Skeleton & Buoyancy
A shark’s body is supported by a skeleton very similar to that of other fishes, except it is made of cartilage rather than bone. Some parts of the skeleton, including the vertebrae and skull, are strengthened by increased calcification.
All sharks are slightly negatively buoyant, which means they sink. Unlike many bony fishes, sharks do not have a swim bladder to provide buoyancy. To help compensate for their tendency to sink, their livers contain large amounts of oil that is less dense than seawater. Pelagic (open water) sharks generally have larger livers, with more and lighter oil, than sharks which live in shallower water or near the ocean bottom.
The combination of a cartilaginous skeleton, which is lighter than bone, and an oil-filled liver work together to increase swimming efficiency and buoyancy.