To breathe underwater is to perform respiration by extracting oxygen molecules that are dissolved in water. This dissolved oxygen (DO) is the same molecular oxygen found in the atmosphere, but it is physically suspended in the water, not part of the water molecule itself. The concentration of dissolved oxygen in water is significantly lower than the concentration of oxygen in air, which presents a major physiological challenge for aquatic life. For instance, the atmosphere is approximately 21% oxygen, while oxygen-saturated water at room temperature holds less than one percent oxygen. Aquatic animals have evolved specialized structures to efficiently capture this scarce resource from the dense, oxygen-poor medium of water.
The Primary Mechanism Gill Respiration
The most widespread and effective method for aquatic respiration is the use of gills, which are complex organs evolved to maximize the surface area for gas exchange. Gills are typically made of delicate, comb-like structures called filaments, which are further covered in tiny, flattened plates known as lamellae. This intricate folding creates an enormous surface area, sometimes covering the equivalent of a small car hood, allowing water to pass over a thin, highly vascularized membrane.
The efficiency of gill respiration relies on a process called countercurrent exchange, which is the physical principle that allows fish to extract 80% or more of the available oxygen from the water. In this arrangement, the blood flowing through the capillaries of the lamellae moves in the direction opposite to the water flowing over the gills. This counter-flow ensures that the blood always encounters water with a slightly higher oxygen concentration, maintaining a continuous diffusion gradient across the entire respiratory surface. This mechanism is the standard for most bony fish, cartilaginous fish like sharks, and many aquatic invertebrates such as crustaceans and mollusks.
Breathing Through the Skin
Cutaneous respiration involves gas exchange occurring directly across the animal’s outer skin or integument. This method is effective only if the animal’s skin is thin, moist, and well-supplied with a dense network of blood vessels just beneath the surface. Effectiveness also depends on the animal having a high surface area relative to its body volume, which allows a sufficient amount of oxygen to diffuse into the body.
Cutaneous respiration is often a supplement to other forms of breathing, but for some amphibians, it is the primary or sole source of oxygen. Lungless salamanders (family Plethodontidae) are a notable example, as they lack lungs entirely and rely on their highly vascularized skin for nearly all gas exchange. Aquatic frogs and certain species of turtles, like the musk turtle, also utilize their skin for a significant portion of their oxygen uptake. Among invertebrates, aquatic earthworms and other annelids depend on their moist cuticle to facilitate the diffusion of oxygen into their circulatory system.
Unique Adaptations in Aquatic Invertebrates
Aquatic invertebrates, particularly insects, have developed diverse solutions for breathing underwater that do not rely on traditional gills or simple skin diffusion. Many aquatic insects, such as mayfly and damselfly nymphs, possess tracheal gills. These are thin outgrowths containing a dense network of internal air tubes called tracheae, allowing oxygen to diffuse from the water across the cuticle directly into the respiratory system.
Physical Gills
Another adaptation is the use of a physical gill, which can be a temporary air bubble or a specialized, permanent air layer called a plastron. Certain diving beetles carry a temporary air bubble trapped beneath their hardened forewings. This bubble acts as a physical gill by allowing dissolved oxygen from the water to diffuse into the bubble to replace the oxygen the insect consumes. A plastron, found in insects like riffle beetles, is a thin, incompressible film of air held against the body by a dense mat of hydrophobic hairs. This permanent air layer functions as a physical gill that does not shrink, allowing the insect to remain submerged indefinitely.
Atmospheric Access
Some insects, like mosquito larvae, use long breathing tubes called siphons. These siphons pierce the water surface tension, allowing the insect to access atmospheric air directly while remaining submerged.
Animals That Live Underwater But Must Breathe Air
Marine mammals, such as whales and dolphins, and aquatic reptiles, including sea turtles and sea snakes, are air-breathing vertebrates that must regularly return to the surface. These animals possess lungs, not gills, and their thick, relatively impermeable skin is not adapted for cutaneous respiration. When they dive, they are essentially holding their breath, relying on physiological adaptations to conserve their stored oxygen.
They have evolved specific mechanisms to maximize the duration of their dives. They possess elevated concentrations of the oxygen-binding proteins hemoglobin in their blood and myoglobin in their muscles, which allow them to store significantly more oxygen than terrestrial mammals. Furthermore, marine mammals trigger a “dive response” that includes a dramatic reduction in heart rate, known as bradycardia. This response also involves the selective shunting of blood flow away from non-essential organs to prioritize the brain and heart. These adaptations allow some species to hold their breath for over two hours.