilobatum.

  • Mollo, E., Gavagnin, M., Carbone, M., Guo, Y.-W., and Cimino, G. (2005) Chemical studies on Indopacific Ceratosoma nudibranchs illuminate the protective role of the dorsal horn. Chemoecology 15: 31-36.

Best wishes,
Bill Rudman

Rudman, W.B., 2008 (Jul 23). Comment on Ceratosoma trilobatum from Mussandam, Oman by Yahia Mokhtar. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21726

Factsheet

Ceratosoma trilobatum

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