Sharks are not boneless; they possess a complete skeleton. However, this internal support structure is not composed of true bone tissue like that of humans or bony fish. Instead, a shark’s skeleton is made almost entirely of cartilage, a much lighter and more flexible material.
The Cartilaginous Skeleton
Cartilage is a highly flexible connective tissue found throughout the shark’s body. It is primarily composed of collagen and elastin fibers embedded in a matrix, lacking the extensive mineralization that defines true bone tissue. Unlike bone, which contains calcium phosphate and a network of blood vessels, cartilage is less dense and avascular, meaning it lacks blood vessels.
However, the shark’s skeleton is not purely soft cartilage; it undergoes a process called calcification, which gives it necessary rigidity. This hardening happens as the cartilage matrix accumulates mineral salts, particularly in areas like the jaws and the vertebral column. The calcified tissue on the surface of the skeleton forms a mosaic of tiny, interlocking, hexagonal plates known as tesserae.
The tessellated structure provides rigidity without transforming the cartilage into true bone. This calcification differs from ossification, the formation of true bone, because the underlying cartilage cells remain intact. The core of the skeleton remains unmineralized cartilage, providing a balance of strength and lightness.
Classification of Chondrichthyes
Sharks belong to the biological class Chondrichthyes, meaning “cartilaginous fish.” This classification groups all vertebrates whose skeletons are primarily composed of cartilage. This includes not only sharks, but also their close relatives, the rays, skates, and chimaeras, sometimes known as ghost sharks.
Cartilaginous fish diverged from the bony fish (Osteichthyes) millions of years ago. This is an ancient lineage that has existed for over 400 million years. The cartilaginous skeleton is not a primitive trait but is considered a derived feature retained by this group.
Functional Adaptations
The lighter density of the cartilaginous skeleton aids in buoyancy. Cartilage is approximately half as dense as bone, helping to counteract the natural tendency of their bodies to sink in water. This is important because sharks lack a swim bladder, the gas-filled organ most bony fish use to maintain neutral buoyancy.
The reduced weight of the skeleton complements the shark’s large, oil-filled liver. This liver contains low-density oils, such as squalene, which further assists in flotation. Despite these adaptations, most sharks are still negatively buoyant, meaning they must swim constantly to avoid sinking.
The flexibility of the cartilage allows for greater maneuverability. The resilient nature of the skeleton permits quick, tight turns and powerful bursts of speed that would be structurally taxing on a rigid bone structure. This combination of lightness, flexibility, and calcified strength enables sharks to be highly efficient and agile aquatic predators.