Osmoregulatory mechanisms, ion transport proteins, and gill physiology in euryhaline fish adapting to varying salinity conditions

Abstract

Euryhaline fish demonstrate remarkable capacity to maintain ionic homeostasis across wide salinity ranges through rapid physiological adjustments in gill ion transport. This research examined molecular and cellular mechanisms underlying salinity adaptation in Japanese sea bass (Lateolabrax japonicus) transferred between freshwater and seawater environments. Expression of Na+/K+-ATPase, NKCC1, and CFTR ion transporters was quantified by immunohistochemistry and Western blotting at multiple time points following salinity transfer. Chloride cell morphology was assessed by scanning electron microscopy and confocal immunofluorescence imaging. Seawater transfer induced 2.8-fold increase in branchial Na+/K+-ATPase activity within 72 hours with corresponding upregulation of alpha-subunit protein expression. NKCC1 cotransporter protein showed 3.1-fold induction while freshwater-associated NCC transporter decreased to 0.3-fold of freshwater values. Chloride cell density increased from 1,247 to 2,034 cells per mm2 of gill filament epithelium and apical pit morphology changed from small accessory cell-covered apertures to large open pits characteristic of secretory ionocytes. Plasma osmolality was transiently elevated at 24 hours post-transfer but restored to baseline by 72 hours, coinciding with peak transporter expression. These findings elucidate fundamental mechanisms enabling euryhaline adaptation with implications for aquaculture management in fluctuating salinity environments.