The Annual Cycles of Two Species of Onchidoris (Opisthobranchia: Nudibranchia)

The annual cycles of the intertidal nudibranchs Onchidoris muricata (Muller) and O. bilamellata (L.) were followed through three generations at Robin Hood's Bay, North Yorkshire. O. muricata , a generalist polyzoan grazer, occurs throughout the range of its preferred prey species - Electra pilosa (L.) - and exhibits a simple annual cycle. O. bilamellata , on the other hand, is narrowly restricted within the distribution of its major prey species ( Balanus balanoides (L.)), and shows local variations in growth rate, levels of fecundity, timing of spawning and, in some localities, precocious maturation. Although the life cycle of O. bilamellata is generally annual, individuals undergoing precocious development spawn and die within approximately 3 months of settling. The planktotrophic larval phase of O. bilamellata is of some 3 months duration, with settlement and metamorphosis of the pediveliger only in the presence of live barnacles. The major aspects of the population ecology of these 2 species are compared and contrasted, and related to the differing pressures placed upon the predators by the morphology and life-cycles of their respective prey items.