Sharks

When the first sharks swam the seas, there were no trees, no insects and no animals of any kind on land. Dinosaurs would not appear for another 200 million years. Sharks have outlasted them all, surviving every mass extinction, and they did it with a body plan so effective that it has barely needed to change. The result is an animal of enormous grace and efficiency with an undeserved reputation as a monster.

A different kind of fish

Sharks are fish, but not like a cod or a goldfish. They have no bones. Their skeleton is cartilage, the same flexible material that forms your nose and ears, which is lighter than bone and lets them turn and accelerate fast. Their skin is covered in tiny tooth-like scales that reduce drag so effectively that swimsuit designers have copied the texture. They have no swim bladder to keep them buoyant; instead they rely on an oily liver and must keep moving, or glide, to avoid sinking.

And the teeth. Shark teeth are not rooted in the jaw but held in the gums, in rows, like a conveyor belt. When a front tooth is lost, the one behind moves forward to replace it, and a shark can go through 30,000 teeth in a lifetime. That is why fossil shark teeth are so common on beaches. The shape varies with diet: triangular and serrated for slicing seal, needle-like for gripping fish, flat plates for crushing shellfish.

Senses

Sharks hunt with a battery of senses finer than ours. Their smell is legendary, able to detect blood at concentrations of a few parts per billion, and their nostrils are so sensitive to timing that they can tell which side a scent reached first and turn toward it. They hear low-frequency sounds from kilometres away. A line of pressure sensors along each side of the body feels the movement of water around a struggling fish.

Most remarkably, pores around the snout filled with a conductive jelly detect the faint electric fields that every living animal produces when its muscles move or its heart beats. A shark can find a flatfish buried in sand by its heartbeat alone. The hammerhead's strange head is partly a wide platform for these sensors, sweeping the seabed like a metal detector.

More than one shark

More than 500 species of shark are known. The whale shark grows to 18 metres and is the largest fish alive, yet it eats plankton and small fish, filtering them from the water as it swims open-mouthed. The dwarf lanternshark would fit in your hand and glows in the dark. The Greenland shark drifts through Arctic waters at a kilometre an hour and lives for 400 years or more, reaching maturity at 150. Wobbegongs lie camouflaged on reefs; thresher sharks stun fish with whip-like tails; cookiecutter sharks take neat round bites out of whales and submarines. The great white, the species of nightmares, reaches six metres, can warm its muscles above the sea temperature, and launches itself clear of the water to take seals from below.

The reputation

Sharks kill on average between five and ten people a year worldwide. Dogs, cows, hippos, bees and falling coconuts each kill more. Most bites are cases of mistaken identity in murky water, and most are not fatal. Sharks have no interest in humans as food; they have been eating fish and seals for hundreds of millions of years and we taste wrong.

The killing runs the other way. An estimated 100 million sharks a year die at human hands, caught deliberately for their fins and meat or accidentally in nets set for other fish. Because sharks grow slowly, mature late and have few young, often fewer than a dozen a year, they cannot recover quickly. A third of shark and ray species are now threatened with extinction, and populations of oceanic sharks have fallen by about 70 percent in fifty years.

Why it matters

Sharks sit at the top of ocean food webs, and removing them has consequences all the way down. Where sharks have been fished out, mid-sized predators multiply and strip the reefs and seagrass beds of the fish and grazers that keep them healthy. Reefs with healthy shark populations are richer in nearly everything. An animal that survived 450 million years of planetary upheaval is now, in a few human generations, being pushed toward the edge, and the oceans will be poorer for it.