Sharks, as fish, do not possess lungs. They rely entirely on a specialized and highly efficient organ system to extract oxygen dissolved in water. This process is complex because dissolved oxygen in seawater is significantly lower—only about one percent—compared to the twenty percent saturation found in the air. Survival depends on the shark’s ability to continuously process large volumes of water across its gills to secure the necessary oxygen.
The Gills: Shark’s Primary Respiratory Organ
The shark’s respiratory organs are its gills, located in a series of distinct openings on both sides of the head. Most species have five pairs of gill slits, though some possess six or seven pairs. Unlike the gills of bony fish, which are protected by a bony plate called an operculum, the shark’s gills open directly to the outside environment.
The gills are structured around stiff, cartilaginous gill arches that provide support. Extending outward are numerous delicate, finger-like projections known as gill filaments. These filaments are the true sites of gas exchange, creating an immense surface area for water contact.
The respiratory surface is maximized by tiny, folded structures called lamellae that cover the filaments. These lamellae are richly supplied with capillaries, separated from the passing water by an extremely thin membrane. This arrangement facilitates the diffusion of dissolved oxygen into the bloodstream and the simultaneous release of carbon dioxide back into the water.
Methods for Moving Water
A shark must maintain a constant, unidirectional flow of water across its gill surfaces to ensure continuous oxygen delivery. Sharks achieve this necessary water movement through two primary behavioral mechanisms: ram ventilation and buccal pumping. The specific method used often determines a species’ lifestyle and whether it can rest stationary on the seabed.
Ram ventilation is a passive method where the shark swims forward with its mouth slightly open, forcing water to flow over the gills. This mechanism is highly efficient for fast-moving, pelagic species, such as the Great White, Mako, and Whale sharks. These sharks are often obligate ram ventilators, meaning they must swim continuously to breathe; stopping movement can lead to asphyxiation.
In contrast, buccal pumping is an active process that allows a shark to remain stationary while breathing. These species use muscular contractions of the mouth and pharynx (the buccal cavity) to actively draw water in and pump it over the gills. The nurse shark is a classic example, frequently seen resting motionless on the ocean floor.
Some bottom-dwelling species, such as rays and skates, utilize specialized openings called spiracles, located behind the eyes, to assist in buccal pumping. Spiracles draw in clean water, bypassing the mouth, which might otherwise ingest sand or debris while the shark rests on the substrate. Many active sharks, like reef sharks, are facultative ventilators, switching between buccal pumping at low speeds and ram ventilation when swimming faster.
The Efficiency of Gas Exchange
The ability of sharks to extract sufficient oxygen from the water is due to the countercurrent exchange system within the gills. This system maximizes the concentration gradient—the difference in oxygen levels between the water and the blood—which is the driving force for oxygen movement.
In the countercurrent system, blood flows through the gill lamellae in the direction opposite to the flow of water. As the water begins its journey, it is highly oxygenated and meets blood that is already nearly saturated with oxygen. This arrangement ensures the blood is constantly exposed to water with a higher oxygen concentration throughout the exchange surface.
Even as the blood picks up oxygen, it continues to encounter progressively fresher water, maintaining a diffusion gradient that never reaches equilibrium. This continuous gradient allows oxygen transfer across the whole respiratory surface. This countercurrent flow is so effective that sharks can extract up to 80% of the available oxygen from the water, far surpassing the efficiency of mammalian lungs.
Contrasting Gills and Lungs in Marine Environments
The differences between gills and lungs represent a fundamental divergence in evolutionary adaptation to two distinct environments. Gills are optimized for the extraction of oxygen from a dense, low-oxygen medium—water—and are external outfoldings of tissue. Their design requires constant movement of that dense medium to function.
Lungs, conversely, are internal organs optimized for extracting oxygen from a light, high-oxygen medium—air. Marine mammals, such as whales and dolphins, evolved lungs to handle the high oxygen content of the atmosphere. These animals must surface periodically to take a single, large breath.
Sharks, as cartilaginous fish, remained in the aquatic environment and perfected the gill system to cope with low oxygen availability. They avoid the trade-off of constantly having to surface that marine mammals face. The gill’s high efficiency made the evolutionary development of a lung system unnecessary for survival.