A fish out of water usually dies because its gills collapse, reducing the area of the respiratory surface, and become dry, effectively stopping the diffusion of oxygen into the blood. Many fishes, however, have evolved methods of extracting oxygen from air. Such adaptations permit these fishes to live in oxygen-poor waters, where they come to the surface to gulp air, or in waters subjected to drying; or they may enable a species to exploit environments, such as damp beaches, unavailable to other fishes. One method of air breathing is the development of gills that either secrete mucus or trap moisture that supports the gills and keeps them wet. Another method is to breathe through a damp skin, as do the freshwater eels. Very commonly, special chambers have been developed in the mouth, throat, or head in which inspired air is brought into contact with moist tissues richly supplied with blood vessels. Some fishes have thin-walled areas in the intestine where oxygen can be extracted from swallowed air. In still others, the swim bladder, often mistaken for a lung because of its inflated shape and shiny, silvery white walls, is modified into an air-breathing apparatus. Air breathing has become so important to some species that they will drown if not allowed access to air.
Distribution - Anatomy - Circulation -
Body Temperature - Water Balance - Swimming - Gas Bladder
Lateral Line System - Evolution - Reproduction
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