Ultrastructural characterization of gills in juveniles of the Argentinian Silverside, Odontesthes bonariensis (Valenciennes, 1835) (Teleostei: Atheriniformes).

An ultrastructural study was performed on the gills of juvenile Argentinian silverside, Odontesthes bonariensis. The gills are composed of two sets of four holobranchs and, in turn, each holobranch consists of a gill arch and two rows of caudolaterally projecting branchial filaments. From the dorsal and ventral surfaces of each filament, branchial lamellae radiate out as foldings of the epithelial layer. Gill rakers are present on each of the gill arches, on the anteromedial side of the arch opposite to the filaments. Gill rakers, gill arches and branchial filaments are covered by a stratified epithelium, whereas branchial lamellae essentially consist of a thin epithelial envelope containing capillaries. In the stratified epithelium, mucous cells, rodlet cells, granular cells, pavement epithelial cells and mitochondria-rich cells are identified. The thin epithelium that lines the lamellae comprises two cell types, outer and inner epithelial cells, and the capillary walls on the inside of the epithelial envelope are defined by pillar cells. The ultrastructure of all these cell types is described and our findings are discussed in light of the existing data on fish gill morphology. In the gills of juvenile Argentinian silverside is of particular interest the characteristics showed by mitochondria-rich cells, such as their arrangement in clusters of 2-3 cells and their small and depressed surface in contact with the aquatic milieu, features which strongly resemble those of euryhaline species.