Intraspecific morphological divergence among chemosymbiotic bivalves (Lucinidae) from anchialine lakes of San Salvador Island, Bahamas

Lakes can serve as cradles of biodiversity by facilitating adaptive radiation linked to distinct environmental conditions and ecological opportunities. On San Salvador Island, the Bahamas, karstification during Holocene transgressive flooding created anchialine lakes that have supported rapid evolutionary diversification, most notably among endemic pupfishes with specialized feeding strategies. To assess whether similar phenotypic divergence occurs in other members of these lake communities, we investigated morphological variation in shells of the chemosymbiotic infaunal bivalve Ctena sp. (Lucinidae). Live and dead Ctena specimens were sampled from muddy sediment filling karst pockets along the shorelines of three anchialine lakes, a habitat type from which chemosymbiotic bivalves have not previously been reported. Using two-dimensional geometric landmarks of shell interiors and valve margins, we found significant differences in shell shape, size, and allometric relationships among living populations from each lake. Death assemblages preserved similar among-lake patterns, suggesting that morphological divergence predates the window of time averaging represented by death assemblages. Living and dead-shell assemblages within each lake also differed significantly in shell shape, and temporal shape-change vectors from the dead to living included both shared and distinct trends among lakes. Ongoing genomic work suggests that lake-dwelling Ctena on San Salvador represent a single endemic species. Thus, spatiotemporal patterns among lake populations likely reflect ecophenotypic responses to distinct sedimentological and geochemical conditions that influence their chemosymbiotic lifestyle. These results demonstrate the potential of combined life and death assemblage analyses to detect patterns of phenotypic divergence over geologically instantaneous timescales.