Conklin, D. J., Mowbray, R. C., and Gingerich, W. H., 1992, Effects of chronic exposure to soft, acidic water on gill development and chloride cell numbers in embryo-larval brook trout, Salvelinus fontinalis: Aquatic Toxicology, v. 22, no. 1, p. 39-52. Abstract In order to evaluate the effects of acid exposure on the respiratory and ionoregulatory systems of developing brook trout, Salvelinus fontinalis , differences in gill morphology and numbers of chloride cells were compared between groups of developing embryo-larval fish continuously exposed to moderately hard well water (130.0 mg/l as CaCO sub(3), pH 7.94) or to reconstituted soft, acidic water (4.4 mg/l as CaCO sub(3), pH 5.25) designed to mimic acidic waters of northern Wisconsin acidified lakes. Exposures were maintained for up to 48 days (82 days after fertilization) during critical periods of growth and differentiation of branchial structures. The second right gill arch of each fish was examined for changes in the development of filaments and lamellae and for differences in numbers of chloride cells. Gills of fish that developed in soft, acidic water contained greater numbers of normal and degenerating chloride cells, exhibited hyperplasia of primary epithelium and multiple fusions of adjacent filaments and lamellar epithelium than the gills of control fish. Filament and lamellar lengths and numbers of lamellae per filament were significantly less (P < 0.05) in fish that developed in soft, acidic water than in fish exposed to well water. Keywords acidity, freshwater-fish, developmental-stages, gills, osmoregulation, toxicity-tests, acids, morphology, Salvelinus-fontinalis, cell-morphology, water-quality; chronic-toxicity; hardness-; chloride-cells