Sharks: 400 Million Years of Survival in the World's Oceans

What Makes a Shark a Shark?

Sharks are fish, but they are very different from the salmon, cod and goldfish most people know. Those familiar fish are bony fishes, with skeletons made of true bone. Sharks belong to a separate group called the cartilaginous fishes (Chondrichthyes), whose skeletons are made of cartilage, the same flexible tissue that shapes the human nose and ears. Within this group, sharks belong to the elasmobranchs, together with their closest relatives, the rays and skates.

Scientists recognise a shark by a set of features working together:

• a skeleton of cartilage with no ribs;
• skin covered in tiny tooth-like scales called dermal denticles;
• five to seven gill slits on each side of the head;
• pectoral fins that are not fused to the head.

That last point is what separates sharks from rays. A ray is essentially a flattened relative whose pectoral fins have joined to the sides of its head, forming the wide "wings" it uses to glide. Modern sharks are grouped by scientists in the division Selachii, and the rays and skates (Batomorphi) are their sister group, meaning the two lineages share a common ancestor and split from each other long ago.

Today there are more than 500 known species of shark, and new ones are still being described, especially from the deep sea. They live in every ocean, from shallow coral reefs and coastal bays to the cold, dark water thousands of metres below the surface.

An Ancient Lineage

Sharks are one of the oldest surviving groups of vertebrates. Tiny fossil scales that look like those of cartilaginous fishes have been found in rocks from the Late Ordovician Period, roughly 450 million years ago. Because cartilage rarely fossilises, much of the early history of sharks is known only from scales, teeth and fin spines rather than complete skeletons.

By the Devonian Period, often called the "Age of Fishes", shark-like animals were clearly present. Doliodus, from the Early Devonian, is one of the oldest known shark-like fishes preserved with much of its body. Cladoselache, a famous Late Devonian fossil from North America, had a streamlined body, a forked tail and a mouth at the front of its head. These creatures were swimming in the seas long before the first dinosaurs appeared.

These early animals were not yet "modern" sharks. The earliest confirmed members of the modern group, the Selachii, are known from the Early Jurassic, around 200 million years ago, and the oldest named one is Agaleus. Some researchers think true modern sharks may go back even further, into the Permian Period, but the evidence is still debated.

Sharks have lived through several of the great mass extinctions in Earth's history, including the one that ended the age of the dinosaurs about 66 million years ago. One of the most famous extinct sharks, Otodus megalodon, lived millions of years later and disappeared only a few million years ago. It is known mainly from its huge, triangular teeth, which can be about the size of a human hand. Estimates of its length vary, but it was far bigger than any predatory shark alive today.

Built for the Water: Shark Anatomy

A Light, Flexible Skeleton

Cartilage is lighter and more flexible than bone, which helps sharks turn and swim efficiently. Parts of the skeleton, such as the jaws and spine, are strengthened with mineral deposits so that they can withstand the forces of biting and swimming.

Skin Like Sandpaper

If you stroked a shark from head to tail, its skin would feel fairly smooth; stroked the other way, it would feel rough, like sandpaper. This is because of the dermal denticles. Each denticle is built much like a tooth, with a core of pulp, a layer of dentine and a hard outer coating. Their shape and grooves help the water flow smoothly over the body, reducing drag. Engineers have studied shark skin when designing swimsuits, ship coatings and surfaces that resist the growth of bacteria.

Floating Without a Swim Bladder

Most bony fishes stay afloat with a gas-filled swim bladder. Sharks do not have one. Instead, many rely on a very large liver filled with oil, which is lighter than seawater. In some species the liver makes up a large share of the body's weight. Sharks also generate lift with their pectoral fins and their tail, much as an aeroplane's wings create lift as it moves forward. The sand tiger shark has an extra trick: it gulps air at the surface and holds it in its stomach to help it hover in the water.

Breathing

Water must flow over a shark's gills for it to take in oxygen. Some species, such as the nurse shark, can pump water over their gills using muscles in their mouth and throat, so they can rest on the sea floor. Others, including the great white and the mako, depend mostly on ram ventilation: they swim with their mouths slightly open so that water is pushed over the gills. For these sharks, swimming and breathing are closely linked.

Endless Teeth

Shark teeth are not fixed in sockets like ours. They grow in rows on the inside of the jaw and move forward like a slow conveyor belt. When a front tooth breaks or falls out, one from the row behind takes its place. Over a lifetime, a shark can go through thousands of teeth, which is why fossil shark teeth are among the most common vertebrate fossils on Earth. The shape of the teeth reveals the diet: needle-like teeth grip slippery fish, serrated triangles slice through large prey, and flat, crushing teeth break open shellfish.

The Sixth Sense and Other Super-Senses

Sharks have an impressive set of senses that allow them to find prey in murky water, in darkness and over long distances.

• Smell: Sharks can detect very small amounts of certain substances dissolved in water, such as blood or fish oils, and they can tell which nostril picks up a scent first, helping them steer towards its source.
• Hearing: They are especially sensitive to low-frequency sounds, such as those made by a struggling fish.
• The lateral line: A line of sensory canals along each side of the body detects movement and pressure changes in the surrounding water.
• Vision: Many sharks have a reflective layer behind the retina, called the tapetum lucidum, which helps them see in dim light. Some species have a protective inner eyelid, the nictitating membrane, while the great white rolls its eyes back in its head when it strikes.
• Electroreception: This is the shark's "sixth sense". Small jelly-filled pores on the snout, called the ampullae of Lorenzini, detect the weak electric fields produced by the muscles and nerves of living animals. A shark can find a fish hidden under sand simply by sensing these signals at close range.

The unusual head of the hammerhead shark, called a cephalofoil, spreads these sensors across a wide area. Scientists believe this helps hammerheads sweep the sea floor for buried prey such as stingrays, a little like a metal detector, and may also improve their manoeuvrability and give their eyes a wider field of view.

A Huge Variety of Shapes and Lifestyles

Modern sharks are usually divided into eight or nine orders, and they vary enormously. The smallest, such as the dwarf lanternshark, are only about 20 centimetres long when fully grown and could fit in your hand. The largest, the whale shark, can reach more than 12 metres and is the biggest fish in the world.

Surprisingly, the largest sharks are gentle filter feeders. The whale shark, the basking shark and the rarely seen megamouth shark swim with their enormous mouths open and strain plankton, small fish and tiny shrimp-like animals from the water.

Other sharks show remarkable specialisations:

• The great white shark, the mako and their relatives can keep some of their muscles warmer than the surrounding water, using a special network of blood vessels that holds on to heat. This helps them hunt actively in cool seas. The shortfin mako is considered one of the fastest sharks.
• The bull shark can survive in fresh water and has been found far up rivers such as the Mississippi and the Amazon.
• The cookiecutter shark, only about half a metre long, attaches to much larger animals, such as whales and big fish, and twists its body to remove a neat round plug of flesh.
• The wobbegongs and angel sharks are flattened ambush predators that lie camouflaged on the sea floor and snap up passing prey.
• Many lanternsharks and the kitefin shark can produce their own light, and some catsharks glow green under blue light, a phenomenon called biofluorescence.

The Greenland shark holds a different kind of record. Living in the icy waters of the Arctic and North Atlantic, it grows extremely slowly. In 2016, scientists used radiocarbon dating on the lenses of the sharks' eyes and estimated that the oldest animal in their study was at least 272 years old, making the Greenland shark the longest-lived vertebrate known.

How Sharks Are Born

Unlike most fish, which release huge numbers of eggs into the water, sharks use internal fertilisation. Males have a pair of organs called claspers, which develop from their pelvic fins. Shark mothers produce relatively few offspring, but each one is large and well developed at birth.

Sharks have three main ways of bringing young into the world:

1. Laying eggs. Some species, such as many catsharks and horn sharks, lay eggs in tough, leathery cases. Empty cases often wash up on beaches, where they are known as "mermaid's purses". Horn sharks lay spiral-shaped cases that can be wedged into rock crevices.
2. Eggs that hatch inside the mother. In many species the young develop in eggs inside the mother's body and are born live once they hatch. In some of these sharks, such as the great white and the mako, the growing embryos feed on unfertilised eggs. In the sand tiger shark, the largest embryo in each of the two uteri eats its smaller siblings, so usually only two pups are born.
3. Placental live birth. In species such as hammerheads and the blue shark, the embryo is connected to the mother by a placenta, similar in function to that of mammals, which supplies nutrients until birth.

Pups are independent from the moment they are born and receive no care from their parents. Many species give birth in sheltered coastal "nursery" areas, where young sharks are safer from larger predators. In aquariums, females of a few species, including the bonnethead and the zebra shark, have even produced young without a male, a process called parthenogenesis.

Why Sharks Matter to the Ocean

Many sharks are top predators, and they help keep marine ecosystems in balance. By hunting sick, weak or abundant prey, they influence the size and health of other animal populations. Their presence also changes how prey animals behave. In Shark Bay in Western Australia, for example, researchers have found that the risk of meeting tiger sharks affects where sea turtles and dugongs feed, which in turn helps protect seagrass meadows from being overgrazed.

Sharks are also great travellers that connect distant parts of the ocean. In the early 2000s, a female great white shark tagged off South Africa swam all the way to Australia and back in less than a year, a round trip of around 20,000 kilometres. Satellite tags have shown that many sharks undertake regular long migrations between feeding and breeding areas.

Fear, Facts and Conservation

Films and news stories have given sharks a fearsome reputation, but bites on humans are rare. Most species are small or live far from people, and only a handful are responsible for most serious incidents. Worldwide, unprovoked shark bites usually cause fewer than ten deaths in a year, far fewer than many everyday hazards.

The real danger runs the other way. Humans kill enormous numbers of sharks: a widely cited study published in 2013 estimated that around 100 million sharks are killed every year. Sharks are caught for their meat, liver oil and especially their fins, which are used in shark fin soup. Many more die accidentally as bycatch in nets and on long fishing lines set for other species.

Sharks are especially vulnerable because of their slow life cycle. Many species take years to reach maturity and produce only a few young at a time, so their populations recover slowly once they have been reduced. A study published in 2021 found that the abundance of oceanic sharks and rays had fallen by 71 percent since 1970. Today, more than a third of all shark and ray species are considered threatened with extinction.

There is also good news. Many countries have banned or restricted shark finning, international agreements now control trade in several threatened species, and marine protected areas give sharks safe places to feed and breed. In some regions, shark-watching tourism earns more money from living sharks than fishing could earn from dead ones. After surviving for hundreds of millions of years, sharks now depend on how well people understand and protect them.