The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body.
The operculum is composed of four fused bones; the opercle, preopercle, interopercle, and subopercle. The posterior rim of the operculum is equipped with a flexible, ribbed structure which acts as a seal to prevent reverse water flow during respiration. The morphology of this anatomical feature varies greatly between species. For example, the bluegill (Lepomis macrochirus) has a posteriorly and dorsally oriented rounded extension with a small black splotch present. In some species, the operculum can push water from the buccal cavity through the gills.
For some fish, opercula are vital in obtaining oxygen. They open as the mouth closes, causing the pressure inside the fish to drop. Water then flows towards the lower pressure across the fish's gill lamellae, allowing some oxygen to be absorbed from the water.
Whereas the cartilaginous ratfishes have soft and flexible opercular flaps, the sharks, rays and relatives (elasmobranch fishes) lack opercula completely and respire instead through a series of gill slits perforating the body wall. Without opercula, other methods of getting water to the gills are required, such as ventilation.