Ancula pacifica from Vancouver, Canada

Dear Paul,
Thanks for these photos of 'Ancula pacifica'. Firstly I find it very difficult to believe the spotted and yellow-lined animals are really both the same species, and  also that either of them are the same as the Atlantic A. gibbosa. Although I have put all these colour forms together on the one Fact Sheet, I suspect some anatomical work might show suggest they should be splits.

Concerning where do they go when you can't find them. Most small nudibranchs live for less than a year, and most have planktonic larvae which swim or drift around in the water for part of their life cycle. If the species is particularly seasonal, absence could mean it is in its larval stage. Since larvae are at the mercy of currents etc, there is no guarantee that a particular species will be found at a particular spot each year. For short lived species - living less than a year - we think the species moves around from place to place by the adults producing larvae at one place, which then drift to another place where they find the right food, settle and grow to adulthood. These adults then produce larvae which float away and the cycle begins again. For species such as the one you describe here which seem present each year at the same spot, its possible the larvae are dependent on a cycle of temperature and currents which combine to bring back the right  conditions each year for the species to thrive. Usually these 'regular returns' are not as clockwork as they appear, but diligent sampling and recording of species is very time-consuming and for that reason is seldom undertaken. Longterm regular recording is the sort of thing dive clubs around the world sometimes undertake, but it takes a great degree of committment by the participants and has to be done for some years.
Best wishes,
Bill Rudman