Osmoregulatory and endocrine relationships with chloride cell morphology and density during smoltification in coho salmon ( Oncorhynchus kisutch)

Abstract

Chloride cell morphology and density in the gill and opercular membrane of coho salmon ( Oncorhynehus kisutch) were examined during the 1982 and 1983 smolt-seasons for structural correlates to smoltification-associated changes in hypoosmoregulatory ability and plasma hormone levels. Although not synchronous in the two years, plasma thyroxine levels displayed two peaks each year. In 1982, changes in gill Na +,K +-ATPase activity were preceded by elevations in plasma thyroxine. In 1983, the increase in enzyme activity corresponded with the April increases in plasma thyroxine and prolactin. Seawater (SW) acclimation experiments showed discrete increases in SW tolerance which coincided with changes in gill enzyme activity. Although these data suggest a relationship between SW tolerance and plasma thyroxine levels, the patterns in the two years were different. A significant peak in plasma prolactin levels in April suggests possible involvement in the parr-smolt transformation. Chloride cells were studied in the opercular membrane, a chloride cell-containing epithelium lining the branchial side of the operculum, and the gill epithelium. Opercular membrane chloride cells (1982), marked with a mitochondrion-specific fluorescent stain, increased nearly two-fold in late May, concurrent with the second increase in gill Na +,K +-ATPase activity and SW tolerance. Gill morphology was examined (1983) with scanning and transmission electron microscopy and light microscopy. Initially, the gill filament surface appeared rough but became smoother during smoltification and rough again toward the end of smoltification. Two mitochondrion-rich (chloride) cell types were present in the gill epithelium during smoltification. The electron-lucent type I cell contained large, circular mitochondria while the electron-dense type II cell contained thin, elongate mitochondria. Ultrastructurally, many type I cells appeared disrupted which may indicate cellular degeneration or a special response of type I cells to fixation. However, it was unusual to find either intact or disrupted type I cells after March. Chloride cell density (type I plus type II) remained constant during smoltification. The inability to distinguish between these cell types with the light microscope may have obscured changes in their individual densities. The data from both the opercular membrane and the gill epithelium and their relationship with gill Na +,K +-ATPase activity suggest that the development of SW tolerance during smoltification is partially due to changes in chloride cell enzyme activity and density. The presence of two mitochondrion-rich cell types is discussed with regard to their possible role in the hypoosmoregulatory changes which occur during smoltification.