The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Each gill is supported by a cartilaginous or bony gill arch. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Īll basal vertebrates breathe with gills. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators. Facultative air breathers, such as the catfish Hypostomus plecostomus, only breathe air if they need to and can otherwise rely on their gills for oxygen. Obligate air breathers, such as the African lungfish, are obligated to breathe air periodically or they suffocate. The red gills detached from the tuna head on the leftĪir breathing fish can be divided into obligate air breathers and facultative air breathers. However, recent studies on gill formation of the little skate ( Leucoraja erinacea) has shown potential evidence supporting the claim that gills from all current fish species have in fact evolved from a common ancestor. Previously, the evolution of gills was thought to have occurred through two diverging lines: gills formed from the endoderm, as seen in jawless fish species, or those form by the ectoderm, as seen in jawed fish. Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians. This opening is hidden beneath a protective bony cover called the operculum. However, bony fish have a single gill opening on each side. Some fish, like sharks and lampreys, possess multiple gill openings. The gills push the oxygen-poor water out through openings in the sides of the pharynx. In some fish, capillary blood flows in the opposite direction to the water, causing counter-current exchange. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide.įish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. Gills are tissues that are like short threads, protein structures called filaments. Most fish exchange gases like oxygen and carbon dioxide using gills that are protected under gill covers (operculum) on both sides of the pharynx (throat). It then pumps it over gills so oxygen enters the bloodstream, and allows oxygen-depleted water to exit through the gill slits (right)įish gills are organs that allow fish to breathe underwater. Respiratory mechanism in bony fish The fish draws oxygen-rich water in through the mouth (left).