Many people assume sharks have bony skeletons like most other fish or humans. However, their skeletal composition is fundamentally distinct. The question of whether sharks possess a backbone, or vertebral column, reveals a fascinating aspect of their biology.
Sharks and Their Vertebral Column
Sharks do possess a vertebral column, a segmented spinal column that runs the length of their body, providing support and protecting the spinal cord much like a bony spine in other vertebrates. The key difference is its composition: a shark’s vertebral column is made entirely of cartilage, not bone. This cartilaginous column classifies sharks as vertebrates, as the definition centers on the presence of a spinal column, regardless of the material. The cartilaginous vertebrae link together, forming a robust yet flexible structure that enables powerful, agile movements. Some areas, like the jaws and parts of the spinal column, are strengthened by deposited calcium salts.
The Nature of Cartilaginous Skeletons
Cartilage is a flexible and durable connective tissue that forms the entire skeleton of sharks, rays, and skates. Unlike bone, cartilage is less dense and lacks true bone marrow. While some areas of a shark’s cartilage may be stiffened by hexagonal, mineralized blocks called tesserae, this is not true ossification into bone. This skeletal material is similar to the cartilage found in human noses and ears, providing support while maintaining flexibility. The cartilaginous skeleton can be remarkably strong, with the material strength and stiffness of some shark vertebral columns comparable to mammalian bone.
Functional Advantages of Cartilage
The cartilaginous skeleton offers several benefits for sharks in their marine environment. Cartilage is approximately half as dense as bone, contributing to a shark’s lighter body weight and aiding buoyancy by lessening the energy sharks need to expend to avoid sinking, especially since they lack swim bladders. The inherent flexibility of cartilage allows sharks to make quick, tight turns, enhancing agility and speed while swimming, which is useful for hunting prey and evading predators. Furthermore, cartilage is more resilient to impact than bone, providing shock absorption during collisions or aggressive hunting maneuvers. The vertebral column’s spring-like quality also helps store and return energy during swimming, contributing to efficient movement.