kieuwdeksel

Kieuwdeksel, or operculum, is a curved bony plate on each side of a fish’s head that covers and protects the gills. In most bony fishes the operculum forms part of a gill-cover apparatus built from several bones, including the main opercular plate (opercle) and adjacent elements such as the subopercular and preopercular bones.

The operculum serves two primary roles: protection of delicate gill tissue and participation in respiration. Most fishes breathe through buccal-opercular ventilation, in which water is drawn into the mouth, passes over the gills, and exits through the gill openings as the operculum moves. The movement of the operculum is driven by muscles attached to the gill-cover bones, enabling a pumping action that helps push water across the gill filaments.

Variation and distribution: The operculum is a characteristic feature of most bony fishes (osteichthyes). Cartilaginous fishes such as sharks and rays lack a full opercular cover and have gill slits that are not fully shielded. Among bony fishes, the shape, size and ornamentation of the operculum vary considerably and can reflect differences in protection, hydrodynamics and respiratory efficiency.

Evolution and significance: The operculum is an important adaptation that allows independent regulation of water flow over the gills, supporting efficient gas exchange and often enabling higher activity levels in diverse aquatic environments. The Dutch term kieuwdeksel simply means gill lid.