Responses of skin mucous cells to aluminum exposure at low pH in Atlantic salmon (Salmo salar) smolts

Atlantic salmon (Salmo salar) smelts were exposed for 80 h to seven water qualities: pH 5.6 with 31 and 46 mu g labile Al . L-1, pH 6.0 with 18 and 24 mu g labile Al . L-1, and pH 6.2 with 12 and 18 mu g labile Al . L-1 and control water at pH 6.8 and <10 mu g labile Al . L-1. The three groups with the highest concentrations of labile Al (31 and 46 mu g labile Al . L-1 at pH 5.6 and 24 mu g labile Al . L-1 at pH 6.0) suffered high mortalities and showed a disturbance in osmoregulation and a massive secretion of mucus, as seen from a decrease in number of skin mucous cells. Furthermore, an increase in skin mucous cells containing acidic mucosubstances was observed. The loss of plasma chloride and skin mucous cells showed a significant linear correlation (R-2 = 0.68, p < 0.001). The increased secretion of mucus on skin and gills and the increase in acidity of mucosubstances are consistent with their prior presumed defensive role in binding of Al.