Fish
Shark
Selachimorpha · also called sharks
A shark is a very old idea that still works. The design — a cartilage skeleton, a battery of replaceable teeth, a nose that reads electric fields — is around 450 million years old, older than trees, and it has carried sharks through every mass extinction since. There are more than 500 species, from a shark that fits in a hand to the largest fish in the sea.
Overview
Sharks are cartilaginous fish, with a skeleton of cartilage, not bone, five to seven exposed gill slits, skin covered in tooth-like scales, and no swim bladder (a big oily liver provides buoyancy instead). Most are predators of fish, squid, and crustaceans, though the giant whale, basking, and megamouth sharks filter plankton. Sharks live in every ocean, from reefs to the deep sea.
Senses and teeth
A shark hunts with smell, then the vibration-sensing lateral line, then vision, and finally the ampullae of Lorenzini — snout pores that detect the electric fields of hidden prey. Its teeth sit in the gum in rows and rotate forward as they wear, so a shark uses tens of thousands in a lifetime.
Records
The mako and great white keep parts of the body warm for fast hunting in cold seas, while the slow Greenland shark is the longest-lived vertebrate known, some individuals 270 to 500 years old.
Status
Sharks as a group are Not Evaluated, but cartilaginous fish are among the most threatened vertebrates — about a third of species are threatened, mostly from overfishing, bycatch, and the fin trade. Sharks bite about 70 people a year; people kill 70 to 100 million sharks a year.
Shark: common questions
What makes a shark different from other fish?
The biggest difference is the skeleton. Sharks, along with rays and chimaeras, are cartilaginous fish, with a frame of cartilage rather than the true bone of most fish. They also have five to seven separate gill slits on each side with no protective gill cover, tooth-like scales called denticles instead of the flat overlapping scales of bony fish, no gas-filled swim bladder for buoyancy, relying instead on a large oily liver, and internal fertilisation, with the male using a pair of claspers. Their teeth sit in the gum and are constantly replaced rather than being fused into a jawbone. All of this marks out a lineage that split from the bony fish very early in vertebrate history.
How do sharks find their prey?
In stages, using several senses that take over as the shark closes in. From far away it detects the scent of blood or body fluids carried on the current, and it can track that scent trail over long distances, though the idea that it can smell a single drop in an ocean is an exaggeration. Closer in, the lateral line, a row of pressure sensors along the body, picks up the vibrations of a struggling or swimming animal. Nearer still, the shark uses vision, which is good in low light. And in the final approach, when the target may be hidden in sand or hard to see, the electrical sensors on the snout home in on the faint field that every living animal produces, guiding the bite.
Are sharks really dangerous to people?
Far less than their reputation. Of more than 500 shark species, only a handful, chiefly the great white, tiger, and bull sharks, are involved in serious bites on humans, and even for these, attacks are rare and usually appear to be cases of mistaken identity or investigation rather than predation, since people are not normal shark food. Worldwide there are roughly 70 to 100 unprovoked bites in a typical year and fewer than ten deaths, which is a much lower risk than that from bees, dogs, or driving to the beach. The far larger story is the reverse one, of people killing tens of millions of sharks a year and pushing many species toward extinction.
Why are so many sharks endangered?
Sharks are slow to reproduce. Many species take years or decades to mature, and produce only a few young at a time, so populations that are fished hard cannot replace themselves. Demand for shark fin soup drives a large trade in which sharks are caught, their fins removed, and the body often discarded. Sharks are also caught in huge numbers as unwanted bycatch in nets and on longlines set for tuna and other fish, and their coastal nursery habitats are being lost to development and pollution. The result is that about a third of assessed shark and ray species are now threatened, and some of the large open-ocean sharks have declined by more than 70 per cent since 1970.