July 9, 2001
From: Bernard Picton
I see the recent correspondence on Cadlina from the NW Atlantic. Here in the NE Atlantic we are only supposed to have Cadlina laevis, but in my experience there seem to be two colour forms. In Scotland they seem to be more intertidal and feed on the sponge Halisarca dujardini (which looks just like Paul Young's sponge incidentally) and they are milky white in colour. On the west coast of Ireland where it is warmer they have many small yellow spots and a thin yellow margin like Paul Young's animal but feed on Dysidea fragilis primarily and live mostly in the sublittoral. Cadlina laevis has direct development and this variation has always been ascribed to the fact that with no planktonic dispersal there is probably a clinal variation in Cadlina laevis. The two forms don't seem to occur together so this is possible. Again I think the DNA or Enzyme experts need to look at this!
UPPER RIGHT: - Nov 1979 - Malin Beg, Co Donegal, Ireland
LOWER LEFT: - July 1981 - Westmann Islands, Iceland
LOWER RIGHT: - July 1991 - Loch Etive, W. Scotland
Picton, B., 2001 (Jul 9) Cadlina laevis & C. cf. luteomarginata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4793
Your message is a nice supplement to Jim Anderson's photos, though as you suggest the identity of these Cadlina is unresolved. Unless Cadlina laevis can happily feed on both Dysidea and Halisarca it seems there may be more than one species present. Perhaps the yellow bordered 'form' from Ireland is the 'form' of C. laevis reported from southern England to the Mediterranean that Bouchet & Moreteau (1976) suggest may be a distinct species. Do we know that all the colour forms have direct development? Before the chemists are called in, perhaps it would be worth correlating food, radular morphology and development type with colour form - though perhaps this has already been done?
• Bouchet, P. & Moreteau, J.C. (1976) Additions a l'inventaire des Molusques de Roscoff: Gasteropodes Opisthobranches. Trav. Stat. biol. Roscoff, 23: 1-8.