Ceratosoma gracillimum from the Solomon Ids

Dear Bruce,
Yes this is C. gracillimum. Why are they together and why haven't you seen it before? We still have much to learn about nudibranch population dynamics. While it has been possible to study some temperate water species in considerable detail, because they have fairly large 'permanent' populations, this has not proved so easy with species which are not so common, or feed on animals which are themselves fairly sparsely distributed, which is the case with many tropical animals.

If you consider the lottery they participate in. If their egg mass survives, they hatch out as planktonic larvae and after some days or weeks drifting in the plankton, they then have to find the right sponge (or other appropriate food) to settle on and grow to adulthood. Keep in mind that the sponge has has also gone through a similar planktonic larval stage. This certainly explains why a species can suddenly arrive at a particular spot, such as your dive site, even though it has been absent from there for many years. Apparently this year the sponge larvae settled, and a while later the nudibranch larvae drifted past and sensed its presence. How come three Ceratosoma settled together? I guess that is one of life's little mysteries. I find it difficult to believe that a group of C. gracillimum larvae stuck together during their planktonic life, but until someone devises a way to study this, it must remain a possibility. However it happened, clearly a number of larvae sensed an appropriate sponge colony, or perhaps a 'garden' of many sponges of the same species, and settles out of the plankton. Again there are very few long-term studies of tropical species but the little evidence we have suggests they stay near their food colonies, which would explain why you found three together. This is just conjecture. It would be nice to have some more long-term studies of tropical species, but such work is note easy.
Best wishes,
Bill Rudman