Cumanotus from Slovenia

Dear Tom,
Although its long cerata are reminiscent of Facelina dubia, I think this is a much more interesting find. I am pretty sure its a species of Cumanotus. Two species have been described from the Atlantic region, and both are considered to be quite rare. One species, Cumanotus cuenoti Pruvot-Fol, 1948, was described from the Atlantic coast of southern France, and the other, Cumanotus beaumonti (Eliot, 1906) is known from the British Isles and Norway.

Whether the two species are distinct is difficult to say as our knowledge of each is based on very few specimens. Thompson & Brown (1984) note that C. cuenoti appears to be smaller, not exceeding 10 mm in length, lacking oral and propodial tentacles, and having fewer denticles on the radular teeth. C. beaumonti is reported to reach 20 mm in length and Picton & Morrow (1994) report it to swim, as do Tardy & Gantes (1980) for C. cuenoti. Cumanotus cuenoti feeds on Tubularia, and Ectopleura dumortieri (see Tardy & Gantes, 1980) and C. beaumonti feeds on Corymorpha nutans (see Picton & Morrow, 1994). I have added a closeup of part of one of your photos alongside as it shows some anemone-like cnidarians. It would be interesting to know what they are and if your animal is feeding on them

Photos of C. beaumonti in Picton & Morrow (1994) and Bernard Picton's website, look almost identical to your animal. It has previously only been reported from the British Isles and Norway, where it is considered to be very rare. If your animal is indeed the same species, it would suggest the species has a much wider distribution  and the rare finds in the northern Atlantic, may suggest a geographic range centered much further south. It is also possible that C. cuenoti is a synonym.

If you find this species again, it would be very interesting to get photos of the head region.

• Eliot, C.N.E. (1906) Notes on some British nudibranchs. Journal of the Marine Biological Association, United Kingdom 7(3), 333-382, Pls.11-12.
• Picton, B. E. (1991) Cumanotus beaumonti (Eliot, 1906), a nudibranch adapted for life in a shallow sandy habitat? Malacologia 32: 219-222.
• Picton, B. E. and Morrow, C. C. (1994) A Field Guide to the Nudibranchs of the British Isles. London, Immel Publishing Ltd.  143pp.
• Pruvot-Fol,A (1948): Deux aeolidiens d'Arcachon. Journal de Conchyliologie 88(3), 97-100
• Tardy J. & Gantes H. 1980. Un Mollusque Nudibranche peu connu : Cumanotus cuenoti A. Pruvot-Fol, (1948) ; redescription, biologie. Bulletin de la Société zoologique de France, 105: 199-207.
• Thompson, T.E. & Brown, G.H. (1984) Biology of Opisthobranch Molluscs, Vol 2. 229pp. 41 Pls., Ray Society: London

Best wishes,
Bill Rudman