Gill morphometry and fish osmoregulation

Morphofunctional parameters were studied during adaptation of rainbow trout (Oncorhynchus mykiss) to different ionic environments: Strasbourg tap water, ion-poor water, and artificial seawater. The gill lamellae displayed large changes in size. Surface area of individual lamellae increased in trout acclimated to ion-poor water or seawater. Conversely, the harmonic mean thickness of the lamellar epithelium decreased in seawater, and to an even greater extent in ion-poor water. The apical surface area of individual branchial filament chloride cells, the number of these cells, and their apical surface density per unit of filament epithelial surface area were calculated in these three conditions. These variables did not differ significantly in Strasbourg tap water or seawater, but increased greatly in ion-poor water. These results are discussed in relation to gill permeability and ionic regulation in fish.