Abstract
The way an animal utilizes and stores energy directly affects its ability to perform essential life functions (e.g., foraging, migration, reproduction), with implications for its health and fitness; therefore, individual variation in energy storage and metabolism within a population leads to intra-population variation in individual fitness and survival. Consequently, understanding the factors that contribute to variation in energy storage and metabolism within a population can aid in identifying factors that influence that population’s fitness. Compared to other taxa, there has been relatively little research to investigate the factors that govern variation in elasmobranch energy use and storage, particularly in wild populations. Furthermore, there has been almost no research conducted to explore how urbanization, a growing threat to marine ecosystems and organisms, affects elasmobranch energy storage and metabolism and, ultimately, their health and fitness. To address this knowledge gap, morphological (i.e. body condition) and biochemical (i.e., plasma cholesterol, free fatty acid, triglyceride, and ketone body concentrations) metrics related to energy storage and utilization were measured for 118 wild-sampled south Florida nurse sharks (Ginglymostoma cirratum). Statistical analyses were performed to identify any correlations between metrics and explore ontogenetic, sexual, seasonal, and spatial variation in body condition and plasma metabolite concentrations within the population. Results demonstrated that several metrics can be significantly linearly correlated with one another, but these relationships are highly dependent on nurse shark life stage and sex. The mean plasma metabolite concentrations for immature, mature female, and mature male nurse sharks was not significantly different; however, mature female nurse sharks had significantly higher body condition than mature male sharks. Further analysis revealed that this difference is driven by seasonal variation in mature female body condition, likely related to the accumulation of energy stores prior to reproduction. Significant seasonal variation also occurred in plasma concentrations of beta-hydroxybutyric acid for mature male nurse sharks, possibly related to energetically exhausting mating activity during the wet season. Significant spatial variation occurred in the plasma concentrations of both triglycerides and free fatty acids for immature nurse sharks. Immature nurse sharks sampled within the more urbanized Metropolitan zone had higher mean triglyceride concentrations, and lower mean free fatty acid concentrations, than their conspecifics sampled within the relatively more natural National Park zone; this may imply that nurse sharks in the Metropolitan zone are feeding more, or more frequently, than nurse sharks in the National Park zone. The results of this thesis contribute to the growing knowledge base on energy storage and metabolism in wild-sampled elasmobranchs, as well as provide the first exploration of variation in elasmobranch body condition and plasma metabolites relative to urbanization, an anthropogenic threat with the capacity to influence elasmobranch fitness and survival.