Unknown nudibranch from south of France

Dear Christian,
I am not an expert on the Mediterranean fauna, but if your slug had no obvious gills then it was probably Phyllidia flava. Have a look at the top of this page, above your message, for a photo and description of P. flava.

There have been a number of studies of this species in scientific journals under the name Phyllidia pulitzeri Pruvot-Fol, 1962, which is now considered an unnecessary later name for P. flava. There is a good illustrated account of this species in the Italian magazine for shell-collectors La Conchiglia with colour photos of it on one of its food sponges and laying eggs.

Reference: Maeri, G. (1986) Notes and remarks on Pruvot-Fol, 1962. La Conchiglia, Number 208-209: 1-5.

When disturbed this species, like most phyllidiids, exudes a milky secretion from skin glands, which is distasteful to fish. I don't know if you handled the animals you saw, but one author reports that "this animal smells disgusting" which is a reference to the smell of the defensive chemical secretion. It is reported to feed on sponges of the genus Axinella.

If you have photos it would be fairly simple for me to confirm that it is Phyllidia flava.

Best wishes,
Bill Rudman.