Polybranchia sp. undescribed

When I first spied this sea slug I just could not believe my eyes. What an astounding and amazing animal. It beggars belief that such an improbable creature exists. Sometimes it is the finding of just one special critter that makes a whole trip worthwhile. In fact I almost wanted to end the dive at that point just so I could get back and see my images on a bigger screen.

Polybranchia sp. is an undescribed and rarely sighted species of sacoglossan sea slug belonging to the Caliphyllidae (Polybranchiidae) family. How do we know this is a Polybranchia? There are features that point us in the right direction. First it has deeply bifurcated (forked) rhinophores (at right) and a dorsum of large crowded cerata. These features give us the family. Second, those cerata are filled with ramifications of the digestive gland showing through as a very fine reticulum. This gives us the genus of Polybranchia rather than Cyerce. A further confirmation, if we could see the foot, would be the lack of a transverse furrow on the sole that is only present in Cyerce. The cerata are thin and transparent and seem to be just a tangled mess - all different sizes projecting at all different angles. However it is that very transparency creating that effect. Close scrutiny reveals the outline of the discrete cerata. Around the margin or "seam" of each can be seen small white pustules that most probably hold glands producing noxious chemicals for defensive purposes if they function in the same manner as their better known sister species. The cerata are graduated in height with the tallest located medially creating a top-heavy appearance. The random bright red markings seem to be a constant characteristic of this particular species.

So far it has only been reported from Indonesia, PNG and the Philippines.

References:

- Burn R. (1998) Order Sacoglossa Pp 961-974 in Beesley, P.L., Ross, G.J.B. & Wells, A. (eds) Mollusca: The Southern Synthesis. Fauna of Australia. Vol. 5. CSIRO Publishing: Melbourne, Part B.
- Rudman 1998 - 2010.Polybranchia sp. 1, Factsheet & Related Messages, Sea Slug Forum, Australian Museum, Sydney.

Dave Mullins on location

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