gillbreathing

Gillbreathing refers to respiration carried out by gills, specialized extensions of the pharyngeal region that extract dissolved oxygen from water and release carbon dioxide. In aquatic animals, water passes over thin-walled gill filaments and their flattened lamellae, where gas exchange occurs across a large surface area.

In most fish, gills are arranged on gill arches and covered by an operculum; gill filaments bear lamellae that increase the surface area available for diffusion. Blood flows through the gill capillaries in a direction opposite to water flow (countercurrent exchange), which maximizes oxygen uptake. Ventilation is typically achieved by buccal pumping or by ram ventilation during swimming. Some amphibians and many aquatic invertebrates possess external gills or gills housed in a mantle or body cavity.

Gillbreathing occurs in fishes and many aquatic larvae; adult amphibians rely on lungs or skin but may retain gill-based respiration during larval stages. In mollusks and crustaceans, gills serve both respiratory and, in some taxa, feeding or filtering roles. Respiratory efficiency depends on water oxygen content, temperature, flow, and the structural arrangement of filaments and lamellae. Compared with lungs, gills extract dissolved oxygen directly from water, enabling life in submerged environments but with different performance constraints.