Adaptation to various aquatic environments and functional diversity of chloride cells in Fish

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In the present study, adaptation to various aquatic environments was examined in three teleost species. Euryhaline species such as killifish and tilapia are adaptable to both freshwater (FW) and seawater (SW) environments, whereas acid-tolerant dace in Lake Osorezan can survive in the acidic lake water of pH 3.6-3.7. The functional diversity of chloride cells is importantly involved in fish adaptation to various environments with different ion composition and concentration.1. Morphological differences between SW - and - FW - adapted killifish were examined with special reference to chloride cells. A rich population of chloride cells was detected in the whole-mount preparations of the gills and opercular membrane in SW and FW fish. In both tissues, the cells were larger in FW than in SW. The apical membrane of chloride cells was invaginated to form a pit in SW fish, whereas it was flat or showed projections and equipped with microvilli in FW. Our findings suggest that SW - and FW - type chloride cells are equally active in both environments, but exerting different ion-transporting functions.2. We succeeded in identifying Na^+/H^+ -exchanger (NHE3) in the gills of Osorezan dace adapted to acid environment. NHE3 is an ion-transporting protein that uptakes Na^+ in exchange of H^+ secretion. The expression of NHE3 in the gills was greater in acidic environment than in neutral water. Immunocytochemical staining showed that NHE3 was localized in the apical membrane of chloride cells. Thus, NHE3 is considered to function as a key molecule for acid-tolerant mechanism in fish.3. When tilapia was exposed to acide condition (pH 4.5), the apical membranes was expanded in some population of gill chloride cells. Such morphological alteration was not observed in the fish maintained in neutral water. As is the case in dace, we speculate that chloride cells play a central role in adaptation to acidic environments in tilapia.