Sharks have a disproportionately large liver, constituting up to 25% of their total body mass. This organ is packed with low-density oils, giving it an inflated size. This immense oil reserve serves two fundamental biological purposes: providing buoyancy to counteract the animal’s tendency to sink and acting as a dense, long-term energy bank.
The Buoyancy Challenge for Sharks
The need for a massive, oil-filled liver stems from a fundamental difference in the skeletal structure of sharks compared to bony fish. Sharks belong to the class Chondrichthyes, meaning their skeletons are made of cartilage rather than bone. While cartilage is less dense than bone, the overall composition of a shark’s body is still denser than the surrounding seawater, meaning it is inherently negatively buoyant.
The majority of fish overcome this density problem by using a specialized, gas-filled organ called a swim bladder. They regulate the amount of gas in this bladder to achieve neutral buoyancy, allowing them to hover effortlessly at any depth. Sharks, however, do not possess a swim bladder.
The absence of a gas bladder allows sharks to rapidly change depth without the risk of the gas expanding or contracting from pressure changes. This freedom of vertical movement is advantageous for a predator, but it forces the shark to find an alternative to stay afloat. For many species, this means they must swim almost constantly, generating hydrodynamic lift with their pectoral fins.
How Liver Oil Achieves Hydrostatic Lift
The oil stored in the liver provides hydrostatic lift, the primary solution to the buoyancy challenge. This mechanism reduces the shark’s overall specific gravity to achieve near-neutral buoyancy with minimal effort. By storing a massive volume of a compound less dense than water, the shark effectively lowers its average body density.
The oil’s low density is due to its composition, largely made up of the hydrocarbon squalene. Squalene is an organic compound with a density of approximately 0.858 g/cm³, whereas the density of seawater is around 1.025 g/cm³. The significant difference between these two values means that every liter of squalene the shark stores displaces a greater mass of water than the oil itself weighs, providing lift.
In some deep-sea sharks, which need greater lift to compensate for their generally heavier bodies, the squalene content in the liver oil can reach up to 90%. Replacing dense tissue mass with a large volume of low-density squalene offsets the shark’s tendency to sink. This biological strategy provides a passive form of buoyancy control that functions across all depths without the pressure limitations of a gas-filled bladder.
The Role of Liver Oil as an Energy Bank
Beyond buoyancy, the enormous, oil-rich liver serves a secondary, yet equally important, metabolic function as a long-term energy reserve. The lipids stored in the liver are a highly concentrated source of fuel, providing more than twice the energy per gram compared to carbohydrates or proteins. This makes oil an ideal substance for long-term energy storage.
This dense energy bank is particularly useful for sharks that undertake extensive migrations across nutrient-poor oceans, where feeding opportunities are infrequent. Studies on species like the Great White Shark show they rely on their liver oil reserves to power non-stop journeys spanning thousands of miles. During these long movements, the stored oil is gradually depleted as the primary metabolic fuel.
The liver oil is also crucial for the reproductive success of female sharks. Gestation periods can be lengthy, and the large store of lipids provides the necessary energy to sustain the developing embryos. For deep-sea sharks living in environments where food is scarce, this energy reserve is vital during prolonged periods between successful hunts.