Fish, like all living organisms, require oxygen to survive. While humans and other land animals extract oxygen from the air using lungs, most fish have developed a highly efficient system to obtain oxygen dissolved in water. However, some fish species possess remarkable adaptations that allow them to breathe atmospheric air, particularly in challenging environments.
How Fish Breathe Water
Most fish breathe underwater through specialized organs called gills. These structures are positioned on either side of a fish’s head, protected by a bony flap known as the operculum. Water, rich with dissolved oxygen, enters through the fish’s mouth and is then pumped over the gills.
The gills are composed of comb-like filaments, covered by plate-like structures called lamellae. Each lamella contains a dense network of capillaries. This arrangement provides a large surface area for gas exchange, maximizing the amount of oxygen a fish can extract from the water.
Within the lamellae, a process called countercurrent exchange occurs, enhancing oxygen absorption. Blood flows through the capillaries in the opposite direction to the water passing over the gills. This creates a constant concentration gradient, ensuring that oxygen diffuses from the water, where it is more concentrated, into the fish’s bloodstream, where it is less concentrated. Once oxygen is absorbed, carbon dioxide, a waste product, diffuses from the blood into the water and is expelled from the fish’s body through the gill openings.
Fish That Also Breathe Air
While most fish rely solely on gills for respiration, certain species have evolved mechanisms to breathe atmospheric air. These adaptations often allow them to survive in habitats with low dissolved oxygen levels or during periods out of water. This ability has evolved independently in fish lineages.
One example is the lungfish, which possesses one or two lung-like organs derived from a modified swim bladder. These internal structures have honeycomb-like cavities lined with blood vessels, enabling gas exchange similar to terrestrial lungs. African and South American lungfish are obligate air-breathers that must periodically surface to gulp air, and some can even survive prolonged dry periods by burrowing into mud.
Another group, known as labyrinth fish, includes popular aquarium species like bettas and gouramis. These fish have a specialized “labyrinth organ” located in a chamber above their gills. This vascularized structure allows them to absorb oxygen directly from gulped air, providing an advantage in stagnant, oxygen-poor waters. Some air-breathing catfishes, such as armored suckermouth catfishes, have even modified stomachs that function as respiratory organs. Mudskippers, amphibious fish, can breathe through their skin and the lining of their mouths and throats when out of water.
Beyond the Gills
Fish primarily obtain oxygen from water using their gills. However, the diverse world of fish includes species that have developed additional respiratory strategies. These air-breathing adaptations, such as modified swim bladders and specialized organs, highlight the evolutionary flexibility of fish. They demonstrate how different species have overcome environmental challenges, enabling them to thrive in various aquatic and even semi-aquatic environments.