Sharks do not have bones in the way humans or most other fish do. Their internal framework is composed of a different material, which provides them with distinct advantages in their marine environment.
Understanding Shark Skeletons
A shark’s entire skeleton is made primarily of cartilage, a flexible and tough connective tissue found in parts of the human body, such as the nose and ears. This material provides structural support throughout their body, including the jaws and fin supports. Unlike true bone, cartilage is less dense, which contributes to the shark’s buoyancy in water.
Although the shark skeleton is cartilaginous, certain areas can undergo calcification. This involves the deposition of calcium salts into the cartilage. Calcified cartilage is particularly evident in areas requiring greater strength, like the jaws and spinal column, providing support without transforming into true bone. This hardening mechanism allows for specialized functions while maintaining the overall benefits of a cartilaginous structure.
How Cartilage Benefits Sharks
Its lighter weight compared to bone helps sharks maintain buoyancy, reducing the energy needed to stay at a desired depth. This reduced density means they do not have to expend as much energy constantly swimming to avoid sinking, a significant advantage for conserving energy over long periods. This inherent lightness aids in their overall energy efficiency.
The flexibility of cartilage also provides sharks with enhanced maneuverability in water. This allows them to execute sharp turns and rapid movements, which are valuable for hunting prey and evading threats. Furthermore, the skeletal structure, particularly the jaw’s flexibility, can contribute to a powerful bite force, enabling them to open their mouths widely and exert significant downward pressure on their prey. The resilience of cartilage also helps absorb impacts, providing protection in their dynamic environment.
Sharks Versus Bony Fish
The fundamental distinction between sharks and most other fish lies in their skeletal composition. Sharks, belonging to the class Chondrichthyes, possess skeletons made entirely of cartilage. In contrast, most other fish, categorized as bony fish (Osteichthyes), have skeletons composed of rigid, calcified bone. This difference extends to other anatomical features that influence their aquatic lives.
A notable difference is how they manage buoyancy. Bony fish typically utilize a swim bladder, an internal gas-filled organ that allows them to control their position in the water column without continuous swimming. Sharks, however, lack a swim bladder and instead rely on a large, oil-rich liver, which can constitute up to 25% of their body weight, to provide buoyancy. They also generate dynamic lift through the shape of their pectoral fins as they swim, necessitating continuous movement for many species to avoid sinking.
Further distinctions are observed in their respiratory systems. Bony fish commonly have a single gill opening on each side covered by a protective bony flap called an operculum. Sharks, conversely, have multiple, exposed gill slits, typically five to seven pairs, directly opening to the outside. This anatomical variation reflects different evolutionary paths and adaptations to their diverse aquatic habitats.