Drinking rate and oxygen consumption in the euryhaline teleost Aphanius dispar in waters of high salinity.

When euryhaline teleosts are adapted to waters of salinities higher than the osmolality of plasma an increase in drinking rate has been observed (Lotan, 1969; Maetz, 1970; Maetz & Skadhauge, 1968; Potts et al. 1967; Shehadeh & Gordon, 1969). This is to be expected since drinking, and absorption of salt (NaCl) and water through the intestinal wall, is the only way for the fish to get water to make up for the osmotic losses. The cyprinodont, Aphanius dispar, can tolerate very high salinities of the surrounding water (Lotan, 1971). It would be of interest to investigate what permits the adaptation to high salinity. It has been suggested that tolerance to high salinity is determined by the capacity of the intestine to absorb the NaCl (Maetz & Skadhauge, 1968; Skadhauge, 1969). It has also been suggested that the overall osmotic permeability coefficient of the integument goes down during adaptation to higher salinity, since the drinking rate is not augmented proportional to the osmolality difference (Shehadeh & Gordon, 1969; Motais et al. 1969). In the work here reported the drinking rate has been studied in Aphanius adapted to higher salinities, because it is a measure both of the effective osmotic permeability of the integument and of the intestinal salt absorption. The latter has already been studied in vitro (Lotan & Skadhauge, 1972). Since it has been reported recently that drinking rates were low in eels at low temperature (Motais & Isaia, 1972) and during inactivation (Kirsch, 1971), probably caused by a low rate of metabolism, the oxygen consumption was also measured.