Changes in aerobic metabolism associated with the settlement transition for the leopard coral grouper (Plectropomus leopardus)

Metamorphosis is a critical aspect of coral reef fish ecology. This developmental milestone marks changes in form and function that permit successful transition of pelagic larvae to the demersal lifestyle on coral reefs. However, we know very little about the physiological changes that occur during this period, specifically potential changes in energetics associated with swimming. This is critical, as swimming is the mechanism by which pelagic larvae find a suitable reef on which to settle. Coral grouper larvae (Serranidae: Plectropomas leopardus) were collected at night as they came into the vicinity of a fringing reef to settle, and their physiological metamorphosis was characterized. Larvae and 24 h-settled juveniles were exposed to an endurance swimming test at ecologically relevant swimming speeds, and oxygen uptake rates were measured during activity. To describe how aerobic and anaerobic properties of tissues change during metamorphosis, we also measured whole body citrate synthase and lactate dehydrogenase activity, respectively, as well as mitochondrial density in the trunk and pectoral fins. Our approach accurately measures the oxygen uptake rates these life stages need during the recruitment process, with larvae having a 74% higher mass-specific oxygen uptake rate (ṀO₂) than settled juveniles despite swimming at speeds that are only 1.5 body-lengths per second (BLs⁻¹) faster. Citrate synthase activity significantly decreased upon settlement; as larvae had 3.7 times higher activities than juveniles, suggesting that rapid changes in aerobic metabolism of tissues may be an important process during metamorphosis in this species. In contrast, lactate dehydrogenase did not significantly differ upon settlement. These findings highlight some physiological modifications that pelagic coral grouper larvae undertake within 24 h that contribute to successfully settling onto a coral reef.