Sacoglossan Opisthobranchs on Northwestern Pacific Shores: Stiliger berghi Baba, 1937, and Elysia sp. on Filamentous Red Algae

At least 20 species of sacoglossan opisthobranchs worldwide feed on delicately branching red algae; these species include members of three genera (Hermaea Loven, 1844; Stiliger Ehrenbergh, 1831; and Elysia Risso, 1818) in three sacoglossan families. The algal hosts include members of three algal families and at least ten algal genera in the order Ceramiales. We studied two sacoglossan species that feed on filamentous red algae: (1) the temperate to boreal Stiliger berghi Baba, 1937, on wave-sheltered shores of Honshu and Hokkaido, Japan, and (2) the subtropical to tropical Elysia sp. on moderately wave-exposed shores of Okinawajima and Honshu, Japan. Preference experiments demonstrated that S. berghi prefers to associate with the alga Dasya when given pairwise algal choices but readily consumes members of several algal genera and exhibits no preferences between algal life-history phases (diploid tetrasporophytes vs. haploid female gametophytes). Elysia sp. is a small sacoglossan that consumes uniseriate and polysiphonous red algae. Given the small size and seasonally abundant populations of organisms that feed on red algae, we predict that these sacoglossans and their ecological analogs on other shores may have an unexpectedly important role in consuming and/or fragmenting ceramialean red algae. Given the known propensity of these algae to be dispersed by international shipping and oyster mariculture, careful malacological consideration should be given to the potential of sacoglossans to be inadvertent "hitchhikers" on a global scale.