Emerging associations on marine rocky shores: Specialist herbivores on introduced macroalgae

Abstract

1. Sacoglossan sea slugs (Gastropoda: Opisthobranchia) can change their highly specialized algal host associations in response to introduced hosts. 2. Adult sacoglossans (Placida dendritica) collected from the introduced green macroalga Codium fragile ssp. tomentosoides in west-coast Scottish sea lochs tended to associate with and consume introduced hosts over the native C. tomentosum in pairwise-choice feeding trials. 3. On west-coast Irish shores, where native congeneric hosts were more common, significantly more P. dendritica attacked C. tomentosum on the shore whereas Elysia viridis disproportionately attacked the exotics. In Lough Hyne, Co. Cork, Ireland, juvenile E. viridis attacked both C. fragile ssp. tomentosoides and the native, sympatric C. vermilara; in pairwise-choice feeding trials, sacoglossans preferred the host species from which they were collected. 4. In the Channel Islands, adult E. viridis were common on C. tomentosum on Guernsey shores. In pairwise-choice feeding trials adults exhibited no preference between native and introduced hosts, indicating that the stenophagous herbivores could change their associations. 5. On temperate Australian shores, the native sacoglossan Placida aoteana attacked the recently introduced C. fragile ssp. tomentosoides as well as native congeners and conspecifics. P. aoteana was common and its herbivory evident in Victoria and Tasmania. Slugs collected from native C. fragile exhibited no preference between algal subspecies in Victoria but a strong preference for the introduced subspecies in Tasmania. 6. Flexibility in host use enables stenophagous marine herbivores to respond within years to the presence of recently introduced hosts. The implicit peril of the host-specificity paradigm - that specialists could change their associations - does occur in stenophagous sacoglossans, and there is considerable intra- and interspecific variation in response. 7. Biological control of invasive green algae using sacoglossan herbivores is fraught with several risks, including taxonomic uncertainty of species' boundaries, ecological flexibility of feeding, unexplored role of host susceptibility and insufficient evolutionary- developmental information to establish coevolutionary associations.