Most fish are aquatic creatures that suffocate rapidly when removed from water. However, some unique species have adaptations allowing them to breathe air and survive out of water for varying periods. This article explores how fish breathe underwater, why this fails on land, and the fascinating ways some fish have evolved to breathe atmospheric air.
The Mechanics of Gill Respiration
Fish primarily breathe using specialized organs called gills, which are highly efficient at extracting dissolved oxygen from water. Gills are typically located on either side of a fish’s head, protected by a bony cover called the operculum. Each gill consists of numerous delicate structures known as gill filaments, which extend into the water. These filaments are further covered with even smaller, thin plates called lamellae, significantly increasing the surface area available for gas exchange.
Water flows unidirectionally over these gill structures, meaning it enters through the mouth and exits through the gill openings. Within the lamellae, a process called countercurrent exchange takes place. Blood flows through capillaries in the lamellae in the opposite direction to the water flow, ensuring that the blood always encounters water with a higher oxygen concentration. This maintains a continuous concentration gradient, maximizing the diffusion of oxygen from the water into the fish’s bloodstream, allowing gills to extract over 80% of available oxygen.
Why Gills Fail on Land
The highly specialized structure of fish gills, while efficient in water, becomes a severe disadvantage in air. Gills are designed to be supported by the buoyancy of water, which keeps the delicate gill filaments and lamellae separated. When a fish is taken out of water, this crucial support is lost, causing the fragile gill structures to collapse and stick together. This physical collapse drastically reduces the surface area available for gas exchange, making it impossible for the fish to absorb sufficient oxygen from the air.
Beyond structural collapse, desiccation, or drying out, is another significant problem. The thin, moist membranes of the gill lamellae, necessary for efficient gas exchange, rapidly dry out when exposed to air. This desiccation further impairs their function, as gases cannot efficiently diffuse across a dry surface. Consequently, even though air contains a much higher concentration of oxygen than water, most fish suffocate on land because their respiratory system is not adapted to an aerial environment.
Air-Breathing Fish
Not all fish are confined to aquatic respiration; some have evolved the ability to breathe atmospheric air. This adaptation often arises from environmental pressures like stagnant, oxygen-poor water or drought. These air-breathing fish survive conditions lethal to most other aquatic species. Prominent examples include lungfish, mudskippers, and walking catfish. Lungfish possess lung-like organs, enabling them to gulp air from the surface. Mudskippers are amphibious, venturing onto land and breathing air through various means. Walking catfish are also notable for their ability to move across land, facilitated by their air-breathing capabilities.
Diverse Adaptations for Air Breathing
Air-breathing fish have developed various specialized organs and methods to extract oxygen from the atmosphere. Many have evolved modified swim bladders, typically buoyancy organs, into functional lungs. Lungfish, for instance, have highly vascularized swim bladders connected to their esophagus, allowing them to gulp air and absorb oxygen. Some species, like gars and bowfins, also use their swim bladders as auxiliary respiratory organs, especially when water oxygen levels are low.
Other fish utilize their skin for gas exchange, known as cutaneous respiration. This method requires the skin to remain moist and is often found in fish with reduced scales and numerous blood vessels close to the surface, such as mudskippers. Mudskippers absorb oxygen through their moist skin and the lining of their mouths and throats, allowing them to remain out of water.
Some air-breathing fish have modified their gill structures or developed specialized linings in their mouths and pharynx. For example, walking catfish possess suprabranchial organs, tree-like structures above their gills, that prevent collapse and facilitate air uptake. These organs are rich in blood vessels and function like lungs, allowing them to survive in oxygen-deficient environments. Additionally, some species employ buccal pumping, actively gulping air into specialized chambers or organs. This involves rhythmically moving the mouth floor to draw air in and force it over respiratory surfaces.