Sharks are older than trees
Sharks have been swimming in the oceans for around 450 million years. The first trees appeared something like 60 million years later, which makes sharks older than trees and far older than the dinosaurs. They have survived several mass extinctions that wiped out most other life.
A shark has no bones. Its skeleton is made of cartilage, the same bendy material as the tip of your nose, which is light and flexible. Its skin is covered in tiny tooth-like scales that make it feel like sandpaper if stroked the wrong way, and its teeth are replaced constantly, in rows, so a single shark can go through thousands in a lifetime. Sharks have no swim bladder, the gas-filled bag most fish use to float; instead a large, oily liver helps keep them from sinking.
A shark that can live for centuries
The GREENLAND shark lives in the cold, dark waters of the Arctic and North Atlantic and grows about a centimetre a year. In 2016 scientists used radiocarbon dating on the lenses of their eyes and estimated that one female was around 400 years old, give or take many decades — making the species the longest-lived vertebrate known.
Other sharks are strange in other ways. The GOBLIN shark shoots its jaws forward out of its mouth to snatch prey. The THRESHER uses its very long tail like a whip to stun fish. The WOBBEGONG lies flat on the seabed disguised by a fringe of skin, and the ANGEL shark buries itself in sand to ambush passing fish. The FRILLED shark looks much as its ancestors did tens of millions of years ago.
A sixth sense
Sharks can detect the tiny electrical signals made by the muscles and heartbeat of other animals, using jelly-filled pores on their snouts. The HAMMERHEAD's wide, flat head spreads these sensors out, letting it sweep the sand like a metal detector for rays hidden underneath. Sharks can also feel vibrations in the water through a line of sensors along each side of the body.
Not every shark is a hunter of big prey. The BASKING shark, the second-largest fish in the world, swims with its huge mouth open, filtering plankton. The MAKO is among the fastest fish in the sea. The MEGALODON, an extinct giant that died out around three and a half million years ago, may have reached fifteen metres or more.
Who should be afraid of whom
Worldwide, sharks bite people roughly seventy or eighty times a year without being provoked, and only a handful of those bites are fatal. People, by contrast, kill tens of millions of sharks every year, many for their fins, and a large number of species are now endangered. Most sharks are wary of people and avoid them.
Peter Benchley, who wrote the novel Jaws, later said he could never write it again knowing what he had learned about sharks, and spent much of his life campaigning to protect them. Our Jaws page covers the film. The rest of the grid names more of the roughly 500 species: the BULL, TIGER, NURSE, REEF, BLACKTIP, WHITETIP, SILVERTIP, SANDBAR, SPINNER, DOGFISH, BRAMBLE and CARPET sharks.