UC San Diego

Ectotherms, including sharks, behaviorally thermoregulate in order to optimize physiological processes. Juvenile sharks often utilize estuaries, which can provide warm water temperatures that increase metabolism and facilitate growth. Both the California Horn Shark (Heterodontus francisci) and the Leopard Shark (Triakis semifasciata) occupy estuaries of southern California as juveniles and represent contrasting activity levels; horn sharks are relatively sedentary while leopard sharks are more active and mobile. The primary aim of this study was to better understand the relationship of behavioral thermoregulation and metabolism in response to temperature in these two species. The objectives were threefold: 1) identify the temperatures juveniles prefer, 2) assess the sensitivity of metabolism (Q10) to temperature via measurements of oxygen consumption, and 3) compare these results between species, sex, and seasonal acclimation treatments. Using a shuttlebox, this study showed juvenile horn sharks and leopard sharks have comparable thermal preferences and temperature ranges. Respirometry trials at different temperatures showed both species exhibited increasing metabolic rates with increasing temperature, and leopard sharks exhibited higher metabolic rates than horn sharks across all temperatures. However, horn sharks exhibited a higher overall Q10 suggesting their metabolism has greater thermal dependence. Underlying acclimation to seasonal temperatures did not appear to have an effect on any of these parameters. Horn sharks likely target environments (i.e., day-time resting places) closer to their preferred temperature and more stable in temperature fluctuation than leopard sharks. Lab-based determination of physiological and behavioral responses to temperature are important in understanding habitat usage during vulnerable life stages.