Phyllodesmium briareum from the Philippines

August 14, 2008
From: Marcel Tanke

Dear Bill,
In Cebu-Moalboal we found in 2 separate locations, a rather large aggregation of Phyllodesmium briareum. In both cases they were on top of a thin layer of sponge (?) that covered the reef. There were maybe several hundreds of them. The first picture attached alone shows some 20 P. briareum already.

We watched- and took pictures for several minutes. We did not see one that moved, not even a little bit. I tried to make them move by waving my hand and move some water over them. They were stuck very strongly to the sponge (?), no movement at all.  We also saw that several had their cerata 'bent' towards their mantle. I read somewhere on your forum (can't find it anymore) that you referred to it as 'sleeping'. Any other / additional information why they are doing this?

Locality: Cebu Island, Moalboal, 10-15 meter, Philippines, 28 April 2008. Length: 4mm - 3 cm. Photographer: Marcel Tanke.

Thanks and regards,
Marcel

marceltanke@cs.com

Tanke, M.A., 2008 (Aug 14) Phyllodesmium briareum from the Philippines. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21778

Dear Marcel,

I did a quick search on the Forum and couldn't find my reference to 'sleeping' but it worth remembering that this is a solar-powered sea slug, so it needs to ensure that the plant cells in its body get plenty of sunlight so they can photosynthesise and provide it with food. I have included a close-up to show the lines of brown specks in the cerata, which are the clusters of microscopic plants [zooxanthellae] living in ducts of the digestive gland. This species gets it zooxanthellae by eating a soft coral which often has a dark purple colour form. I can't see any sign of its food in your photo so its possible the Phyllodesmium have completely eaten out the soft coral colony and are now acting as plants by 'sunning' themselves in an unshaded place. If you look through some of the earlier messages on the Forum about this species, you will find they are often found in large aggregations. I wouldn't be surprised if this indicates that they have either direct development or an abbreviated development, which would mean all the hatchlings from one egg ribbon grow up together in the same area, presumably on one soft coral colony. 

Interestingly the other major group of solar-powered slugs - the sacoglossans which keep chloroplasts alive in their tissues - have members which also laze about in the sunlight, much for the same reason.

Best wishes,
Bill Rudman

Rudman, W.B., 2008 (Aug 14). Comment on Phyllodesmium briareum from the Philippines by Marcel Tanke. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21778

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