ENVIRONMENTAL-EFFECTS ON FISH GILL MORPHOLOGY

Fish gill morphology and function are intricately related. Adjustments in gill morphology during adaptation of fish to environmental changes are often instrumental in conserving physiological functions. But optimization of one particular gill function by an environmentally induced morphological modification may be detrimental to other gill functions. Moreover, changes in gill morphology in response to a particular environment may not always be beneficial and indeed may contribute to the death of fish in polluted or acidified waters. In this paper we discuss both adaptive and nonadaptive changes in gill morphology when fish encounter changing environments. Emphasis is placed on recent findings concerning environment oxygen, ionic composition, and xenobiotics. An assimilation of the results of these studies reveal a common response to widely different environmental disturbances (e.g., hyperoxia, hypercapnia, acidification, low [NaCl], low [Ca2+], freshwater-to-seawater transfer), namely branchial chloride cell proliferation. We suggest therefore that the chloride cell is multifunctional and plays a role in Ca2+ uptake, acid-base balance, NaCl uptake in freshwater fish, and NaCl excretion in seawater fish.