Buoyancy is a fundamental challenge for all marine life that is denser than water. Most bony fish (class Osteichthyes) solve this problem with a gas-filled organ that allows them to achieve neutral buoyancy with minimal effort. Sharks, members of the class Chondrichthyes (cartilaginous fish), do not possess this specialized organ. They rely on a combination of different biological adaptations rather than a swim bladder to stay suspended in the water.
The Basic Answer and What Sharks Lack
Sharks do not have a swim bladder, an internal, gas-filled sac found in the majority of bony fish. This organ functions as a hydrostatic device, allowing bony fish to precisely regulate their position in the water column without needing to swim constantly. By adjusting the volume of gas inside the bladder, bony fish can change their overall density relative to the surrounding water.
This gas-filled structure is an efficient way to achieve neutral buoyancy, letting the fish hover at a specific depth while conserving energy. Cartilaginous fish, including sharks, skates, and rays, lack this air sac entirely. The absence of a swim bladder forces sharks to evolve entirely different mechanisms to counteract their natural tendency to sink.
The Primary Buoyancy Mechanism
The most significant adaptation sharks use for static buoyancy is a massive liver filled with low-density oil. In many species, this organ can account for up to 25% of the shark’s total body weight. This large, oil-filled liver provides substantial lift, effectively reducing the overall density of the shark’s body.
The oil is primarily composed of a hydrocarbon called squalene, which has a density considerably lower than that of water. Squalene is a lipid found in high concentrations within the liver cells, often making up 80% or more of the liver’s volume in some species. Deep-sea sharks, which need greater buoyancy at depth, often have livers with a higher percentage of this low-density squalene.
By storing this large volume of oil, sharks achieve a degree of lift that offsets a major portion of their body weight. This fatty liver also serves as a metabolic reserve, allowing some sharks to go for extended periods without feeding.
Buoyancy Through Structure and Movement
Beyond the oily liver, two other factors contribute significantly to a shark’s buoyancy: its skeleton and its hydrodynamics.
Skeletal Structure
A shark’s skeleton is composed of cartilage, a flexible material that is approximately half the density of bone. This lighter skeletal structure reduces the shark’s overall mass compared to bony fish of a similar size.
Dynamic Lift
The second aid is dynamic lift, which requires continuous forward movement. Sharks possess large, rigid pectoral fins that work much like the wings of an airplane. As the shark swims, the angle and shape of these fins generate an upward force, or lift, that counteracts the remaining negative buoyancy.
Furthermore, the shark’s tail, or caudal fin, is typically heterocercal, meaning the upper lobe is larger than the lower lobe. This asymmetrical design produces a downward thrust as the shark swims, which causes the head and body to be tilted upward, further contributing to hydrodynamic lift. This combination of a lighter frame and lift-generating fins means the shark must swim to maintain its position.
The Physiological Cost
The reliance on a buoyancy system that is not fully neutral creates a significant physiological cost for most sharks. Since their oil-filled liver only provides static lift and is not adjustable, most sharks remain slightly negatively buoyant. This means that if they stop moving, they will gradually sink.
To avoid sinking, many pelagic sharks must swim without rest, incurring a continuous energy expenditure. This constant swimming is also necessary for many species to force water over their gills for respiration, a process known as ram ventilation. The shark’s lifestyle is metabolically more demanding due to the need for constant motion to generate lift and breathe.
The inability to adjust buoyancy instantly also limits the shark’s depth control compared to bony fish. While deep-sea sharks have evolved larger livers to achieve near-neutral buoyancy, many fast-swimming sharks maintain a degree of negative buoyancy, which may be more efficient for rapid, accelerated movements used in hunting.