The family Lucinidae is one of the largest and oldest bivalve lineages. All adult members of this family have been shown to contain intracellular chemoautotrophic bacteria in the gill tissue. This dissertation investigates the nutritional and respiration strategies employed by the chemosymbiotic associations between the gill endosymbiont and its lucinid host.

Despite the focus on chemoautotrophy as the primary nutritional mode for this family, there is evidence that particulate feeding remains an important nutritional source for all lucinids. To learn more about the nutritional dependence of L. aequizonata on its symbiont, the transmission mode of the symbiont and the feeding abilities of veliger and adult L. aequizonata were examined. A diagnostic assay was used to examine the life stages of L. aequizonata for the presence of the symbiont. Examination of mature gonads, eggs, and veligers indicated that the symbiont was acquired from the environment at a later stage. Contrary to a previous study, observation of gut content and radiolabeled feeding experiments showed that adult animals have maintained the ability to assimilate carbon from particulate organic matter. These studies demonstrate that L. aequizonata has maintained a mixotrophic diet, relying on reduced carbon from both chemosynthesis and feeding to meet its nutritional requirements.

Alternative sources of nitrogen, outside of particle ingestion, in lucinids have not been previously investigated. Ammonia and nitrate were tested as possible nitrogen sources for the lucinid Codakia orbicularis using stable isotopes and LC-MS analysis of free amino acids. Nitrogen from nitrate was not assimilated while ammonia was rapidly metabolized. Ammonia is assimilated via glutamine synthetase in both gill and body tissue, comprising up to 46 % of the L-glutamine pool within 1 hour exposure to 15N-ammonium. Three hours later, label is found in both L-glutamate and L-alanine. The turnover rate is rapid; the label is almost completely removed after a 24-hour pulse with unlabeled ammonium. Data from these studies suggest that the clam is partly responsible for uptake and metabolism of ammonia. The assimilation of ammonia most likely plays an important role in the nitrogen metabolism of C. orbicularis chemosymbiosis since it is present at environmental levels similar to those used in this study.

Nitrate respiration was previously suggested as an important electron acceptor among chemosymbionts. Respiration strategies utilized by the C. orbicularis symbiont were explored using a combination of biochemical methods on mechanically purified symbionts, intact clam-symbiont associations, and habitat analysis. The presence of low nitrate levels in the environment, non-detectable levels of nitrate in the hemolymph of the clam, and the low nitrate reductase in symbiont containing tissue all support the hypothesis that oxygen is the primary electron acceptor for the symbiont. Microelectrode studies of purified symbionts showed that the symbionts are capable of oxygen respiration. These studies suggest that the respiration strategies used by lucinid symbionts may be more reflective of the respective habitat rather than a characteristic trait of the chemosymbioses. Like other chemosymbioses tested, hydrogen sulfide is produced when the animal/symbiont is exposed to anoxia. This is the first study to show by direct measurements that the symbionts are the actual source of the sulfide production.