Osmotic regulation in the Sheepshead Minnow Cyprinodon variegatus Lacépède

Plasma osmotic regulation was evaluated in Cyprinodon variegatus acclimated to salinities from fresh water to 120%. These fish showed roughly stable regulation in the range from 0.3% to 70%. Fish acclimated to fresh water showed an increased plasma concentration over those in 0.3%, thought to result from a higher Ca⁺⁺ level in fresh water. Plasma concentrations rose abruptly at salinities above 70%. Osmotic regulatory characteristics of C. variegatus were compared with data for other extremely euryhaline species with expectation of a graded series of responses, species showing greatest fresh water tolerance demonstrating widest ranges of regulation and with lowest plasma concentrations. Results were not consistent with expectations. The range of ambient salinities over which good regulation of plasma concentration was shown was found to be directly correlated with the magnitude of the salinity tolerance range of all of the species compared. Forms with greater fresh water affinity generally, but not uniformly, showed the lowest plasma concentrations in the region of hyperosmotic regulation.