Sacoglossan Opisthobranchs Associated with the Green Macroalgae Codium spp. on Pacific Rocky Shores of Japan

Diverse and abundant populations of sacoglossan opisthobranchs inhabit Japanese rocky shores. The species of sacoglossans that feed on the green macroalgae Codium spp. were investigated in and around Sagami Bay on the Pacific coast of Honshu, Japan. On the Miura and Izu Peninsulas, two sacoglossan species were particularly common: Elysia trisinuata Baba, 1949 and E. setoensis Hamatani, 1968. Phenology and algal host use of these species are described. Elysia trisinuata grew substantially larger than sympatric sacoglossan species on Codium with maximum weight of 821 mg. Small E. trisinuata recruited to algal hosts primarily in summer. Some grew and spawned by autumn whereas others apparently over-wintered and spawned in spring. This sacoglossan associated with and fed on 6 of 8 Codium spp. investigated in this study and 4 additional Codium species in other studies. The congener E. setoensis was substantially smaller (max. 64 mg) than E. trisinuata and was frequent on C. fragile and C. intricatum in summer as well as present on 4 other genera of coenocytic green algae (Bryopsis, Caulerpa, Caulerpella, and Cladophoropsis). Although peak abundance of E. setoensis was in spring and summer, this species also persisted through the winter. Five additional species of sacoglossans associated with Japanese Codium spp. are briefly summarized.