Abstract

Lucinidae are the most specious family of chemosymbiotic bivalves and have a fossil record extending to the late Silurian. The relationship between lucinid hosts and endosymbionts is inferred to have existed since at least the Jurassic based on habitat associations (occurrence at hydrocarbon seeps). Our understanding of the evolutionary relationships among lucinids and their relationships with endosymbionts is limited, however, as modern lucinid phylogenies are primarily derived from sequence data and are, therefore, limited to extant taxa. In order to fully understand the rich evolutionary history of lucinids, fossil taxa must be incorporated into analyses and, thus, the phylogenetic signal of conchological traits needs to be tested. This dissertation examines how lucinid shell morphology, as described by categorical character states and geometric morphometrics, corresponds with molecular phylogenies and endosymbiont diversity at several scales. Chapter 1 examines congruence and resolution in molecular tress versus trees based on categorical shell character states. A matrix of 58 shell characters and 52 species of lucinids from the Neogene and Quaternary of the Western Atlantic was used to construct morphological cladograms that were compared to published molecular phylogenies. Results indicate that cladograms based on shell characters are often congruent with molecular phylogenies at the genus-level, but are poorly resolved at more basal nodes. Nonetheless, the inclusion of shell characters provides a means to include extinct taxa in phylogenetic analyses. Chapter 2 further examines morphological and phylogenetic relationships using geometric morphometric analyses of shell interiors of eight lucinid species to test for phylogenetic signal in shape based on a landmark configuration focused on adductor muscle scars and shell outline. Results confirm a strong phylogenetic signal in landmark data, however, this signal does not represent shape variation for any particular morphologic feature(s). Finally, in Chapter 3, population-level host molecular phylogeny, host shell morphology (geometric morphometrics), and gill-associated bacterial relative abundances are compared in Ctena n. sp. occurring in anchialine ponds and coastal lagoons of San Salvador Island, the Bahamas as well as in Ctena orbiculata from coastal Florida and lagoons of San Salvador Island, the Bahamas. No significant differences in sequence data among populations were found, although there were differences in both host shell morphology and gill-associated bacterial relative abundance by locality, whereas host shell morphology and gill-associated bacterial relative abundance were not significantly correlated. This dissertation provides 1) a new phylogeny for lucinids incorporating morphological characters, 2) the first used of COI in a molecular phylogeny for the family, 3) the first test for phylogenetic signal in lucinid morphology, 4) the first test of a relationship between host morphology and gill-associated bacterial relative abundance, and 5) a description of and justification for a new lucinid species of Ctena.