TROWBRIDGE, C.D.* ; HIRANO, Y.J.: Marine specialist herbivores on high-diversity shores: Ecological notes of Japanese Ascoglossan (= Sacoglossan) Opisthobranchs.

Most marine specialist herbivores belong to the molluscan order Ascoglossa, a group of suctorial sea slugs that show intriguing parallels with phytophagous insects. We investigated the ecology of ascoglossans in a region of high species richness: the temperate rocky shores of Sagami Bay, NE Japan. The wealth of taxonomic descriptions about Japanese sea slugs contrasts with the meager ecological information. Based at Misaki Marine Biological Station (Univ. Tokyo), we investigated three ascoglossan assemblages: (1) Placida dendritica, Elysia trisinuata, and Elysia setoensis on Codium spp., (2) Ercolania boodleae, Aplysiopsis minor, and Elysia ?nigrocapitata in high intertidal pools associated with filamentous green algae, and (3) Stiliger berghi associated with polysiphonous red algae. The small (<15 mg) Placida dendritica was most abundant on subtidal C. fragile whereas the large (<800 mg) Elysia trisinuata was most frequent on the encrusting C. arabicum. The pool-dwelling Ercolania boodleae formed high-density populations of small animals (<10 mg) whereas the other two sympatric species were sparsely distributed and considerably larger. Finally, the very small (<4 mg) Stiliger berghi occurred on red algae epiphytic on subtidal Codium. The feeding specificity of the Codium-associated slugs was partially elucidated: recent feeding history of the slugs and age and physiological condition of the algae influenced slugs’ host-plant choice. Given the high species richness of ascoglossans on NW Pacific shores, interspecific interactions may influence coexistence among these sympatric species. Additional ecological study of these species is clearly merited.