INTRASPECIFIC VARIABILITY OF VALVE MORPHOLOGY AND ENDOSYMBIONT DIVERSITY AMONG LUCINIDAE FROM SHALLOW MARINE HABITATS

All modern Lucinidae species host environmentally-acquired chemosymbiotic, sulfur-oxidizing bacteria in gill bacteriocytes. Lucinids generally share anatomically similar reduced digestive tracts and gill features adapted to accommodate symbionts. However, other functional anatomical features and corresponding valve characteristics differ among lucinids, such as the presence and placement of mantle gills that may affect the position and shape of the pallial line. These features are likely related to ecological niche divergence and adaptation, due in part to hosting endosymbionts. Inter- and intra-specific variability of anatomy, valve morphology, and endosymbiont diversity and ecology, however, have not been systematically evaluated. To address this deficit, we analyzed valve morphology and endosymbiont genetics from ten lucinid species in diverse shallow marine habitats, including dense populations of Phacoides pectinatus, Stewartia floridana, and Ctena orbiculata. Endosymbiont taxonomic diversity was largely comprised of uncultured Gammaproteobacteria within the genus Sedimenticola, but that diversity differed among lucinid species and even within a single population. Metagenomics analysis of endosymbionts recovered from P. pectinatus, S. floridana, and C. orbiculata reveal diverse metabolic capabilities associated with sulfur, nitrogen, and carbon fixation pathways. Moreover, endosymbiont diversity based on single gene analysis (e.g., 16S rRNA genes) did not correlate with host ecology, habitat type, or shell morphology. For example, among C. orbiculata collected in different Bahamian marine lakes, genetically similar endosymbionts did not correlate with valve morphology that differed among lakes. Also, S. floridana valve morphology differed by microhabitat, whereby individual morphologies within seagrass were distinct from those within unvegetated sediment, despite low endosymbiont diversity with similar metagenome profiles. Collectively, these data suggest that there may not be a common suite of traits that indicate chemosymbiotic associations within lucinids, as endosymbiont metabolic capabilities and gene expression are tied to environmental conditions, but functional anatomy and other host-specific traits may not connect with endosymbiont activity.