How Many Bones Do Sharks Have?

Sharks have zero bones. Their entire skeleton is made of cartilage — the same flexible tissue in your nose and ears. This puts them in the class Chondrichthyes, meaning "cartilaginous fish", along with rays, skates and chimaeras.

Why cartilage is an advantage, not a limitation:

• It's lighter. Cartilage is roughly half the density of bone. Sharks have no swim bladder, so a lighter skeleton is what stops them sinking and lets them stay buoyant with less effort.
• It's more flexible, allowing the tight turns and body undulation that make sharks efficient swimmers.
• It's cheaper to grow, needing less calcium and energy.
• It heals well and is durable across a long lifespan.

Far from being primitive, this is a highly refined design. Sharks have been around for roughly 450 million years — longer than trees, and they predate the dinosaurs by about 200 million years.

Is any part hardened? Yes, partly. Some regions — particularly the vertebrae and the jaws — undergo calcification, where calcium salts are deposited into the cartilage to strengthen it. This is why a shark can bite with enormous force without its jaw collapsing. But calcified cartilage still isn't bone: it lacks the internal structure, the marrow, and the living cell network of true bone.

Shark teeth are the exception, and that's why we find them. Teeth are made of dentine coated in enamel, both far harder than cartilage and genuinely mineralised. As a result:

• Teeth fossilise readily, while the cartilage skeleton usually decays without a trace.
• Sharks shed teeth constantly, growing replacements throughout life — some species lose tens of thousands over a lifetime.
• Almost everything known about extinct sharks such as megalodon comes from teeth and a handful of calcified vertebrae.

So the fossil record of sharks is essentially a record of their teeth.

Do sharks have a skull? They have a chondrocranium, a single solid cartilage braincase protecting the brain. It's a skull in function, but it isn't the multi-plated bony skull of mammals or bony fish.

Sharks compared with bony fish:

How sharks stay afloat without a swim bladder. Bony fish adjust a gas-filled bladder. Sharks instead rely on a combination of the light cartilage skeleton, an enormous oil-rich liver — squalene is less dense than water, and the liver can be a quarter of the shark's body weight — and hydrodynamic lift from their pectoral fins while swimming. Some species must keep moving to avoid sinking, though many can rest on the seabed.

Their skin is a surprise. Shark skin is covered in dermal denticles, microscopic tooth-like scales with the same dentine-and-enamel structure as teeth. They point backwards, which makes shark skin feel smooth one way and like sandpaper the other, and they reduce drag and turbulence. The design has been studied and copied for competitive swimwear and aircraft surfaces.

Which other animals have no bones? Rays, skates and sawfish share the cartilage skeleton, as do lampreys and hagfish. Beyond fish, all invertebrates — insects, molluscs, jellyfish, worms — lack bones entirely, which is the vast majority of animal species on Earth.

A common misconception: cartilage doesn't make sharks weak or fragile. A great white's bite force is estimated in the thousands of pounds per square inch, delivered by calcified cartilage jaws. The material is simply doing a different job from bone, and doing it well.