Salinity Tolerance and the Role of Cortisol during Osmotic Adjustments of an Air-Breathing Catfish, <i>Clarias batrachus</i>

Abstract

Sudden transfer of freshwater catfish, Clarias batrachus to media of various salinities ranging from 10-100% sea water (SW) establishes that it is stenohaline fish with upper limit of tolerance up to 30% SW (298 mmol/kg) which gets marginally elevated upon its acclimation in sub-lethal salinities. No appreciable change in plasma osmolarity following transfer of the catfish to 15% SW (132 mmol/kg) even after 14 days reveals that it can actively osmoregulate up to 15% SW. Unlike other teleost, this fish lacks the bimodal adaptive and regulatory phases as evident from the increase in the plasma osmolarity within 0.5 h of transfer to higher salinities which remained elevated throughout the experiment. The changing profile of plasma cortisol following transfer to higher salinities suggests the involvement of this hormone in SW adaptation.