Respiratory Surfaces of Aquatic Animals

How do aquatic animals breathe?

Some aquatic invertebrates, such as cnidarians and some flatworms, are relatively small and have thin-walled bodies whose outer surfaces are always wet. These animals rely on diffusion of oxygen and carbon dioxide through their outer body covering. A few aquatic chordates, including lancelets, some amphibians, and even some sea snakes, rely to varying extents on gas exchange by diffusion across body surfaces.

For large, active animals, however, skin respiration alone is insufficient. Many aquatic invertebrates and most aquatic chordates other than reptiles and mammals exchange gases through gills. As shown in Figure 27–6, gills are feathery structures that expose a large surface area of thin, selectively permeable membrane to water. Inside gill membranes is a network of tiny, thin-walled blood vessels called capillaries. Many animals, including aquatic mollusks and fishes, actively pump water over their gills as blood flows through inside. This helps maintain differences in oxygen and carbon dioxide concentration that promote diffusion. Aquatic reptiles and aquatic mammals, such as whales, breathe with lungs and must hold their breath underwater. Lungs are organs that exchange oxygen and carbon dioxide between blood and air. You will learn more about lungs shortly.