Environmental Controls on the Diversity and Distribution of Endosymbionts Associated with <i>Phacoides pectinatus</i> (Bivalvia: Lucinidae) from Shallow Mangrove and Seagrass Sediments, St. Lucie County, Florida

Lucinid bivalves are capable of colonizing traditionally inhospitable shallow marine sediments due to metabolic functions of bacterial endosymbionts located within their gills. Because lucinids can often be the dominant sediment infauna, defining their roles in sediment and pore fluid geochemical cycling is necessary to address concerns related to changes in coastal biological diversity and to understanding the sensitivity of threatened coastal ecosystems over time. However, there has been limited research done to understand the diversity and distribution of many lucinid chemosymbiotic systems. Therefore, the goals of this thesis were to evaluate the distribution of Phacoides pectinatus and its endosymbiont communities from Ft. Pierce, St. Lucie County, Florida, and to define the environmental controls on potential free-living P. pectinatus endosymbionts to understand symbiont distribution patterns and host acquisition. Nearly all of the encountered P. pectinatus hosts were confined to within one meter of mangrove canopy. The distribution of P. pectinatus correlated to concentrations of organic carbon in the sediments, but not to total dissolved sulfide or sea grass vegetation densities. Sequencing of bacterial 16S rRNA genes from sediment and pore fluids from the P. pectinatus habitat revealed that Proteobacteria dominated the communities, including Alpha-, Delta-, and Gammaproteobacteria classes. Other major phyla included the Bacteroidetes, Chloroflexi, Planctomycetes, and Spirochaetes. 16S rRNA genes for both P. pectinatus gills and feet were closely related to novel bacterial communities comprised of Sedimenticola, Kistimonas, Methylomarinum, and Spirochaeta spp., as well as unclassified Rickettsiales (Alphaproteobacteria) and unclassified Lentisphaerae. This level of potential endosymbiont diversity has not been previously reported for lucinids. Moreover, potential endosymbiont populations differed by lucinid location, as gills containing higher Rickettsiales occurred in areas with the lowest clam density and gills in deeper sediments contained a higher proportion of Methylomarinum. Despite broad sediment and pore water bacterial diversity, no environmental sequences genetically matched those from P. pectinatus at the genus level. These results, while strengthening our understanding of a lucinid-symbiont system, still highlight how limited our knowledge is about these systems. These results provide new directions for future research and can be used to help understand how sensitive these systems are to environmental change.