Acid tolerance of Japanese dace (a cyprinid teleost) in Lake Osorezan, a remarkable acid lake

The Japanese dace (Tribolodon hakonensis) is the only teleost species that inhabits Lake Osorezan, a remarkable acid lake in Japan with the water pH o f 3.4-3.8. In the present study, physiological changes following transfer o f the dace acclimated to neutral stream water (pH 6.8-7.2) to acid lake wat er (pH 3.6-3.7) were examined with special reference to changes in gill chl oride cell morphology. The dace survived direct transfer to acid lake water for 3 days. Plasma [Na+] and [Cl-] showed transient decreases at 1 hr afte r transfer; however, the decrease in [Na+] was greater than that in [Cl-]. The recovery of [Cl-] was more evident than that of [Na+]. The transient de creases of plasma [Na+] and [Cl] were followed by acidosis. Blood pH was de creased by 0.13 unit at 6 hr, but partially restored by 24 hr. In the dace acclimated to neutral stream water, chloride cells were scattered in the gi ll filament. Following transfer to acid lake water, however, well-developed chloride cells were arranged in a radial fashion, forming follicular or gl and-like structures in the gills. Each chloride cell was equipped with an a pical pit, which faced a common lumen leading to the external environment. These findings provide morphological evidence for a significant role(s) of gill chloride cells in ion and acid-base regulation in the acid-tolerant da ce.