Operculum

The operculum is a bony flap covering and protecting the gills in most bony fishes. Its movement helps draw water across the gills for respiration.

Buccal pumping: A respiratory mechanism in which coordinated movements of the mouth and throat move water across the gills. The operculum works with mouth movements to maintain water flow across the gills.
Fish gill: A respiratory organ that extracts dissolved oxygen from water and releases carbon dioxide. The operculum is the protective cover over this respiratory organ.
Osteichthyes: The vertebrate group comprising bony fishes and their descendants, including tetrapods. The operculum is characteristic of many living bony fishes within this group.
Spiracle: An opening behind the eye in some fishes that can admit water to the gills. Sharks use spiracles for water intake rather than a bony opercular cover.
Countercurrent exchange: An exchange process in which two fluids flow in opposite directions, sustaining a concentration gradient between them. Water moved by the operculum meets blood flowing oppositely in the gill lamellae.
Gill filament: A slender projection from a fish’s gill arch that bears numerous gas-exchanging lamellae. Filaments form the exposed respiratory tissue beneath the operculum.
Placodermi: An extinct group of armored jawed fishes that lived from the Silurian to the Devonian periods. Their gill coverings differed from the familiar bony operculum of many modern fishes.
Gill slit: An external opening through which water leaves or enters a fish’s gill chamber. Sharks expose multiple gill slits instead of covering them with one operculum.
Gill arch: A skeletal support bearing the gill filaments and associated structures in fishes. The operculum covers the gill arches that support the respiratory surfaces.
Lamella: A thin plate-like structure in a fish gill where gases move between water and blood. These delicate surfaces are protected behind the operculum.

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