Cartilaginous fish, such as sharks, rays, and skates, face a fundamental challenge in the marine environment. Unlike the majority of bony fish, they do not possess a gas-filled swim bladder for buoyancy control. Cartilaginous fish must instead rely on static physiological mechanisms and continuous movement. Their survival depends on adaptations that both reduce their overall density and allow them to generate lift while swimming.
Reducing Body Weight Through Internal Structure
The primary static mechanism cartilaginous fish employ to reduce their overall body density is the accumulation of low-density lipids within the liver. This organ can be exceptionally large, sometimes constituting between 25% and 30% of the animal’s total body mass. The liver is heavily saturated with oil, primarily squalene, a hydrocarbon with a density significantly lower than that of seawater.
This oil-filled liver acts like an internal flotation device. Deep-sea species, in particular, may have liver oil that is up to 90% squalene, allowing them to conserve energy in cold, nutrient-poor waters. Storing these low-density lipids shifts the fish’s overall center of buoyancy closer to its center of mass, reducing the energy cost of remaining suspended in the water column.
A second physiological factor contributing to reduced density is the composition of the skeleton itself. Cartilaginous fish possess skeletons made of cartilage and connective tissue rather than true bone. Cartilage is roughly half the density of bone, providing a foundational buoyancy advantage that requires less energy to overcome.
Cartilaginous fish retain high concentrations of organic molecules like urea and trimethylamine N-oxide (TMAO) in their tissues to balance internal salt levels with the surrounding seawater. While their main function is to maintain internal fluid balance, these solutes also reduce the overall density of the internal body fluids. This accumulation contributes to buoyancy, acting as a distinct adaptive function beyond their role in osmoregulation.
Generating Lift Through Hydrodynamic Design
Despite these density-reducing adaptations, most cartilaginous fish remain slightly negatively buoyant. To overcome this residual weight, they rely on dynamic lift generated by their body shape and fins. The large, rigid pectoral fins function as fixed hydrofoils, similar to the wings of an airplane, generating upward lift when the fish swims forward at a specific angle of attack.
The body itself also plays a role, with some species maintaining a slight positive angle of attack during horizontal swimming to generate lift along the body surface. This lift helps balance the forces acting on the animal and maintains its position in the water column. The combination of the body’s angle and the pectoral fins ensures that the fish can actively control its vertical position while moving.
The characteristic asymmetrical caudal fin, known as the heterocercal tail, is also involved in generating dynamic lift. This tail features an upper lobe that is longer than the lower lobe. When the tail beats from side to side, the angle of the fin pushes water backward and slightly downward, resulting in a reaction force that is directed forward and upward.
This reaction force from the tail provides a vertical lift component that helps propel the fish upward. This upward force creates a pitching moment, which is countered by the lift generated by the pectoral fins and the body’s angle. This continuous, active process of lift generation means that swimming is directly tied to buoyancy control for most species.
Species Specific Buoyancy Strategies
The balance between static buoyancy and dynamic lift varies significantly across cartilaginous fish, reflecting their ecological niche. Pelagic sharks, such as the Great White and Mako, are fast, open-ocean swimmers that often have greater tissue density, resulting in a more negative buoyancy. These species rely heavily on their streamlined bodies and continuous movement to generate hydrodynamic lift for hunting and cruising.
In contrast, deep-sea sharks, which inhabit cold, resource-scarce environments, rely more on static buoyancy to conserve energy. Species like the Greenland shark or sleeper sharks possess large, oil-rich livers that bring their buoyancy closer to neutral, allowing them to glide or swim slowly without sinking. Some deep-sea species, such as the sixgill shark, have even been observed to be positively buoyant, suggesting they can glide upward without active swimming.
Benthic species, which include rays and skates, exhibit a different strategy as they spend most of their lives resting on the seafloor. The need for neutral buoyancy or constant dynamic lift is minimal. Their flattened body shapes and expansive pectoral fins are adapted more for locomotion along the substrate or in short bursts, rather than for long-term suspension in the water column.