INFLUENCE OF ENVIRONMENTAL SALINITY ON ROUTINE METABOLIC-RATE AND CRITICAL OXYGEN-TENSION OF CYPRINODON-VARIEGATUS

We examined the influence of a wide range of environmental salinities on routine metabolic rate and critical oxygen tension in Cyprinodon va riegatus, a common coastal resident of the western Atlantic Ocean and Gulf of Mexico. C. variegatus lives in ambient salinities ranging from freshwater to 142 parts per thousand, with fish used in this study ob tained from a Gulf of Mexico salt marsh near Cedar Key, Florida. In a steady state experiment, routine metabolic rate and critical oxygen te nsion were determined at salinities ranging from 0 parts per thousand to 100 parts per thousand. Measures of routine metabolic rate and crit ical oxygen tension were unaffected by changes in salinity between 0 p arts per thousand and 40 parts per thousand. However, routine metaboli c rate declined and critical oxygen tension increased progressively at salinities above 40 parts per thousand. The reduction in routine meta bolic rate and rise in critical oxygen tension correlates with a reduc ed ability of C. variegatus to osmoregulate effectively at high salini ties. The variations in routine metabolic rate and critical oxygen ten sion at high salinities suggest that C. variegatus responds to high sa linities by reducing energy expenditures, effectively increasing the t ime that individuals can tolerate hypersaline conditions.