Q. How do fishes do gas exchange?

Fishes "breath" through gills, branchiae or Gills, are highly vascularized organs specialized in gas exchange under water and present in aquatic animals (crustaceans, marine annelids, tadpoles and fishes). Gills are a respiratory organ (analogous, for instance, 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 as operculum, in chondrichthyes there are no opercula.