Sharks use massive, oil-filled livers to stay buoyant in the ocean.

Sharks belong to the class Chondrichthyes, which includes fish with cartilaginous skeletons rather than bone. Because cartilage is lighter than bone, sharks already have a head start on buoyancy, but they still require a specialized mechanism to stay afloat. While teleost (bony) fish use a swim bladder to regulate depth, sharks evolved a massive liver that can account for 25% to 30% of their total body weight.The secret to this buoyancy is squalene, an organic compound and hydrocarbon with a specific gravity of approximately 0.855. Since squalene is significantly less dense than seawater, it provides the necessary lift to keep the shark from sinking to the bottom. This is particularly vital for deep-sea species like the Portuguese dogfish, whose livers are exceptionally large to compensate for the high pressure of the deep ocean.Marine biologists at institutions like the Scripps Institution of Oceanography have studied how this liver-based system affects shark behavior. Because oil is incompressible, sharks can change depths rapidly without the risk of a gas bladder rupturing due to pressure changes. However, this system is static, meaning sharks must often use their pectoral fins like airplane wings to create dynamic lift while swimming.Beyond buoyancy, these livers serve as a critical energy reserve during long-distance migrations. Species like the Great White shark can travel thousands of miles across the Pacific Ocean, metabolizing the lipids stored in their livers when food is scarce. This dual-purpose organ is one of the most efficient evolutionary adaptations in the marine world.