Sharks exhibit unique biological features that set them apart from many other fish. Unlike most bony fish, sharks do not possess a swim bladder. This gas-filled organ regulates buoyancy, allowing aquatic animals to control their position in the water column. Buoyancy, the upward force exerted by a fluid, is crucial for survival, enabling efficient movement and depth maintenance. Without a swim bladder, sharks have evolved alternative adaptations to stay afloat.
How Sharks Maintain Buoyancy
Sharks rely on biological and physical adaptations to maintain their buoyancy. A primary mechanism involves their large, oil-rich liver. This organ can constitute a significant portion of a shark’s body weight, sometimes up to 30% in certain species. The liver produces and stores squalene, a low-density oil lighter than water, which helps reduce the shark’s overall body density and provides static lift. In some deep-sea sharks, squalene can make up as much as 90% of the liver oil, providing substantial buoyancy.
Sharks also possess a cartilaginous skeleton, which is considerably lighter than the bone found in most fish. Cartilage is about half the density of bone, contributing to a reduced body weight that aids buoyancy. This lighter skeletal structure, along with their streamlined body shape, helps sharks manage their density.
Sharks utilize dynamic lift, a mechanism requiring continuous forward movement. Their pectoral fins act like airplane wings, generating lift as water flows over them while the shark swims. The heterocercal tail, characterized by its uneven lobes, also generates upward thrust and forward propulsion. This combination of fin and tail movement allows sharks to actively counteract their tendency to sink.
Additionally, sharks retain urea and trimethylamine oxide (TMAO) in their tissues for osmoregulation, which helps maintain the balance of fluids with the surrounding seawater. While primarily for osmotic balance, these compounds also contribute slightly to their buoyancy by reducing tissue density.
The Role of Swim Bladders in Bony Fish
The swim bladder is a gas-filled internal organ found in most bony fish. Its primary function is to enable fish to precisely control their buoyancy, allowing them to remain at a desired water depth without expending constant energy. By adjusting the amount of gas within the bladder, fish can increase or decrease their density, effectively rising, sinking, or hovering motionless.
There are two main types of swim bladders: physostomous and physoclistous. Physostomous bladders retain a connection to the gut via a pneumatic duct, allowing fish to gulp air at the surface to inflate the bladder or burp gas to deflate it. This type is common in more primitive fish and those living in shallow waters.
In contrast, physoclistous bladders are closed systems, lacking a direct connection to the digestive tract. These fish regulate gas volume by exchanging gases directly with their bloodstream through a specialized network of capillaries called the rete mirabile and a gas gland. This allows for more precise and rapid buoyancy control across a wider range of depths, making it common in many advanced bony fish.
Adaptations for a Swim Bladder-Free Life
The absence of a swim bladder influences the lifestyle and behavior of sharks. Many active pelagic sharks must swim continuously to maintain dynamic lift and to breathe. This continuous movement, known as ram ventilation, forces water over their gills, ensuring a constant supply of oxygen. If these obligate ram ventilators stop swimming, they risk suffocating.
While constant swimming requires more energy, it also confers predatory advantages. The ability to sink quickly allows sharks to execute ambush attacks from below or rapidly dive to pursue prey. Their inherent negative buoyancy means they are always in a state of sinking, which is an advantage for efficient diving without needing to expel gas.
Not all sharks are constant swimmers; some bottom-dwelling species exhibit different adaptations. Sharks like nurse sharks and wobbegongs can actively pump water over their gills using buccal pumping, allowing them to rest motionless on the seabed. These species often have less dense bodies and may utilize their pectoral fins to “walk” along the bottom.