Osmotic and volaemic effects on drinking rate in elasmobranch fish

Abstract

An increase in drinking rate of two species of marine elasmobranch fish, Scyliorhinus canicula and Triakis scyllia, acclimated to 80 % sea water was observed following the introduction of 100 % sea water to experimental tanks. The drinking response in both species was found to be maximal within 6 h, and a significant increase was sustained for up to 24 h in T. scyllia. Plasma osmolality was significantly increased within 6 h following introduction of 100 % sea water, and this increase was principally due to elevated plasma Na(+) and Cl(-) concentrations. Administration of 2 mol l(-1) mannitol, 75 % sucrose and vehicle (elasmobranch Ringer) did not induce a significant increase or decrease in the drinking rate of S. canicula. However, injection of 20 % NaCl was found to decrease drinking rate significantly in S. canicula 60 min after administration. Controlled haemorrhage of approximately 5.7 % of total blood volume in S. canicula induced a rapid 36-fold increase in drinking over basal levels. The present study demonstrates a physiological dipsogenesis in response to hypovolaemia in marine elasmobranch fish as part of their overall iso/hyperosmoregulatory strategy.