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The life history of anadromous Atlantic salmon (Salmo salar) entails an initial freshwater stage that precedes their migration to marine environments. The development of physiological systems that will ultimately support the survival of juvenile salmon upon their entry into marine habitats is termed parr‐smolt transformation. This springtime transformation (smoltification) includes the acquisition of seawater (SW) tolerance. Thus, the ability of smolts to successfully acclimate to SW is attained through the development of branchial pathways for ion‐secretion. In addition, the capacity to rapidly attenuate ion‐absorptive processes upon exposure to SW is a vital aspect of SW tolerance. In the current study, we investigated the branchial gene expression patterns of three Na+/HCO3−cotransporter 1 isoforms (nbce1.1, nbce1.2a, and nbce1.2b) during smoltification and following seawater challenges. Given the proposed roles of Nbce1s in the absorption of environmental Na+ by branchial ionocytes in other teleosts, we hypothesized that nbce1s would be downregulated in salmon undergoing smoltification and/or following SW exposure. In two separate years, we observed springtime increases in branchial Na+/K+‐ATPase activity, Na+/K+/2Cl−‐cotransporter 1 and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no seasonal changes in the expression of nbce1 isoforms. On the other hand, we observed diminished nbce1.2a expression within 2 days after the transfer of parr and smolts to SW that remained low throughout the experiment (>2 weeks). Collectively, our results suggest that while there are no clear seasonal nbce1 dynamics during smoltification, the modulation of nbce1.2a expression upon exposure to SW may underlie the attenuation of environmental Na+ uptake. Support or Funding Information Supported by the National Science Foundation (IOS‐1755131).

Euryhaline fishes maintain hydromineral balance in a broad range of environmental salinities through the functional coordination of multiple tissues, including the gill, intestine, kidney, and urinary bladder. Fishes inhabiting freshwater (FW) environments are faced with the diffusive loss of ions and the osmotic gain of water. In turn, both kidney and urinary bladder support the reabsorption of Na+ and Cl− and the excretion of excess water. While functional responses by these two organs to FW conditions have been previously described, the subcellular pathways for Na+ and Cl− reabsorption are not well understood. We hypothesized that euryhaline tilapia (Oreochromis mossambicus), employ Na+/Cl− cotransporter 1 (Ncc1) and Clc family Cl− channel 2 (Clcn2c) for the active reabsorption of Na+ and Cl− and thus their respective gene transcripts are expressed under FW conditions. In the current study, we report ncc1 and clcn2c gene expression patterns in both steady‐state animals and animals abruptly transferred from seawater (SW) to FW. In kidney and urinary bladder, ncc1 and clcn2c gene expression was markedly enhanced (>30‐fold) in FW‐ versus SW‐acclimated tilapia. By 6 hours after transfer from SW to FW, ncc1 and clcn2c in both kidney and urinary bladder were coordinately elevated compared with controls continuously maintained in SW. These patterns in the kidney and urinary bladder occurred in concert with branchial processes supportive of Na+ and Cl− absorption from the external environment. Collectively, our results suggest that both Ncc1 and Clcn2c contribute to the functional plasticity of kidney and urinary bladder that underlies acclimation to variable environmental salinities. Support or Funding Information Supported by the National Science Foundation (IOS‐1755131 and ‐1755016).