The Mozambique tilapia, Oreochromis mossambicus , is a teleost fish native to estuarine
waters that vary in salinity between fresh water (FW) and seawater (SW). The neuroendocrine
system plays a key role in salinity acclimation by directing ion uptake and extrusion
in osmoregulatory tissues such as gill. While most studies with O. mossambicus have
focused on acclimation to steady-state salinities, less is known about the ability
of adult fish to acclimate to dynamically-changing salinities. Plasma osmolality,
prolactin (PRL) levels, and branchial gene expression of PRL receptors (PRLR1 and
PRLR2), Na + /Cl − and Na + /K + /2Cl − co-transporters (NCC and NKCC), Na + /K
+ -ATPase (NKAα1a and NKAα1b), cystic fibrosis transmembrane conductance regulator
(CFTR), and aquaporin 3 (AQP3) were measured in fish reared in FW and SW steady-state
salinities, in a tidal regimen (TR) where salinities changed between FW and SW every
6 h, and in fish transferred from FW or SW to TR. Regardless of rearing regimen, plasma
osmolality was higher in fish in SW than in FW fish, while plasma PRL was lower in
fish in SW. Furthermore, branchial gene expression of effectors of ion transport in
TR fish showed greater similarity to those in steady-state SW fish than in FW fish.
By 7 days of transfer from steady-state FW or SW to TR, plasma osmolality, plasma
PRL and branchial expression of effectors of ion transport were similar to those of
fish reared in TR since larval stages. These findings demonstrate the ability of adult
tilapia reared in steady-state salinities to successfully acclimate to dynamically-changing
salinities. Moreover, the present findings suggest that early exposure to salinity
changes does not significantly improve survivability in future challenges to dynamically-changing
salinities.