Relationships of Salinity Tolerance to Immunolocalization of Na+,K+-ATPase in the Gill Epithelium during Seawater and Freshwater Adaptation of the Guppy, Poecilia reticulata

The relationships of salinity tolerance to immunolocalization of Na+,K+-ATPase in the gill epithelium were examined during seawater and freshwater adaptation of the guppy. In fresh water, immunoreactivity for Na+,K+-ATPase appeared in two types of chloride cells, which are located on the primary lamellae of the gills. Immunoreactivity was strong in the chloride cells located at the base of the secondary lamellae and weak in the chloride cells located at the interlamellar region. During seawater adaptation, the strongly-immunoreactive chloride-cells increased in number and size while the weakly-immunoreactive chloride-cells decreased in number with an increase in salinity tolerance. In the fish of the seawater-adapted strain, on the other hand, most of the chloride cells were located at the base of the secondary lamellae and showed strong immunoreactivity. During freshwater adaptation, the strongly-immunoreactive chloride-cells decreased in number and size while the weakly-immunoreactive chloride-cells increased in number with a decrease in salinity tolerance. A positive correlation was observed between the salinity tolerance and the occupying area of the strongly-immunoreactive chloride-cells while a negative correlation was observed between the salinity tolerance and the occupying area of the weakly-immunoreactive chloride-cells during the seawater and freshwater adaptation. These results directly suggested that not only the occupying area of chloride cells but also the expression of Na+,K+-ATPase protein in the cells is important with respect to the osmoregulatory function in the gills and hypoosmoregulatory ability at the individual level.