Copper toxicity across salinities from freshwater to seawater in the euryhaline fish Fundulus heteroclitus: is copper an ionoregulatory toxicant in high salinities?

Two waterborne Cu exposures were performed to investigate if Cu is an ionoregulatory toxicant at all salinities in the killifish, Fundulus heteroclitus. A 30-day flow through exposure in 0 (FW), 5, 11, 22, and 28 ppt (SW) and three [Cu]'s (nominal 0, 30, and 150 microg Cu L(-1)) revealed no apparent Cu induced mortality at the intermediate salinities and high mortality in FW and SW. Fish were sampled at 4, 12, and 30 days after the start of the exposure and both Na+/K+ adenosine triphosphatase (Na+/K+ ATPase) and carbonic anhydrase (CA) activity in the gill and intestine as well as whole body [Na+], and [Cl-] were measured. At the high [Cu] a reduction of whole body [Na+] after 4 days of exposure in FW was the only physiological parameter influenced. A second static 24h Cu exposure was performed in FW, 5, 13, and 29 ppt (SW) and two [Cu]'s (nominal 0 and 110 microg Cu L(-1)). In addition to the parameters listed above, ammonia flux was measured at all salinities and Na+ flux was measured in FW fish. Cu affected ionoregulation in FW where decreased Na+ uptake associated with inhibition of Na+/K+ ATPase led to decreased whole body [Na+] after 24h. The only affected parameter in SW was net ammonia excretion suggesting that Cu is not an ionoregulatory toxicant in SW at the concentrations employed. We propose that physiology rather than chemistry explain much of the variation in Cu toxicity seen across salinities.