THE STRESS RESPONSE OF MOZAMBIQUE TILAPIA (OREOCHROMIS MOSSAMBICUS) TRANSFERRED TO DYNAMICALLY CHANGING SALINITIES

Abstract

An organism’s survival depends on effectively responding to stressors. One of the many stressors aquatic organisms face is osmotic stress due to differences between their internal and environmental solute concentrations. In estuarine environments, euryhaline fish can maintain a stable internal osmolality despite daily changes in environmental salinity. The capacity of euryhaline fish to maintain homeostasis in a wide range of salinities is largely facilitated by the endocrine system. The hypothalamus-pituitary-interrenal (HPI) axis plays a role in the hypo- and hyperosmotic stress responses by facilitating ion and water transport at the gill while also modifying glucose metabolism to provide energy. I characterized responses of the HPI axis and glycogen metabolism in the gill and liver of the euryhaline Mozambique tilapia (Oreochromis mossambicus) under salinity stress. Specifically, in one study I analyzed fish transferred from freshwater (FW) to seawater (SW) and from SW to FW and, in the other, from steady state salinities to salinities that changed between FW and SW every 6 h to simulate a tidal environment. In both studies, I found that the effects of salinity in the HPI-axis were mainly characterized by changes in branchial corticosteroid receptor expression. Moreover, a robust rise in plasma glucose was observed when fish were transferred to FW, and in the FW phase of the tidal cycle, suggesting that glucose may play an osmotic role in compensating for the fall in plasma osmolality triggered by FW environments. Given the natural changes in salinity that occur in a range of wild and controlled environments, identifying changes in the underlying components of the stress response to perturbations in salinity can help mitigate the maladaptive effects of stress, thereby informing environmental management and aquaculture practices.