Annual Cycles of Some Manx Nudibranchs, with a Discussion of the Problem of Migration

Abstract

Monthly samples of twelve common species from intertidal and offshore populations were examined. Two groups were recognized Group 1 species have usually 1, sometimes 2, generations a year and exploit a stable food supply; Group 2 species have 2 or more, often several generations a year and depend on a fluctuating food supply. Populations of the species in group 2 are smallest in winter with seasonal variation greatest on the shore. Life-spans are short; those of group 2 are generally shorter than group 1. Death does not always follow breeding. Various theories of nudibranch migration are discussed. It is concluded that there is no shoreward migration for breeding and that, colonization of the shore is normally by the pelagic larva. Local movements bring individuals together for copulation and also help in finding food.