Sharks have an internal framework that allows them to thrive in marine environments. Unlike most fish, their skeletal system is not composed of true bone. Instead, it features a flexible material providing support and agility. Individual segments of their spinal column, known as vertebrae, contribute to their body shape and movement.
The Cartilaginous Skeleton
Sharks are cartilaginous fish, meaning their internal skeleton consists of cartilage. This tissue is lighter and more flexible than bone. This composition offers advantages for buoyancy, as sharks lack the swim bladders found in bony fish, and allows for quicker movements and tighter turns in the water. While cartilage is softer than bone, certain areas of a shark’s skeleton, such as the jaws and spinal column, undergo calcification. This involves the deposition of calcium salts into the cartilage matrix, increasing its strength and durability, making it resemble bone without added weight.
Anatomy of a Shark Vertebra
An individual shark vertebra appears as a solid, disc-like structure, commonly referred to as a centrum. These structures are discovered washed ashore by beachcombers, often mistaken for fossilized teeth due to their hardened nature and shape. The centrum serves as the main body of the vertebra, providing axial support for the shark’s body. The size of these centra can vary depending on the shark species, with larger species possessing larger vertebrae.
Determining a Shark’s Age
Scientists use shark vertebrae to estimate a shark’s age, a technique known as sclerochronology. They examine concentric growth bands visible on a cross-section of the centrum. These bands resemble annual rings in tree trunks, providing a historical record of the shark’s growth. Traditionally, each pair of alternating opaque and translucent bands represented one year of growth. Recent research indicates that while this may be true for younger individuals, band pair deposition links more closely to changes in the shark’s length and girth, especially in older sharks where growth slows.
To validate these age estimates, scientists use methods like bomb radiocarbon dating. This technique uses the spike in carbon-14 levels from nuclear bomb testing in the 1950s and 1960s, which was absorbed into calcified tissues, including sharks. By comparing carbon-14 levels in specific vertebral bands to known historical records, researchers establish a precise timestamp, confirming the accuracy of growth band counts for certain periods. Accurate age and growth rate information is important for conservation and management of shark populations, as it informs sustainable fishing quotas and population resilience to environmental pressures.