Juvenile of Ceratosoma bicolor from Lembeh?

October 27, 2009
From: Zeineb Alhaidari

Concerning message #5061:

Dear Bill,
I am unable to identify this little Ceratosoma observed during a dive on Critter Hunt , Lembeh , Sulawesi . Due to its small size , I guess it could be a juvenile , and may be due to the bright orange rhinophores and dorsal horn , could it be Ceratosoma bicolor?

Locality: Dive Site : Critter Hunt , Lembeh Strait, 16 metres, North Sulawesi , Indonesia, Molucca sea, 8 october 2009, Sandy bottom with scattered reefs. Length: 1 cm. Photographer: Zeineb Alhaidari.

Many thanks for your help,
Zeineb

z.alhaidari@wanadoo.fr

Alhaidari , Z., 2009 (Oct 27) Juvenile of Ceratosoma bicolor from Lembeh?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/22720

Dear Zeineb,

Juvenile specimens of Ceratosoma never fail to amaze me. Why should such a defenceless juvenile want to flaunt itself with such bright colours - even to having an uproght dorsal horn of a different colour almost advertising to fish that here is a tasty item to eat. But if you look carefully you can see the small whitish spheres packed in a row along the edge of the dorsal horn. These spheres are packed full of distasteful poisons the nudibranch has accumulated from its sponge food and it seems there sole purpose is to defend the nudibranch from attack. So what at first looks like an accident waiting to happen is a cunning lure enticing potential prey to take a bite and learn a salutary lesson.

Now to the question of what species it is. As I have said elsewhere, I think we have to learn a bit more about C. bicolor before we can be sure it is a good species and not just a colour form of C. trilobatum. My feeling is that most species of Ceratosoma cannot be distinguished by their colour pattern, most species having a similar range of colour variations. It seems to me that species of Ceratosoma are best distinguished by the shape of their bodies. For example your animal has 1 dorsal lobe and 1 lateral mantle lobe on each side [= 3 lobes] which makes it C. trilobatum [tri = 3; lobatum = lobes].

Concerning its food sponge. I suspect it may be the blue-grey material on the left of your photo. If by chnace you have a photo which shows the sponge more clearly I would love to see it. I suspect at this magnification that if the sponge is in focus the nudibranch wont be.

Best wishes,
Bill Rudman

Rudman, W.B., 2009 (Oct 27). Comment on Juvenile of Ceratosoma bicolor from Lembeh? by Zeineb Alhaidari. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/22720

Factsheet

Ceratosoma trilobatum

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