BRANCHIAL MORPHOLOGICAL AND ENDOCRINE RESPONSES OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) TO A LONG-TERM SUBLETHAL ACID EXPOSURE IN WHICH ACCLIMATION DID NOT OCCUR

Changes in branchial morphology and in plasma cortisol, adrenaline, and noradrenaline were quantified throughout an 81-d exposure of rainbow trout (Oncorhynchus mykiss) to sublethal acidity (pH 4.8) in artificial soft water and after a 5-h acid challenge (pH 4.0) of naive fish and 81-d acid-preexposed fish. Changes in branchial morphology at pH 4.8 were generally very mild and characterized by slight increases in filamental mucous cells and decreases in lamellar mucous cells. Chloride cell numbers and branchial Na+-K+- and total ATPase activities did not change. The filamental epithelium thickened, but the water-blood diffusion distance in the lamellae decreased during chronic exposure. Cortisol was significantly elevated throughout whereas catecholamines exhibited relatively little response. Response to acute pH 4.0 challenge was similar in naive and 81-d acid-exposed fish: epithelial damage, increase in visible mucous cells, loss of chloride cells by necrosis, and high cortisol levels but no changes in lamellar or filamental epithelial thickness, diffusion distance, ATPase activities, or catecholamine levels. Previously reported physiological data from these same trout demonstrated that sensitization rather than acclimation had occurred. Therefore, these observations support the view that acclimation does not occur in the absence of significant branchial damage and repair.