Gills

Gills are respiratory organs specialized for extracting dissolved oxygen from water and releasing carbon dioxide. They are found in most aquatic vertebrates—including fish and many amphibians—and in a variety of invertebrates such as some mollusks and crustaceans. In vertebrates, gills are typically paired organs located in the pharyngeal region behind the mouth, and they enable gas exchange while the animal remains largely aquatic.

In fish, gills are arranged on gill arches on either side of the pharynx. Each gill arch bears gill filaments, which in turn bear numerous microscopic secondary lamellae that increase surface area. The thin epithelium of the lamellae is highly vascularized, containing a dense network of capillaries.

Gas exchange operates by countercurrent exchange: blood in the gill capillaries flows in the opposite direction to the water passing over the gill surfaces, maximizing oxygen uptake and carbon dioxide removal. Water is moved across the gills by the fish's mouth and operculum (gill cover) through buccal-opercular pumping; some species use ram ventilation, drawing water in as they swim with mouths open.

Developmentally, gills arise from pharyngeal arches during embryonic development and have evolved multiple times in different lineages. External gills, such as those seen in larval salamanders or aquatic larvae of some amphibians, are projections that remain outside the body, while other gills are internal.

Gill structure and function can vary widely among species, reflecting adaptations to different aquatic environments, such as fast-flowing rivers, stagnant water, or high-altitude lakes. Gills are sensitive to water quality, temperature, and pollutants, and gill health is a common indicator of aquatic health in ecological assessments.