Despite the fact that aquatic organisms are mostly poikilothermic and environmental temperature variations can have considerable impact on chemical toxicity, toxicity studies are mainly performed at the species' specific standard or optimal temperature. Since the zebrafish is a recommended test species for use in toxicity tests, we investigated the temperature dependence of 96 h cadmium accumulation and toxicity in zebrafish acclimated to 18, 26, 30 or 34°C. Zebrafish showed high cadmium tolerance with acute 96 h LC50 values of 121.5, 102.4, 124.6 and 126.7 μM at 18, 26, 30 and 34°C respectively. Differences in cadmium toxicity at the different temperatures were small and toxicity did not increase with increasing temperature as is often suggested. We did however observe an interesting concentration dependent crossover pattern in which the temperature dependence at the highest exposure concentrations was exactly opposite to the pattern at the lower concentrations. At the highest concentrations the following order of toxicity was observed: 26°C>18°C>30°C>34°C. Possibly, either the warm acclimation provoked a general stress response which protected organisms against future severe stress situations, or resulted in specific defence mechanisms which also provided protection against cadmium exposure. Although at 18°C cadmium accumulation decreased more than would be expected based on the metabolic rate, cadmium toxicity was not proportionately decreased. This increased cadmium sensitivity in the cold was likely due to the combined effect of low temperature and cadmium exposure on sodium loss. This study shows that the temperature dependence of cadmium toxicity results from the combination of altered cadmium accumulation and sensitivity. Inclusion of the temperature effect in the calculation of environmental quality standards may have to be considered to ensure that more sensitive species are also protected at suboptimal temperatures.
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