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 variegatus, 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 obtained from a Gulf of Mexico salt marsh near Cedar Key, Florida. In a steady state experiment, routine metabolic rate and critical oxygen tension were determined at salinities ranging from 0 parts per thousand to 100 parts per thousand. Measures of routine metabolic rate and critical oxygen tension were unaffected by changes in salinity between 0 parts per thousand and 40 parts per thousand. However, routine metabolic rate declined and critical oxygen tension increased progressively at salinities above 40 parts per thousand. The reduction in routine metabolic rate and rise in critical oxygen tension correlates with a reduced ability of C. variegatus to osmoregulate effectively at high salinities. The variations in routine metabolic rate and critical oxygen tension at high salinities suggest that C. variegatus responds to high salinities by reducing energy expenditures, effectively increasing the time that individuals can tolerate hypersaline conditions.