Endocrine and osmotic responses to tidally-changing salinities in Mozambique tilapia, Oreochromis mossambicus.

Variations in salinity are among the main physical parameters that drive the capacity of fish to survive and thrive in a range of environments. Acclimation to changes in environmental salinity is regulated by the interplay of local osmotically-induced cellular responses and the systemic regulation by the neuroendocrine system, which together, direct ion extrusion or uptake responses via the gill and other osmoregulatory tissues. Many euryhaline teleost fish are native to waters in which salinity varies tidally between that of fresh water (FW) and seawater (SW), such as estuaries. The physiology of salinity acclimation of euryhaline teleosts has been studied extensively in steady-state salinities, and after one-way transfers between steady-state salinities. Fewer studies, however, have addressed salinity regimes that reflect or simulate the continuous dynamic changes by which euryhaline fishes may be subject to in a native environment. Experimentally, the simulation of a changing environment was obtained by rearing fish in salinities that changed between those of FW and SW every six hours, in a tidal regime (TR). An overview of the main differences in the regulation of transcripts involved in ion balance between euryhaline Mozambique tilapia, Oreochromis mossambicus, responding to TR and those acclimating to steady-state salinities is provided. Transcripts analyzed include branchial Na+/K+-ATPase (nka), Na+, Cl- co-transporter (ncc), Na+/H+ antiporter 3 (nhe3), 2Cl- co-transporter (nkcc), cystic fibrosis transmembrane conductance regulator (cftr), and aquaporin 3 (aqp3). In particular, both cftrand aqp3were highly sensitive to changes in salinity in fish acclimated to a TR, indicating indispensable roles in rapidly maintaining hydromineral balance. The pituitary hormones, prolactin and growth hormone, known to play osmoregulatory roles in Mozambique tilapia, were also compared along with their receptors between TR and steady-state salinity paradigms. Specifically, through the dynamic changes in hormone receptor transcription observed in the TR rearing paradigm, the endocrine control of osmoregulatory outcomes appears shifted from systemic to local regulation at the level of target tissues. Together, these studies indicate that fish are able to compensate for broad and frequent changes in external salinity while keeping osmoregulatory parameters within a narrow range.