SALINITY TOLERANCE AND OSMOREGULATION IN THE SILVER PERCH, BIDYANUS-BIDYANUS MITCHELL (TERAPONIDAE), AN ENDEMIC AUSTRALIAN FRESH-WATER TELEOST

Juvenile silver perch, Bidyanus bidyanus, were subjected to direct tra nsfer from fresh water to various test salinities. No mortality was ob served when the fish were transferred from fresh water to a salinity o f 12, but 40% mortality was observed at a salinity of 15 after seven d ays. Pre-acclimation of silver perch to a salinity of 12 for seven day s resulted in only marginally better survival at higher salinities. Pl asma osmotic concentrations of silver perch rose slightly in salinitie s below 9 but rapidly at higher salinities, following the same track a s the iso-osmotic line. Minimum body water content was observed in ind ividuals subjected to a salinity of 15 for 24 h. As found in other fre shwater teleosts, chloride cells were found in the branchial epitheliu m of silver perch. Accessory cells were observed beside the chloride c ells in both freshwater and salt-water conditions. Fish subjected to a salinity of 12 for seven days showed chloride cells with a more devel oped tubular system than controls. The length of the junctions between chloride cells and accessory cells was significantly shorter in fish adapted to a salinity of 12 than in controls. The ultrastructural feat ure of 'interdigitations' of accessory cells was not observed during s alt-water adaptation. These data indicate that silver perch is the lea st tolerant of high salinities and the most truly freshwater Australia n teleost species examined to date.