Morphometrical analysis chloride cell activity in the gill filaments and lamellae and changes in Na+, K+-ATPase activity during seawater adaptation in chum salmon fry

Changes in gill chloride cell morphology and Na+,K+-ATPase activity were examined following transfer of chum salmon (Oncorhynchus keta) fry from fresh water (FW) to seawater (SW). The gill Na+,K+-ATPase activity in chum salmon fry weighing 0.6-3 g had already been increased prior to SW entry, being twice as high as that in late alevin stages (0.3–0.4 g). The activity further increased gradually when the fry (2–3 g) were transferred to SW. Using immunocytochemical staining with an antiserum specific for the α-subunit of Na+,K+-ATPase, chloride cells were detected in both filament and lamellar epithelia in FW fry. The number of chloride cells in the filament was not affected by SW transfer, whereas lamellar chloride cells were significantly decreased. The size of filament chloride cells was 1.8-fold larger than that of FW fish 21 days after SW transfer, whereas there was no change in the size of lamellar chloride cells throughout the experiment. The intensity of immunoreaction gradually increased after transfer to SW in filament chloride cells. Electron-microscopic observations revealed that mitochondria-rich chloride cells were moderately developed in both filaments and lamellae of FW fish. In the SW-adapted fish, well-developed chloride cells were detected only in the filament. These results suggest that chloride cells in the lamella may be the site of ion uptake in FW chum salmon fry and that the well-developed chloride cells in the filament are responsible for salt secretion in SW. The rich population of filament chloride cells, concomitant with enhanced gill Na+,K+-ATPase activity, may explain the strong seawater adaptability in chum salmon fry prior to SW entry. © 1996 Wiley-Liss, Inc.