Okenia cupella? from French Mediterranean

Dear Marina,
Thanks for these interesting photos. Your animal could be one of two western Atlantic species, which have also been reported from the eastern Atlantic - O. cupella, and O. impexa. Valdes & Ortea (1995) describe specimens of both, O cupella from Spain, and O. impexa from Cuba and Cape Verde Islands. Although they cite earlier records of O. cupella from the western Mediterranean, they consider their record of O. impexa to be the first from the eastern Atlantic, however Schmekel (1979) had already described specimens from Banyuls and Naples in the Mediterranean.

In comparing the two species, Valdes & Ortea note that the major external difference is the shape of the lateral papillae - 'pike-shaped' in O. impexa, and club-shaped in O. cupella. From my experience with species of Okenia I find the shape of the lateral papillae to be a fairly risky character to use because the shape of the papillae is quite plastic in living animals, and also in your photos I can't see any of the lateral papillae which are the 'pike-shaped' ones.

Looking at the original descriptions of the two species, two other external characters seem to be quite important. Firstly, in general shape O. cupella has a narrow elongate body and the mantle edge, between the lateral papillae seems to form a 'webbing'. O. impexa, on the other hand is a shorter, higher animal and the lateral papillae seem to arise directly from the body wall with no remnant mantle skirt between them. The other important difference seems to be in colour. From my reading of the various descriptions, O. cupella seems to have scattered brown spots, especially in the mid dorsal area, while O. impexa has a pattern of brown 'patches' which Ortea and Valdes described as a 'dark brown patch .... which bears pale spots'  and Marcus (1957) describes as 'a net of superficial brown pigment forming patches ...'

I am pretty sure your animals are Okenia impexa. If you have a photo showing the shape of the lateral papillae from above or below it would be nice.

• Marcus, E. (1957) On Opisthobranchia from Brazil (2). Journal of the Linnean Society of London, Zoology 43: 390-486.
• Valdes, A. & Ortea, J. (1995) Revised taxonomy of some species of the genus Okenia Menke, 1830 (Mollusca: Nudibranchia) from the Atlantic Ocean, with the description of a new species. The Veliger, 38(3): 223-234.
• Schmekel, L. (1979) First record of Okenia impexa Marcus, 1957 from the Western Atlantic in the Mediterranean. The Veliger 21: 355-360.
• Vogel, R. M. and Schultz, L. P. (1970) Cargoa cupella, new genus and new species of nudibranch from Chesapeake Bay and the generic status of Okenia Menke, Idalia Leuckart and Idalla Orsted. The Veliger 12: 388-393.
  
  Best wishes,
  Bill Rudman