HORMONAL CONTROL OF GROWTH AND OSMOREGULATION UNDER DYNAMICALLY CHANGING SALINITIES IN MOZAMBIQUE TILAPIA Oreochromis mossambicus

Estuarine fishes, such as the Mozambique tilapia, are suitable models to study the capacity of organisms to acclimate to a range of salinities. Acclimation to salinity is largely mediated by the neuroendocrine system, and involves the regulation of pituitary hormones involved in growth and osmoregulation. Mozambique tilapia reared at the Hawaii Institute of Marine Biology grow faster in seawater (SW) than in fresh water (FW). The elevated growth observed in SW has been shown to be mediated, at least in part, through the growth hormone/insulin-like growth factor (GH/IGF) system. On the other hand, transfer from SW to FW triggers the release of prolactin (PRL), which acts to promote ion uptake and maintain salt-and-water balance.

To date, most studies have reared fish in a steady-state environment, (e.g., FW or SW), or following one-way transfer between salinities, an approach that does not reflect actual conditions of their native habitat, which is characterized by tidal variations in salinity. We compared plasma osmolalities, plasma PRL levels, pituitary PRL mRNA levels, and branchial PRL receptor mRNA levels in tilapia reared in FW, SW, and under a tidal regimen (TR), characterized by changes in salinity between FW and SW every six hours. We also measured the growth rate, feed conversion ratio (FCR), condition factor (K), gene expression of GH in the pituitary, and GH receptor and IGFs in the liver and muscle.

In fish reared in TR, while the expression and plasma levels of GH and PRL were steady, the expression of their receptors in target tissues changed dynamically between each phase of the tidal cycle. Moreover, tilapia reared under TR showed higher growth rates and pituitary GH mRNA expression, and lower FCR and K, when compared with either FW- or SW-acclimated fish. This study indicates that tidal fish are able to compensate for large changes in external osmolality while growing faster and maintaining hormones involved in growth and osmoregulation within a narrow range.