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Invasion of the bryozoan Membranipora membranacea to the Gulf of Maine provided an additional prey resource to the dorid nudibranch Onchidoris muricata (Muller, 1776). We examined prey choice based on past diet history and compared growth and reproductive output among laboratory-reared O. muricata on exclusive bryozoan diets of M. membranacea and the native, historically preferred prey, Electra pilosa. Adult nudibranchs were collected from Cape Neddick, York, Maine for the prey choice experiment and post-metamorphic juveniles were collected from Jaffery Point, Newcastle, New Hampshire for the growth and reproduction experiments. Nudibranchs were laboratory reared under controlled conditions for up to 10 months until completion of spawning. Growth was monitored biweekly, and reproductive performance was assessed with a weight-adjusted dimensionless reproductive index (ΣRI) of each individual's spawn summed over the reproductive period. Prey choice experiments showed nudibranchs tended to select p...

The invasion of the bryozoan Membranipora membranacea to the Gulf of Maine in 1987 eventually provided an additional prey resource to the dorid nudibranch Onchidoris muricata (Muller, 1776). The impact of this novel prey on the population and reproductive biology of O. muricata was investigated at an intertidal site in the southern Gulf of Maine from January 2005 to October 2009. Nudibranchs were found associated with a variety of bryozoans, with a seasonal shift from Electra pilosa (December to May) to M. membranacea (June to November). Juveniles (< 3 mm) were found in all months except May, June, and July and mature nudibranchs were present in all months. Copulating pairs were observed between lanuary and July and spawn masses were found between March and September on the undersides of rocks usually attached to the surface of bryozoans. The population structure and reproductive phenology of O. muricata at this intertidal site shows changes from historical reports. Whether adaptive change of th...

Membranipora membranacea is an invasive bryozoan that was first found in the Gulf of Maine in 1987 and within two years became the dominant organism living on kelps. Membranipora may have become dominant so quickly because it had little competition in a relatively unoccupied niche; however, lack of predation has also probably played a major role. Where Membranipora is native, there is usually a specialist nudibranch predator that keeps the population in check. For example, in European populations, the nudibranch Polycera quadrilineata prefers Membranipora while Onchidoris muricata is known to prefer another bryozoan, Electra pilosa. Electra, Membranipora, and Onchidoris are all now found in the Gulf of Maine while Polycera is not. We tested whether Onchidoris would (1) eat Membranipora at all, (2) eat Membranipora and Electra at different rates, and (3) show a preference for eating Membranipora or Electra when given a choice. We found that Onchidoris does eat Membranipora, and it generally eats Membranipora faster than Electra. However, when given a choice, Onchidoris prefers Electra. Onchidoris typically reproduces in the spring and grows over the fall and winter, but has recently been found reproducing in the winter in New Hampshire. Although it does not survive the winter as well as Electra, Membranipora is the dominant organism living on many macroalgae in the late summer and fall. Thus, the large Membranipora food source now available in the summer and fall may allow Onchidoris to reproduce earlier.

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.