The effect of salinity on the acute toxicity of cadmium to the tropical, estuarine, hermaphroditic fish, Rivulus marmoratus : a comparison of Cd, Cu, and Zn tolerance with Fundulus heteroclitus

The mangrove-dwelling fish, Rivulus marmoratus, is the only vertebrate that is a synchronous, internally self-fertilizing hermaphrodite. This unique reproductive mode yields offspring with little genetic variation, which offers significant advantages for the use of this species in bioassays. We conducted acute (96 h) LC50 tests of Cd toxicity under four different water chemistry conditions, representing fresh water (low [Ca+Mg] and low [Na+K]), 14 ppt sea water simulated with Cl salts (high [Ca+Mg] and high [Na+K]) and two artificial conditions (high [Ca+Mg], low [Na+K] and low [Ca+Mg], high [Na+K]). Two replicates were conducted at different times for each of the four treatments and the results were very reproducible. The mean LC50's as mg total Cd/L were 2.96 (fresh water), 21.12 (high [Ca+Mg], high [Na+K]), 17.86 (high [Ca+Mg], low [Na+K]) and 12.67 (low [Ca+Mg], high [Na+K]). An additional test in 14 ppt sea water (made up from Instant Ocean salts) yielded a 96h LC50 of 24.48 mg Cd/L, and was thus similar to the high [Ca+Mg]-high [Na+K] treatment, despite some differences in anion and cation concentrations. The degree to which [Ca+Mg], [Na+K] and [Cl] interact to determine Cd toxicity is still unclear, although the role of [Cl] is likely to be equal to or greater than that of [Ca+Mg]. When all solutes are high, it is likely that the formation of a Cd complex with Cl (248 mM) leads to the observed reduction in Cd toxicity in comparison with hard fresh water, not the increased divalent [Ca+Mg] levels.