Pleurobranchus mamillatus from Okinawa

Dear Bob,
Thanks for the photos and information. Two general points it raises is 'mass events' such as mass mortality, mass spawning etc., and predation.

We normally associate 'mass events' with Sea Hares but I am sure we all can remember a few occasions when we have witnessed an event such as you describe for P. mamillatus. I can remember in Tanzania on one occasion [lasted about a month] where shallow reef flat was a mass of large Pleurobranchus forskalii. I had never seem them in such numbers before or since. And Scott Johnson's message about Gymnodoris ceylonica reports a similar 'mass' event. I guess the other think to realise is that population explosions like this are commonplace among small sea slugs like sacoglossans, aeolids etc. They only seem special when the animals involved are large.

While mass events in Sea Hares are a fairly normal part of the life cycle, in large pleurobranchs and nudibranchs they seem to be rare events, probably occurring through a lucky combination of normal events.

The other interesting point you raise is possible evidence of predation by something unknown. One big gap in our knowledge is what actually eats Sea Slugs. Except in rare cases such information, where it exists, is published not in the malacological literature, where we would look, but in the literature associated with predator, for example fish biology journals. Another example of mine about Pleurobranchus forskalii in Tanzania, illustrates this problem. In desperation a visiting turtle specialist asked me to look at some strange chitinous plates from the stomach contents of turtles which he had been unable to identify.
They were the jaw plates of large pleurobranchs, probably Pleurobranchus forskalii. This information was published in some report known to turtle biologists but is effectively lost to malacology. I think this is a good time to add a page for predation records where we can gradually accumulate such information on who/what eats sea slugs. any records/observations would be appreciated.

Best wishes,
Bill Rudman.