Fishes “breath” through gills. Gills, or branchiae, are highly vascularized organs specialized in gas exchange under water and present in aquatic animals (marine annelids, crustaceans, fishes and tadpoles). Gills are a respiratory organ (analogous, for example, to lungs) containing very thin lamellae with many apparent blood vessels in direct contact with water. In osteichthyes the gills are covered by a bony flap that protects them called operculum. In chondrichthyes there are no opercula. Fish Facts - Image Diversity: gills