Feeding Specificity of the Photosynthetic Sacoglossan Sea Slug \u3cem\u3eElysia papillosa\u3c/em\u3e

The sacoglossan sea slug, Elysia papillosa is associated with two species of the siphonaceous green alga, Penicillus, which co-occur in mixed macrophyte beds at Tarpon Springs, FL, USA. Field collections revealed that Elysia papillosa is frequently collected from the alga, P. capitatus, but is also present in much lower abundance on P. lamourouxii. Past studies have shown that some species of sacoglossans consume algal species that differ from the algae from which they are collected so, in order to determine whether E. papillosa was consuming either or both species of Penicillus, total DNA was extracted from individual slugs collected from each alga. Identity of the algal species matching the gene sequence of the chloroplast genomic gene, rbcL (large subunit of ribulose bisphosphate carboxylase) was determined. Molecular data indicated that individuals of E. papillosa were consuming the same algal species from which they were collected. To determine if identity of algal food resource had any impact on performance of E. papillosa, feeding trials were conducted in the laboratory to measure growth of slugs (body length, cm) when feeding on P. capitatus compared to P. lamourouxii. Over a three week period, E. papillosa fed P. lamourouxii achieved a mean body length that was 1.5-2X larger than recorded for slugs fed P. capitatus; these differences between feeding groups were significant (One way ANOVA, p\u3e0.001, F = 20.730). Although P. capitatus is clearly more attractive to slugs when in natural settings these results suggest that although P. lamourouxii can offer some kind of nutritional advantage slugs are choosing to consume the algal species that they inhabit in the field. Algal morphology and cellular structures are potentially important factors that may explain E. papillosa’s higher abundance on P. capitatus. Also, E. papillosa could be choosing to utilize P. capitatus, the less nutritious alga resource, because it may offer increased refuge benefits