What makes a shark a shark, and why do they have no bones?
A shark is a fish whose whole skeleton is cartilage, the same bendy stuff in your nose and ears, and the family goes back hundreds of millions of years.
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Which one is it?
A shark is a fish with a skeleton made of instead of bone. Cartilage is the flexible, rubbery tissue that shapes your nose and ears, and in a shark it forms everything from the skull to the spine. Biologists call sharks, skates and rays cartilaginous fishes. A shark also has five to seven gill slits on each side of its body, pectoral fins that are not joined to its head, and skin covered in tiny tooth-like scales called .
is about half as dense as bone, so a cartilage skeleton is lighter and saves energy. It does come with a price: sharks have no rib cages, so on land they can be crushed under their own weight. In the sea the water holds them up. Unlike most fish, sharks also have no gas-filled to keep them afloat. Instead they rely on a big liver filled with oil, and the liver is limited as a float, so sharks also use lift from swimming to hold their depth.
Step 1: Light skeleton
About half the density of bone.
Step 2: A big oily liver
Filled with oil, it helps keep the body up.
Step 3: Lift from swimming
Moving forward gives lift, like a wing, to hold depth.
Sharks are very old. Their oldest relatives, known as spiny sharks, appeared around 439 million years ago. The earliest confirmed modern sharks are known from about 200 million years ago.
The word covers far more than the film monster. Sharks range from a deep-sea species only 17 centimetres long to the whale shark, the largest fish in the world, which feeds on . Until the 16th century, sailors simply called them sea dogs.
Try it
When does a body float?
It sinks
- Under water
- 100%
- Water's push
- 85% of its weight
Challenge · Lower the density until the block just floats in sea water. A shark's oily liver and light cartilage push it that way but not all the way, which is why it also swims for lift.
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Recap
A shark is a fish with a cartilage skeleton, no swim bladder and a family tree that reaches back about 439 million years.
💡 A trick to remember it · Think nose and ear: the bendy stuff you can fold is what a shark is built from, head to tail.
Surprising fact · The whale shark, the largest fish in the world, is a shark.
Connects to
- ⚡ How do sharks find prey in dark water: smell, electric sense and pressure?
- 🦈 How does a great white shark hunt, and where does it live?
- 🦵 What holds a joint together and stops bone grinding on bone?
- 🛳️ How can a ship made of iron float when an iron block sinks?
- 🦋 How does natural selection actually change a species?
Sources (1)
No source, no claim. Every fact in this lesson (12 claims) cites at least one of these.