Sharks are highly successful marine predators that have roamed the oceans for millions of years. Their ability to thrive in diverse aquatic environments is partly attributed to a skeletal structure that sets them apart from most other vertebrates. Unlike fish and land animals with skeletons primarily composed of bone, a shark’s internal framework is fundamentally different. This unique composition is central to understanding their biology.
The Primary Material: Cartilage
A shark’s skeleton is made almost entirely of cartilage, a strong yet flexible connective tissue also found in parts of the human body like the nose and ears. Cartilage contains collagen fibers and a high water content. These components make it resilient, strong, and notably lighter than bone.
Unlike bone, cartilage lacks a rigid, mineralized matrix and marrow cavity. It is less dense and does not contain blood vessels in the same way bone does. This makes cartilage more pliable, allowing for greater movement and shock absorption compared to bony frameworks.
Why Cartilage? Evolutionary Advantages
The cartilaginous skeleton offers several functional benefits for sharks in their aquatic habitat. Its inherent flexibility allows sharks to execute tight turns and rapid maneuvers, contributing to their agility when hunting prey or evading threats. This adaptability in movement is a significant advantage for active predators.
The lighter weight of cartilage, roughly half as dense as bone, plays a role in buoyancy control. Unlike most bony fish that use a swim bladder, sharks do not possess this organ. The reduced density of their skeleton, combined with a large, oil-filled liver, helps them maintain buoyancy and reduces the energy needed to stay afloat. This energy saving enhances their swimming efficiency and speed. The resilience of cartilage also helps the skeleton absorb impacts during hunting or navigating dynamic underwater environments.
The Role of Calcified Cartilage
While primarily cartilaginous, a shark’s skeleton is not entirely soft. Certain areas undergo calcification, where calcium salts are deposited within the cartilage matrix. This process increases the rigidity and strength of the cartilage, making it more durable, but it is distinct from true bone formation. Calcification provides mechanical support while retaining some flexibility and lighter weight.
Calcified cartilage is prominent in parts of the shark’s body requiring greater structural integrity and resistance to force. Examples include the jaws, which withstand powerful bite forces, and the vertebral column, which provides central support. Fin supports also contain calcified cartilage for necessary stiffness. This adaptation allows sharks to have robust skeletal components in high-stress areas without fully transitioning to a bony skeleton.