Distinct seawater and freshwater types of chloride cells in killifish, Fundulus heteroclitus

Physiological and morphological differences between killifish adapted to seawater (SW) and fresh water (FW) were examined with special reference to chloride cells. There was no difference in plasma osmolality between SW- and FW-adapted fish, reflecting their euryhalinity. A rich population of chloride cells was detected in whole-mount preparations of the gills and opercular membrane from SW- and FW-adapted fish. There was no difference between SW- and FW-adapted fish in gill Na+,K+-ATPase activity or oxygen-consumption rates. The gill chloride cells were located mostly in a flat region of the afferent-vascular edge of the filaments. In both tissues, the cells were larger in FW- than in SW-adapted fish. The apical membrane of chloride cells was invaginated to form a pit in SW-adapted fish, whereas it was flat or showed projections and was equipped with microvilli in FW-adapted fish. Chloride cells often interdigitated with neighboring accessory cells in SW-adapted fish, forming multicellular complexes. In FW-adapted fish, on the other hand, a pair of chloride cells that were similar in size was occasionally associated to form twin cells. Thus, distinct SW and FW types of chloride cells were defined. Our findings suggest that SW- and FW-type chloride cells are equally active in the two environments, but exhibit different ion-transporting functions.