Animals in aquatic environments have developed diverse strategies to obtain oxygen. Some truly “breathe underwater” by extracting dissolved oxygen directly from the water. Others, despite spending significant time submerged, must periodically return to the surface to access atmospheric air. Understanding these mechanisms involves recognizing the specialized organs and adaptations that allow life to thrive in watery habitats.
Gill Respiration
Gills are a highly effective and common method for aquatic animals to acquire oxygen from water. These branching organs, often located on the sides of an animal’s head or within specialized cavities, are rich in tiny blood vessels called capillaries. As water flows over these thin, feathery or comb-like structures, dissolved oxygen diffuses from the water into the blood, while carbon dioxide is simultaneously released. The extensive surface area provided by gill filaments and lamellae maximizes this gas exchange.
Many aquatic animals employ countercurrent exchange to optimize oxygen absorption. In this mechanism, blood within the gill capillaries flows in the opposite direction to the water passing over the gills. This opposing flow maintains a continuous and steep oxygen concentration gradient across the respiratory surface. This ensures oxygen always moves from the water, where it is more concentrated, into the blood, where it is less concentrated. This efficient system allows some animals to extract up to 90% of the oxygen present in the water flowing over their gills.
Fish, including bony species that pump water over their gills and cartilaginous fish like sharks that use ram ventilation, are prime examples of gill breathers. Beyond fish, numerous other aquatic creatures utilize gills. Crustaceans, such as crabs, lobsters, and shrimp, possess gills within their thoracic cavity or on their appendages. Many mollusks, including clams, snails, and octopuses, use specialized gills called ctenidia, housed within their mantle cavity. Even some amphibian larvae, like tadpoles and the axolotl, retain external, feathery gills throughout their aquatic life stages for efficient oxygen uptake.
Cutaneous Respiration and Specialized Organs
Beyond gills, some aquatic animals extract oxygen directly from their watery surroundings through alternative methods. One such method is cutaneous respiration, where gas exchange occurs across the skin or outer body surface. This breathing requires the skin to be thin, moist, highly permeable, and often with a rich network of blood vessels close to the surface. Many amphibians, including frogs and salamanders like the axolotl, rely significantly on cutaneous respiration, which can account for a substantial portion of their oxygen intake. Some aquatic insects, worms, certain fish, and even some turtles also utilize this method.
In addition to skin breathing, unique internal or external structures have developed for underwater respiration. Aquatic insect larvae, such as dragonfly nymphs, possess specialized tracheal gills, often located internally. Dragonfly nymphs have a rectal gill system, drawing water into their anus, where oxygen is absorbed across the gill surfaces. This process also provides jet propulsion, allowing them to move quickly through the water.
Cloacal respiration is found in certain freshwater turtles. Species like the Fitzroy River turtle can absorb up to 70% of their oxygen through specialized structures in their cloaca, a multipurpose opening at the base of their tail. Water is actively pumped into two sac-like organs called bursae within the cloaca, which are lined with numerous blood vessels and finger-like projections called papillae. This allows for effective oxygen diffusion into the bloodstream, enabling these turtles to remain submerged for extended periods, particularly during colder months when their metabolic rate is low. Similarly, sea cucumbers utilize internal “respiratory trees” that branch off their cloaca, drawing water in and out for gas exchange.
Air-Breathing Aquatic Animals
While many animals extract oxygen from water, a significant number of aquatic species do not breathe underwater. Instead, they are air-breathers that have evolved adaptations for spending extended periods submerged, but must return to the surface to inhale atmospheric oxygen using lungs. This category includes marine mammals, aquatic reptiles, and diving birds. These animals rely on efficient breath-holding and oxygen management rather than direct aquatic respiration.
Marine mammals, such as whales, dolphins, seals, and sea lions, possess lungs and must periodically surface to breathe. Their bodies are specialized for diving. They can exchange a large percentage of the air in their lungs with each breath, sometimes up to 90%, allowing for efficient oxygen uptake. These animals also have elevated concentrations of oxygen-carrying proteins like hemoglobin in their blood and myoglobin in their muscles, enabling them to store more oxygen than terrestrial mammals.
Physiological adjustments enhance their underwater endurance. When diving, marine mammals experience bradycardia, a slowing of their heart rate, and peripheral vasoconstriction, which restricts blood flow to non-essential areas. This redirects oxygenated blood primarily to vital organs like the brain, heart, and lungs. Their lungs and rib cages are designed to collapse under pressure, helping them withstand extreme depths and minimizing nitrogen absorption into their bloodstream, reducing decompression sickness risk. Some species even exhale before a deep dive to further reduce nitrogen in their lungs.
Aquatic reptiles, including sea snakes, marine iguanas, crocodiles, and sea turtles, rely on lungs for breathing and must surface for air. Similarly, diving birds like penguins and cormorants are atmospheric breathers. While these animals can hold their breath for impressive durations, their fundamental respiratory process involves gas exchange with air, not water. Their adaptations allow them to exploit aquatic environments for food and shelter, but their oxygen source remains the atmosphere above the water’s surface.