Life in an aquatic environment requires a specialized interface to extract the relatively low concentration of dissolved oxygen from water. Gills are the biological solution that evolved across numerous aquatic animal groups to meet this demand. These delicate, highly vascularized organs maximize the surface area for gas exchange, transferring dissolved oxygen into the circulatory system and releasing metabolic carbon dioxide back into the water.
How Gills Facilitate Gas Exchange
The physical challenge of extracting oxygen from water is amplified because water holds significantly less oxygen than air. Gills overcome this limitation through a sophisticated anatomical arrangement that maximizes efficiency. In fish, the respiratory surfaces are supported by bony or cartilaginous structures called gill arches, typically four on each side.
Extending from each arch are numerous thin structures known as gill filaments, which are the primary respiratory surfaces. These filaments are covered with tiny, stacked folds called lamellae, which dramatically increase the total surface area available for gas exchange. The lamellae are extremely thin, often only one or two cell layers thick, ensuring a minimal barrier for gas exchange.
The most powerful adaptation is the principle of countercurrent exchange. This mechanism dictates that the water flowing over the lamellae moves in the opposite direction to the blood circulating within them. This opposing flow maintains a steep concentration gradient across the entire respiratory surface.
Even as the blood absorbs oxygen, it continuously encounters water with a slightly higher oxygen concentration, preventing the oxygen levels from reaching equilibrium. This highly efficient process allows fish to extract up to 80% or more of the available oxygen from the water passing over their gills.
Animals with Internal Gills
Many aquatic vertebrates possess internal gills, protected within a body cavity or covered by a flap. Bony fish feature gills housed in a branchial chamber on each side of the head, covered by a hard, protective flap called the operculum.
Most bony fish have four pairs of gill arches. The operculum’s movement allows them to pump water over the gills without needing to swim constantly, ensuring a continuous, unidirectional flow. This contrasts with cartilaginous fish, such as sharks and rays, which generally lack an operculum.
Sharks typically have five to seven pairs of separate, exposed gill slits. Many active species employ ram ventilation, forcing water over their gills by swimming forward. Jawless fish, including lampreys and hagfish, possess a unique structure where the gills are located in spherical pouches that connect to the outside via separate openings.
Animals with External and Specialized Gills
Many aquatic animals utilize external or highly specialized gills. Many invertebrates have gills that are modified appendages. Crustaceans, such as crabs and lobsters, typically have their gills positioned beneath the carapace, which offers protection while allowing water circulation.
Mollusks, including clams, snails, and squid, use specialized gills known as ctenidia. These feather-like structures are located within the mantle cavity and facilitate gas exchange. In bivalves like clams, ctenidia also function in filter-feeding, demonstrating the dual roles respiratory structures can play.
Insects with aquatic larval stages often possess tracheal gills. These are thin-walled extensions containing a network of air-filled tubes (tracheae) that draw oxygen from the water. These structures are unique because they are part of the insect’s air-delivery system rather than a blood-dependent circulatory system.
Larval amphibians also exhibit external feathery gills protruding from the side of the head. These exposed, highly branched structures offer a large surface area for respiration in their aquatic youth, and are typically reabsorbed or replaced by lungs during metamorphosis. Examples include:
- Tadpoles of frogs.
- Juveniles of some salamanders like the axolotl.
- Mayfly nymphs.
- Dragonfly nymphs.
Respiratory Structures That Are Not Gills
Not all animals living in water rely on gills. Lungs are the primary alternative, found in air-breathing vertebrates like reptiles, birds, and mammals. These internal, vascularized sacs are optimized for extracting oxygen from the atmosphere, which has a much higher oxygen concentration than water.
Marine mammals, such as whales and dolphins, retain lungs and must periodically surface to breathe air. This distinguishes them from fish, which draw oxygen directly from the water. Insects use a tracheal system, a network of tubes that delivers oxygen directly to tissues, bypassing the need for a large circulatory system to transport gases.
Some aquatic and semi-aquatic animals also rely on cutaneous respiration, exchanging gases directly through the skin. Amphibians like frogs utilize their moist skin for a significant portion of their oxygen intake, especially when submerged. This method is also employed by small, inactive aquatic invertebrates whose surface-area-to-volume ratio favors simple diffusion.