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The nudibranch mollusk Onchidoris muricata known from boreal waters of the western Pacific, North American Pacific shore, and both coasts of the northern Atlantic was first recorded from the Sea of Japan (East Sea). This discovery extends its distribution range southwards along the Asian Pacific coast from cool temperate waters of Kamchatka and Kommander’s Islands to cool temperate sub-zone of the Sea of Japan. Three discovered populations in the northern Sea of Japan are confirmed to be conspecific with other Pacific and Atlantic populations of O. muricata because of external and radular morphology along with low interpopulation genetic variation (

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.

Onchidoris muricata (Muller) and Adalaria proxima (Alder and Hancock) are sympatric, potentially competing species of dorid nudibranchs, which preferentially graze the cheilostome polyzoan Electra pilosa (L.). O. muricata is small and lays small eggs which hatch as poorly-developed planktotrophic veliger larvae. A. proxima is larger and reproduces by means of larger eggs which hatch, as well-developed lecithotrophic larvae, that can metamorphose within approximately 24 h of release. A. proxima larvae can feed in the plankton, but do not require extrinsic nutrition to undergo complete development. Both species spawn in February–april, and have a strictly annual life-cycle. Comparisons of the calorific content of spawn have shown that A. proxima apportions a greater number of calories to reproduction, but that O. muricata makes a greater relative effort. A. proxima shown considerable individual variability in reproductive effort, which fails to correlate with, body size or rate of spawning. A more deterministic situation applies to O. muricata, because body size and fecundity follow an allometric relationship. It appears that there is a threshold of absolute energy required to support the lecithotrophic larval strategy in nudibranchs, and that this is not attained by the smaller species, O. muricata. A. proxima thus appears to have both reproductive strategies open to it, and to have adopted lecithotrophy in order to offset the unpredictability of energy available for reproduction.