Deep-water sharks solve a basic problem of life in the ocean in a very different way from most bony fish. Instead of using a gas-filled swim bladder to control depth, they depend on an unusually large liver packed with buoyant oil. In some species, that liver can make up as much as 30 per cent of total body mass, effectively serving as the animal’s buoyancy organ.
The idea is simple but striking: a metabolic organ takes on a second, mechanical role. Because sharks do not have a swim bladder, the oil stored in the liver helps offset their weight in water and allows them to remain closer to neutral density. That means they can stay suspended in the water column with less effort than if they had to rely on constant swimming alone.
This adaptation helps explain an important difference between major groups of fish. Bony fish typically use gas to fine-tune their position in the water, while sharks, rays and chimaeras use a different body plan and a different buoyancy strategy. In deep-water environments, where efficient movement matters, an oil-rich liver becomes a practical substitute for the missing bladder.
The result is an elegant example of evolutionary engineering. A liver best known for its internal physiological role also becomes a structural solution to life at depth, letting deep-sea sharks stay afloat through chemistry and anatomy rather than gas storage.
