Sharks do not possess bones like most other vertebrates. Their skeletons are not composed of true bone. Instead, these ancient marine animals rely on a different material for structural support, distinguishing them from most bony fish.
The Cartilaginous Skeleton
A shark’s skeleton is primarily made of cartilage, a flexible and tough connective tissue also found in parts of the human body, such as the ears and nose. This material provides structural integrity but differs significantly from bone, which is rigid and composed of mineral compounds like calcium phosphate. Sharks, rays, skates, and chimaeras belong to the class Chondrichthyes, meaning “cartilage fish.” This classification highlights their skeletal system, which lacks the dense, mineralized framework found in bony fish (Osteichthyes).
Their entire skeleton, including the skull and spinal column, is cartilaginous. This contrasts sharply with bony fish, where bone forms the primary skeletal structure. The absence of true bone means shark skeletons do not fossilize as readily as those of bony fish, with their teeth being the most common fossilized remains. Despite being softer than bone, cartilage is strong and durable, providing necessary support and shape.
Why Cartilage?
The cartilaginous skeleton offers several evolutionary advantages for sharks as aquatic predators. Cartilage is approximately half as dense as bone, making sharks inherently lighter. This reduced density assists with buoyancy, a significant benefit as sharks lack a swim bladder, the internal organ most bony fish use to regulate their position in the water column. Instead, sharks rely on the buoyancy from their lighter skeletons and large, oil-filled livers.
The flexibility of cartilage allows sharks exceptional maneuverability in water, enabling quick turns and powerful swimming movements. This agility is crucial for hunting prey and evading threats. The flexibility of their skeletal structure also contributes to predatory efficiency, allowing some shark species to open their mouths wider and exert a greater bite force. The cartilaginous skeleton can also be more resilient to impact, providing shock absorption.
Calcification: Not True Bone
While sharks do not possess true bone, their cartilage can undergo calcification. This process involves the deposition of calcium salts within the cartilaginous matrix, increasing its hardness and rigidity. Areas requiring additional support, such as the jaws, cranium, and vertebrae, often exhibit higher degrees of calcification. This hardening provides localized reinforcement without transforming the cartilage into bone.
Calcified cartilage is fundamentally different from true bone. In calcification, the cartilaginous structure and its cellular composition remain intact; the cartilage is not replaced by bone cells or a bone matrix. This process reinforces the existing cartilage, making it tougher and more resistant to mechanical stress. This adaptation allows sharks to maintain the benefits of a lightweight, flexible skeleton while still achieving necessary strength in specific body regions.