TROWBRIDGE, C.D.: Emerging associations: sacoglossan opisthobranchs associated with introduced macroalgae

On temperate European shores, the native stenophagous marine herbivore Placida dendritica associates with the green macroalga Codium fragile introduced from north Pacific shores. On Scottish coasts, adult P. dendritica collected from introduced hosts prefer to associate with and consume the introduced hosts than the native C. tomentosum. On Irish west-coast shores, where the native hosts are common, significantly more P. dendritica on the shore associate with the native C. tomentosum than with the introduced hosts. Elysia viridis, however, disproportionately attacks the exotics, especially C. fragile ssp. tomentosoides. In Lough Hyne, Ireland, juvenile E. viridis attack both native and introduced Codium hosts on the shore and, in lab preference experiments, prefer the algal host from which they are collected. On temperate Australian shores, the native stenophagous marine herbivore Placida aoteana associates with the introduced C. fragile ssp. tomentosoides as well as with native congeners and conspecifics. Placida aoteana is common and its herbivory evident in Port Phillip Bay, Victoria and on both sides of Bass Strait. Slugs collected from native C. fragile exhibit no preference between algal subspecies in Victoria but a strong preference for introduced ssp. tomentosoides in Tasmania. Seasonal slug recruitment to available hosts coupled with an apparent flexibility in host use indicates that stenophagous marine herbivores can rapidly respond to introduced hosts on ecological time scales. Thus, the implicit peril of the host-specificity paradigm – that specialists could change their associations – does occur in these stenophagous sacoglossan herbivores.