Abundance and size distribution of the sacoglossan Elysia viridis on co-occurring algal hosts on the Swedish west coast

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.

Figure 1. Abundance on different algal hosts.

Field abundance of Elysia viridis (E. viridis. g algae⁻¹; mean + SE) on different algal hosts at 2 sites (Yttre Vattenholmen and Saltö Lyngnholmen) during 4 surveys: (A) autumn 2010, (B) spring 2011, (C) summer 2011, and (D) autumn 2011. Numbers below bars indicate the number of samples of each algal host searched. ND denotes no data for a particular host, i.e. no samples of the algal host were collected. Algal hosts within a site and survey sharing the same letter do not significantly differ at α = 0.05 (Dunnett's T3 multiple comparisons test).

Figure 2. Colonisation of different algal hosts.

Thalli occupied by Elysia viridis (%) on different algal hosts at 2 sites (Yttre Vattenholmen and Saltö Lyngnholmen) during 4 surveys: (A) autumn 2010, (B) spring 2011, (C) summer 2011, and (D) autumn 2011. Numbers below bars indicate the number of samples of each algal host searched. ND denotes no data for a particular host, i.e. no samples of the algal host were collected.

Figure 3. Size on different algal hosts.

Size of Elysia viridis (mg or mm, mean + SE) collected from different algal hosts at 2 sites (Yttre Vattenholmen and Saltö Lyngnholmen) during 4 surveys: (A) autumn 2010, (B) spring 2011, (C) summer 2011, and (D) autumn 2011. Numbers below bars denote the total number of E. viridis measured from each algal host. Data were included in the statistical analysis when at least 20 E. viridis were collected from a particular host, site and survey. Algal hosts within a site and survey sharing the same letter do not significantly differ at α = 0.05 (Tukey's honestly significant difference, HSD, multiple comparisons test).