Ultrastructural changes in gill chloride cells during smoltification in wild and hatchery-reared masu salmon Oncorhynchus masou

Abstract

Ultrastructural changes in gill chloride cells during smoltification were examined in wild and hatchery-reared masu salmon. On the filament, two types of chloride cells (α and β) and accessory cells were observed in wild fish from January (parr) to May (full-smolt), while only α and accessory cells (α-a cells) were observed in hatchery-reared fish from March (pre-smolt) to May (full-smolt). Among α-a cells of both types of fish, development of rough endoplasmic reticulum (ER) and Golgi apparatus was detected in pre-smolt before full-smolt. However, ultrastructural changes were not revealed in β cells of wild fish during smoltification. During smoltification, the α-a cell number increased and β cell number decreased. On the lamella, only one type of chloride cell was observed in both wild and hatchery-reared fish during smoltification. Their ultrastructural changes were almost the same as those in filament α-a cells, and their number declined during smoltification. Although there was almost no difference between changes in ultrastructure or in the number of chloride cells in both wild and hatchery-reared fish during smoltification, the chloride cell number of hatchery-reared fish varied widely in comparison to that in wild fish. The present study is the first report for wild salmonids of ultrastructural changes in gill chloride cells during smoltification, and indicated that there were some ultrastructural differences between gill chloride cells of wild masu salmon and hatchery-reared fish.