The Effects of Temperature and Salinity Acclimation on Metabolic Rate and Osmoregulation in the Pupfish Cyprinodon salinus

Abstract

Alterations in standard metabolic rate (.ovrhdot.VO2), plasma osmolality and critical thermal maxima were examined in pupfish, C. salinus, acclimated to freshwater and to Salt Creek [Death Valley National Monument, Inyo County, California, USA] water adjusted to salinities of .apprx. 1/2 sea water (1/2 SW; 560 mosm/kg) and sea water (SW; 1168 mosm/kg). Three acclimation temperatures, 15, 25 and 30.degree. C were maintained at each salinity. The .ovrhdot.VO2 of 1/2 SW and SW fish was significantly greater than that of freshwater (FW) fish at each acclimation temperature. .ovrhdot.VO2, measured at the acclimation temperature, increased in FW and 1/2 SW fish as acclimation temperature increased. The .ovrhdot.VO2 of SW fish was not significantly different between the 25-30.degree. C acclimation groups. Thus, SW fish appear to fully compensate for the effects of increased temperature between 25-30.degree. C. Plasma osmolality increased significantly with acclimation to increased salinity. As the acclimation temperature was increased from 25-30.degree. C the plasma osmolality of the SW fish dropped significantly and that of the FW fish rose significantly. Plasma osmolality of the 1/2 SW fish was statistically identical at each acclimation temperature. The ability to osmoregulate in hypersaline environments then requires the elevated temperatures associated with this condition. This is consistent with natural history observations on C. salinus which only encounters high salinities during summer months when high temperatures occur. The critical thermal maximum (CTM) was significantly elevated by increasing acclimation temperature at each salinity. The 1/2 SW acclimated fish had higher CTM values than FW acclimated fish at each acclimation temperature. CTM was not further elevated by acclimation to salinities higher than 1/2 SW.