Acid-Base Regulation in Three Marine Teleosts: The Oyster Toadfish (\u3cem\u3eOpsanus tau\u3c/em\u3e), the Winter Flounder (\u3cem\u3ePseudopleuronectes americanus\u3c/em\u3e), and the Long-Horned Sculpin (\u3cem\u3eMyoxocephalus octodecimspinosus\u3c/em\u3e)

Abstract

In this study, three species of marine fish were exposed to a variety of dilute salinities in order to determine what effects exposure to diluted seawater may have on acid-base and ion balance. The oyster toadfish (Opsanus tau) and the winter flounder (Pseudopleuronectes americanus) are considered to be able to live in a broad range of salinities, euryhaline (Evans, 1979). The long-homed sculpin, Myoxocephalus octodecimspinosus, is considered to be a stenohaline fish, not able to withstand drastic changes in ambient salinity (Claibome and Evans, 1988). Initially, toadfish exposed to 20 mM diluted seawater took up ΔH+ from the surrounding environment. However, after seven additional days in this salinity, net transfers returned to control levels. This indicates that toadfish are able to regulate the loss of ΔNH4+ and ΔHCO3- to the environment. Preliminary blood data show that these fish can also regulate [Cl-] loss during exposure to low salinities. In 5 mM diluted seawater, ΔH+ uptake increased approximately three times that observed in the 20 mM group. After an additional four days in 5 mM, net transfers were still significantly below control excretion rates and mortality was noted. Flounder exposed to 20 mM diluted seawater did not show any significant change in excretion rates of the fish that were maintained in a tank of 20 mM diluted seawater an additional week, only one fish survived the entire week. Sculpin were able to withstand exposure to 20 mM diluted seawater for periods of 24 hours. During this 24-hour period fish exhibited a rapid loss of ΔHCO3-, but no change in plasma pH. Sculpin survived 100 mM diluted seawater very well. Following 11 days of exposure to 100 mM seawater, excretion rates were not significantly different from seawater control values. When placed in 20 mM diluted seawater following a long 100 mM adaptation, fish did begin to lose ΔHCO3-, but this rate was significantly lower (p≤0.05) than the rate for fish that were transferred directly from seawater to 20 mM diluted seawater. Adjusting osmolarity and injections of epinephrine did not assist the sculpin in 20 mM diluted seawater. The kidneys are able to make up for 35% of the total ΔH+ lost from the fish. The ability of the toadfish and flounder to adjust acid-base losses in dilute salinities may be the key to their survival. It is interesting that the sculpin is able to maintain a constant pH during the acidosis that occurs in dilute salinities. Our indirect evidence suggests that bone demineralization is a source of the observed acidbase alterations during 20 mM exposure