Osmoregulation of Some Fishes from the Bolgoda Estuary

Abstract

The three species of fish studied here, £troplus maculatus, Etroplus suratensis and Puntius filamentosus are teleosts indigenous to Sri Lanka. The three species are found in abundance in the Bolgoda estuary and specimens for the present study were obtained from this estuary. The sal inity of the lagoon water varies from fresh water to 100% sea water in the different parts of the estuary and fluctuates periodically. Although these fishes are commonly found in both fresh and brackish waters, no work has been reported in respect of their osmoregulatory abilities and salinity tolerances. This paper reports on some aspects of osmoregulation in E. maculatus, E. suratensis and P. filamentosus. The two Etroplus species were able to tolerate salinities ranging from 0-60% sea water, while P.filamentosus was able to tolerate only the narrow range from 0-20%sea water. In all species, sodium salts accounted for most of the osmotic concentration, which is indicative of an evolution in fresh water. Metabolic rates were measured for each group of fish in various salinities to determine the relationship between the medium osmotic concentration and the osmotic work done. E.maculatus showed a significant reduction in energy expenditure at 30% sea water, the concentration isosmotic to its plasma osmotic concentration. E.suratensis and P.filamentosus showed no significant changes in metabolic rates at the different salinities tolerated by them. Salinity preferances were determined for the three species using a horizontal salinity gradient chamber. The Etroplus species were found to prefer salinities ranging from 10-20% sea water, while P. filamentosus was found to prefer fresh water.