For her M.S. thesis, Elena studied population connectivity in two Caribbean sea slugs, Elysia tuca and E. crispata, which differ in larval dispersal potential. E. tuca has short-lived, non-feeding larvae that settle only upon contacting the host alga Halimeda incrassata. In contrast, larvae of E. crispata metamorphose within their benthic egg capsules and hence have no free-swimming period. Elena sequenced a portion of the cytochrome oxidase I gene from slugs collected at a dozen sites across the Caribbean, ranging from Panama to Bermuda. She found 6 highly divergent clades within E. crispata that are largely restricted to different islands or regions, with limited gene flow as expected for a species lacking a dispersing larval stage.
In contrast, there was no phylogeographic structure in E. tuca. Using population genetic methods of data analysis, Elena showed that most sites in the central Caribbean are connected and share a common haplotype, whereas populations at the range edges and in the northern Bahamas experience little gene flow with the rest of the basin. Her data suggest that Panama and Bermuda may have retained ancestral polymorphism that was lost from the central Caribbean during Pleistocene sea level fluctuations. These results do not fit the predictions of biophysical coupling models, currently being used to predict source-sink relationships for Caribbean marine animal populations based on ocean circulation and larval lifespan, and suggest phylogeographic patterns may be quite idiosyncratic among different taxa.