Abstract
Sacoglossans are specialized marine herbivores that tend to have a close evolutionary relationship with their macroalgal hosts, but the widely distributed species Elysia viridis can associate with several algal species. However, most previous investigations on the field abundance and size distribution of E. viridis have focussed on Codium spp. in the British Isles, and algae from this genus are considered superior hosts for E. viridis. In the present study, we investigated the abundance and size distribution of E. viridis on 6 potential host algae with differing morphologies (the septate species Cladophora sericea, Cladophora rupestris, Chaetomorpha melagonium, and Ceramium virgatum, as well as the siphonaceous species Codium fragile and Bryopsis sp.) at 2 sites on the Swedish west coast over the course of a year. In spring, slugs were almost absent from all algal hosts. In summer and autumn, E. viridis consistently occurred on several of the algal species at both sites. The highest number of small E. viridis were found on C. sericea, intermediate numbers of significantly larger E. viridis were found on C. rupestris, while fewer, intermediate sized animals were found on C. fragile. Throughout the study period, only a few E. viridis individuals were found on C. melagonium, Bryopsis sp., and C. virgatum. Our results indicate that E. viridis is an annual species in Sweden, capable of exploiting co-occurring congeneric and intergeneric algal hosts with differing morphologies. These results corroborate previous findings that E. viridis can exploit several different algal species, but does not indicate that C. fragile is a superior host.
Highlights
Sacoglossans are an intriguing clade of predominantly herbivorous opisthobranch molluscs that are considered one of the few examples of specialized herbivores in the marine environment
The abundance of E. viridis was low on the green algal species Bryopsis sp. and C. melagonium, and on the dominant red alga C. virgatum
E. viridis numbers were substantially higher on the green algal hosts C. sericea, C. rupestris and C. fragile (Fig. 1)
Summary
Sacoglossans are an intriguing clade of predominantly herbivorous opisthobranch molluscs that are considered one of the few examples of specialized herbivores in the marine environment. Many species are capable of sequestration and/or conversion of algal chemical defence compounds [3], retention of functional algal chloroplasts from their macroalgal diets, known as kleptoplasty [4], and demonstrate crypsis in both morphology and colouration [3]. Despite these adaptive traits having caught the imagination of many researchers, interpretation of their ecological relevance has been limited. A substantial amount of data are available on sacoglossan-algal host associations in the laboratory [6,10], with more insight being gained from the use of molecular techniques to accurately identify sacoglossan diets (e.g. [11,12,13,14,15]), quantitative observational field data of abundance and size distribution on algal hosts are still limited for most species (but see [5,7,16,17,18,19,20])
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