ANCHIALINE LAKES SERVE AS NOVEL HABITATS FOR RELICT TAXA, INCLUDING CHEMOSYMBIOTIC MARINE BIVALVES

Lucinidae are the most diverse family of chemosymbiotic marine bivalves with sulfide-oxidizing gill endosymbionts and typically occupy shallow marine habitats, such as seagrass beds and mangrove swamps, or deeper sea floor habitats. However, dense populations of Ctena spp. have also been found in inland marine lakes on San Salvador, The Bahamas. These anchialine lakes, formed from extensive carbonate dissolution due to freshwater-saline water mixing, are also colonized by endemic pupfishes; clam and fish communities likely became isolated in the lakes after the most recent sea-level highstand, approximately 6,000 years ago. In this study, live Ctena were collected from open ocean habitats and anchialine lakes (Crescent Pond, Moon Rock Pond, Pain Pond) that each have distinct aqueous and sediment geochemistry and coastal connectivity through karst conduits. Both coastal and lake Ctena share similar bacterial endosymbionts based on analysis of 16S rRNA gene sequences, but metagenomics analyses, including phylogenetic reconstruction of partial eukaryotic cytB genes, suggest genetic drift between lake and coastal clam populations. Although valve features characteristic of Ctena are similar among all specimens, morphometric analyses indicate subtle morphological variations in each habitat, most notably the elongation of the inhalant channel—a respiratory feature linked to lucinid chemosymbiosis—in lake clams. Concentric growth rings on valves of lake clams are also highly pronounced, possibly due to more extreme episodic geochemical and physicochemical differences within and among lake habitats. These novel anchialine occurrences of chemosymbiotic bivalves provide key information to help resolve phylogenetic relationships and speciation history within the Ctena species complex in the Western Atlantic.