All living organisms extract oxygen for energy. Humans and other terrestrial animals use lungs to process oxygen from air, but fish have specialized gills designed for aquatic environments. This fundamental difference in respiratory structures explains why fish cannot survive out of water, highlighting distinct adaptations for life in different mediums.
How Fish Breathe Underwater
Fish use gill structures to extract dissolved oxygen from water. Gills are on either side of a fish’s head, supported by bony or cartilaginous gill arches. Each arch has comb-like gill filaments, covered in plate-like lamellae. This creates a large surface area for gas exchange.
Water enters the fish’s mouth and flows over these filaments, where a dense network of capillaries is present. Inside the lamellae, blood flows opposite to the water’s flow, a process called countercurrent exchange. This system maintains a constant concentration gradient, allowing oxygen to diffuse from water into the bloodstream and carbon dioxide to diffuse out, making it a highly efficient method for aquatic respiration.
How Land Animals Breathe Air
Terrestrial animals, such as humans, have lungs for gas exchange with the atmosphere. Air enters the body through the nasal cavity or mouth, traveling down the trachea and branching into smaller airways called bronchi and bronchioles. These passages lead to millions of air sacs known as alveoli within the lungs.
Alveoli are cup-shaped structures with thin walls, surrounded by a dense network of capillaries. Oxygen from inhaled air diffuses across the thin membranes of the alveoli and capillaries into the bloodstream, while carbon dioxide, a metabolic waste product, moves from the blood into the alveoli to be exhaled. This extensive surface area facilitates the efficient uptake of oxygen from the air, which has a higher oxygen concentration than water.
Why Gills Don’t Work in Air
Fish gills are ineffective in air due to their delicate structure and reliance on water for support. When a fish is removed from water, the buoyant force that typically keeps the gill filaments separated is lost. This causes the numerous, thin lamellae to collapse and stick together, similar to the pages of a wet book when it dries. This collapse drastically reduces the functional surface area available for gas exchange, making it impossible for the fish to absorb sufficient oxygen.
Another significant issue is desiccation, or drying out. Gills are designed to be constantly moist, and their thin, delicate membranes quickly dry when exposed to air. This rapid drying damages gill tissues and further impairs their ability to facilitate gas transfer. Even though air contains more oxygen than water, the structural failure and desiccation of gills prevent fish from utilizing this atmospheric oxygen.
Fish That Can Survive Out of Water
While most fish cannot breathe air, some species have developed unique adaptations that allow them to survive outside of water for varying periods. Lungfish, for example, possess true lung-like organs that enable them to gulp air directly from the atmosphere. These specialized lungs allow them to endure periods of drought or low oxygen conditions in their aquatic habitats.
Mudskippers are amphibious fish that can spend significant time on land. They achieve this by retaining water in their enlarged gill chambers to keep their gills moist, allowing some continued gill respiration. Additionally, mudskippers can absorb oxygen through their skin and the lining of their mouths and throats, a process known as cutaneous respiration, provided their skin remains wet.
Walking catfish have evolved accessory breathing organs, such as the suprabranchial arborescent organ. These highly vascularized structures function like lungs, enabling them to breathe air and move across land.