Anti-bacterial activity of molluscan egg masses in the San Juan Islands, WA

Gelatinous egg masses are used by several species of marine invertebrates to encapsulate embryos until hatching. The high protein and mucopolysaccharide content of these egg masses make them particularly hospitable to microbial growth and therefore susceptible to microbial infection. Previous studies have isolated compounds from egg masses that deter bacterial infection, but have not examined quantitative differences among species or the effects of micro- or macro-environmental variation on these defenses. We compared the anti-bacterial activity of egg masses from 16 molluscan species around the San Juan Islands, WA. In addition, several species were sampled from different habitats or substrates to compare within and between-population variation in these defenses. Egg masses were extracted with ethyl acetate (EtOAc) and methanol (MeOH) solvents and then tested for anti-bacterial activity against Vibrio harveyi and Bacillus subtilis in a 96-well plate growth assay. Three sediment characteristics—organic content, percent silt and clay, and bacterial load—were positively correlated across habitats with anti-bacterial activity in egg masses of the cephalaspid Melanochlamys diomedea. Egg masses of another cephalaspid, Haminoea vesicula, varied significantly in anti-microbial activity when laid on different substrates at a single field site and on a common substrate at different field sites. In addition to differences among species we also found patterns at higher taxonomic levels, with cephalaspids showing the highest level of anti-bacterial activity and nudibranchs showing surprisingly low levels. These results suggest wide interspecific and intraspecific variation in the degree of anti-bacterial protection associated with embryo encapsulating structures used by diverse molluscs. Levels of anti-bacterial protection appear to depend both on phylogeny and on environmental conditions at small and large spatial scales.