Sharks have skeletons made entirely of cartilage rather than bone.

Sharks and rays belong to the class Chondrichthyes, a group of cartilaginous fish that diverged from bony fish over 400 million years ago. Unlike the hydroxyapatite-based bone found in mammals, shark skeletons are composed of a dense network of collagen fibers and proteins. This unique structure is approximately half the density of bone, which is crucial because sharks lack a swim bladder to keep them afloat.Research published in journals like 'Nature' highlights that while their skeletons are cartilaginous, certain areas like the jaw and vertebrae are reinforced with calcium salts. This process, known as tessellated calcification, provides the strength needed for powerful bites without the heavy weight of a full bony skeleton. This evolutionary trade-off allows for incredible burst speeds and maneuverability in a high-pressure aquatic environment.The flexibility of cartilage also acts like a spring. When a shark swims, its skeleton stores and releases elastic energy, making its movements remarkably efficient. This anatomical advantage has remained largely unchanged for millions of years, proving that a lightweight, flexible frame is a superior design for apex predators in the open ocean.