Some aspects of adaptation in Chanos chanos (Forskal)

Abstract

A study has been made of the adaptive responses of Chanos to variations in external salinity and temperature. In external salinities varying from 0·l% NaCl (hard tap water) to 5·3% NaCl (sea-water plus common salt),Chanos fingerlings retain their blood serum chloride values fairly constant over a narrow range of 0·3% NaCl. In dilutions of tap water corresponding to a chloride content of 5 p.p.m. and less, the serum chloride values suggest a breakdown of the osmoregulatory mechanism. The period of survival of Chanos fingerlings in glass-distilled water is prolonged if they have been preacclimatised to fresh-water or if sodium or calcium chloride has been added to the medium. The serum chloride values for fingerlings kept in solutions of calcium chloride (144 p.p.m. of Ca) indicate that calcium in the external medium aids the fish in the retention of blood salts. The chloride and calcium contents of natural fresh-waters appear to be of critical significance when Chanos is transferred direct from high salinities. The heat death temperatures of Chanos fry and fingerlings (43 and 39° C. respectively) are higher than those reported for marine fishes. The heat death temperature of the fry is not significantly influenced by the salinity of the medium, the per cent, survival of the fry at any temperature being dependent only on the temperature of the water and the time of exposure. The fry and fingerlings are able to withstand momentary exposures to 18°C. and 14°C. respectively, both in sea-water and in tap water.