PLASMA OSMOTIC REGULATION AND ROUTINE METABOLISM IN THE EUSTIS PUPFISH, CYPRINODON-VARIEGATUS-HUBBSI (TELEOSTEI, CYPRINODONTIDAE)

We assessed the influence of salinity on plasma osmotic regulation and routine metabolism in the freshwater Eustis pupfish Cyprinodon varieg atus hubbsi and compared these results to published data for the widel y distributed, euryhaline sheepshead minnow C. v. variegatus. The Eust is pupfish was able to tolerate salinities twice that of normal seawat er. Plasma osmotic concentration increased with increasing salinity. I n freshwater, Eustis pupfish maintained plasma osmotic concentrations lower than sheepshead minnow. However, this pattern reversed with incr easing salinity. Routine metabolism did not differ between the Eustis pupfish and sheepshead minnow when the effect of body mass was statist ically removed. The Eustis pupfish, although long isolated in fresh wa ters, has retained the physiological plasticity generally characterist ic of the cyprinodontoids.