By D. Calamari, R. Marchetti, G. Vailati
Water Research, Volume 14, Issue 10, 1980, Pages 1421-1426, ISSN 0043-1354, DOI: 10.1016/0043-1354(80)90006-8.

Acute toxicity of cadmium to Salmo gairdneri is increased by a reduction in water hardness. The role of the chemical species of the metal in the intoxication processes in waters of different levels of hardness, is considered as well as the reasons for explaining the observed effects by biological mechanisms.
The theoretical distribution of the chemical species of cadmium in water of different level of hardness (320, 80 and 20 mg CaCO3 1-1) at pH 7.2 was calculated. The results show that similar concentrations of the same form of cadmium (Cd2+) gave different levels of mortality and that similar acute toxic effects were caused by different amounts of ionic form (Cd2+). Fish acclimated at 320 mgCaCO3 1-1 but tested at 20 mg CaCO3 1-1 reacted in an intermediate way, confirming the importance of the biological hypothesis.
Chloride cell proliferation in gills is a common defence response to intoxication processes. Thus the presence of a higher number of chloride cells in fish acclimated to hard water would have explained the lower sensitivity to cadmium. Fish kept in water with a wide difference in hardness and acclimated, so as to have different ionic contents in the blood, had an equal number of chloride cells in the gills. A detoxification mechanism based mainly on the increase in the number and activity of chloride cells should therefore be independent per se, of hardness and rather related to the presence of metals. On the contrary, the possibility of action by cadmium could depend upon the role of calcium in regulating gill permeability.

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