Shark

Sharks are a group of elasmobranch cartilaginous fishes. They have a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not joined to the head. Modern sharks belong to the division Selachii. They are the sister group to the Batomorphi, which includes rays and skates. Some people also use the word "shark" for extinct fish from the Chondrichthyes group that look like sharks, such as hybodonts.

Shark-like fish such as Cladoselache and Doliodus first appeared in the Devonian Period, about 419 to 359 million years ago. Some fossil scales are even older, dating to the Late Ordovician. The earliest known modern sharks (Selachii) are from the Early Jurassic. The oldest known member is Agaleus. True sharks may go back even further, to the Permian.

Sharks come in many sizes. The small dwarf lanternshark (Etmopterus perryi) lives in the deep sea and is only 17 centimetres (6.7 in) long. The whale shark (Rhincodon typus) is the largest fish in the world. It can grow to be very long. Sharks are found in all seas, and they are common down to depths of 2,000 metres (6,600 ft). Most sharks do not live in freshwater, but a few are exceptions. The bull shark and the river sharks can live in both seawater and freshwater, and the Ganges shark lives only in freshwater. Sharks are covered in placoid scales, which protect their skin and help them move through water. They have numerous sets of replaceable teeth.

Several shark species are apex predators, which means they sit at the top of their food chain. Examples include the bull shark, tiger shark, great white shark, mako sharks, thresher sharks, greenland sharks, and hammerhead sharks. Some sharks are filter-feeding planktivores, which means they eat tiny floating animals. Examples are the whale shark and the basking shark. These two are among the largest fish that ever lived.

Humans catch sharks for their shark meat and shark fins. Many shark populations are in danger because of human activities. Since 1970, shark numbers have fallen a lot, mostly because of overfishing and harmful practices such as shark finning.

Etymology

Until the 16th century, sailors called sharks "sea dogs". This can still be seen in several species called dogfish, or the porbeagle.

No one is completely sure where the word shark comes from. One likely idea is that it first meant "a predator, one who preys on others." It came from the Dutch word schurk, meaning 'villain, scoundrel'. Later, people used it for the fish because of how it hunts. You can compare it with card shark and loan shark (cf.). Some people have suggested that it comes from the Yucatec Maya word xoc or xooc, which means 'shark'.

Evolutionary history

See also: Evolution of fish

The oldest total-group chondrichthyans, called acanthodians or "spiny sharks", appeared in the early Silurian period, about 439 million years ago. The oldest confirmed members of Elasmobranchii, the group of cartilage fish that is more closely related to modern sharks and rays than to chimaeras, appeared during the Devonian. The oldest likely relatives of modern sharks and rays, Anachronistidae, date to the Carboniferous. Some scientists think modern sharks and rays split apart during the Triassic. The earliest true sharks may have appeared during the Permian, based on fossils of "synechodontiforms" found in Russia. These sharks were not very varied. Modern shark orders first appeared in the early Jurassic, and true sharks became very varied during the Jurassic. They mostly replaced the hybodonts, which had been a common group of shark-like fish during the Triassic and early Jurassic.

Sharks belong to the division Selachii in the subclass Elasmobranchii within the class Chondrichthyes. Elasmobranchii also includes rays and skates, and Chondrichthyes also includes Chimaeras. It was once thought that sharks were not one single family group, but newer studies show that sharks and rays are each single groups. Selachii is split into the superorders Galeomorphi and Squalomorphi. The galeomorphs are Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and carcharhinoids are usually placed in one clade. The squalomorphs include Hexanchiformes and a group that contains Squaliformes, Squatiniformes, Pristiophoriformes, and the Batomorphi. There are more than 500 species of sharks in thirteen orders, including several extinct orders:

• Carcharhiniformes: Commonly called ground sharks, this order includes the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip reef, and oceanic whitetip sharks (called the requiem sharks). It also includes houndsharks, catsharks, and hammerhead sharks. These sharks have a long snout and a nictitating membrane that protects their eyes during a hunt.
• Heterodontiformes: Generally called the bullhead or horn sharks.
• Hexanchiformes: Examples include cow sharks and frilled sharks, which look somewhat like a sea snake.
• Lamniformes: Commonly called mackerel sharks. They include the goblin shark, basking shark, megamouth shark, thresher sharks, shortfin and longfin mako sharks, and great white shark. They have large jaws and ovoviviparous reproduction, where eggs hatch inside the mother's body. The order also includes the extinct giant megalodon (Otodus megalodon).
• Orectolobiformes: Commonly called carpet sharks. This group includes zebra sharks, nurse sharks, wobbegongs, and the whale shark.
• Pristiophoriformes: Called sawsharks. They have a long snout with teeth along its edge, which they use to catch prey.
• Squaliformes: Includes dogfish sharks and roughsharks.
• Squatiniformes: Called angel sharks. They are flat and look much like stingrays and skates.
• Echinorhiniformes: Includes the prickly shark and bramble shark. Scientists have had trouble placing this group, so it is sometimes given its own order.

Anatomy

Further information: Fish scale § Placoid scales

Shark teeth sit in the gums rather than being fixed to the jaw, and they are replaced throughout life. New rows grow in a groove inside the jaw and slowly move forward like a conveyor belt. Some sharks lose 30,000 or more teeth in their lifetime. Most species replace teeth one at a time, but the cookiecutter shark replaces a whole row at once. The shape of a shark's teeth depends on what it eats. Sharks that eat mollusks and crustaceans have flat, dense teeth for crushing. Sharks that eat fish have needle-like teeth. Sharks that eat larger prey, such as mammals, have pointed lower teeth and triangular upper teeth with serrated edges.

A shark's skeleton is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish, such as skates and rays, have skeletons made of cartilage and connective tissue. Cartilage is flexible and strong, and it is about half as dense as bone, which saves energy. Sharks have no rib cages, so on land they can be crushed under their own weight. Their jaws are not attached to the cranium. The jaw surface has tiny hexagonal plates that make it strong. In the rostrum, or snout, the cartilage is spongy and flexible, which helps absorb impacts.

Sharks have a flexible outer covering of fibers that works like an outer skeleton. It gives their swimming muscles something to attach to. Their skin is covered in dermal teeth, which reduce turbulence and help them swim more smoothly through the water (hydrodynamic advantages). Some species have spots, like the Zebra shark (Zebra shark), and others have stripes, like the Tiger shark. These markings help with camouflage.

Tail shape affects a shark's speed and acceleration, and caudal fin shapes vary between species. Many sharks have a heterocercal caudal fin, where the upper part is usually larger because the vertebral column extends into it. This allows more efficient locomotion. Tiger sharks have a large upper lobe, which helps them cruise slowly and then speed up suddenly. The porbeagle shark hunts fish such as mackerel and herring. It has a large lower lobe that helps it keep pace with its fast prey.

Physiology

Sharks do not have gas-filled swim bladders like bony fish. Instead, they rely on a large liver filled with oil, which contains squalene. Their skeletons are made of cartilage, which is much lighter than bone. Their liver can make up to 30% of their body weight, but it does not work perfectly, so sharks use dynamic lift to stay at depth while swimming. Sand tiger sharks store air in their stomachs, which works somewhat like a swim bladder. Bottom-dwelling sharks like the nurse shark are slightly heavier than water, which lets them rest on the ocean floor. Some sharks enter a calm, still state called tonic immobility when they are turned over or stroked on the nose. Researchers use this to handle sharks safely.

Like other fish, sharks take oxygen from seawater as it passes over their gills. Their gill slits are not covered, and they lie in a row behind the head. A special slit called a spiracle lies just behind the eye and helps with respiration. It is important for sharks that live on the bottom, but it is smaller or missing in active pelagic sharks. Most sharks pump water over their gills while resting. Some kinds must keep swimming or would presumably asphyxiate.

The respiratory and circulatory process begins when venous blood reaches the two-chambered heart. The heart pumps the blood to the gills through branchial arteries. Gas exchange happens there. The blood then flows to the dorsal aorta and around the body. Used blood returns through posterior cardinal veins and sinuses to the heart ventricle.

Most sharks are poikilothermic, which means their body temperature matches the water around them. However, members of the family Lamnidae, such as the shortfin mako shark and the great white shark, are homeothermic. This means they stay warmer than the water. They do this with a strip of aerobic red muscle near the center of the body. A countercurrent exchange system of blood vessels, called the rete mirabile, helps keep the heat in. The common thresher and bigeye thresher have a similar system.

Unlike most bony fish, except the coelacanth, the blood of sharks and Chondrichthyes is isotonic with seawater. This is because it contains high levels of urea and trimethylamine N-oxide (TMAO). This helps them stay in osmotic balance, so they are mostly limited to marine environments. The bull shark is an exception. It changes how its kidney works so it can remove large amounts of urea.

Research by Homer W. Smith in 1930 showed that shark urine does not hold enough sodium to prevent hypernatremia. In 1960, researchers at the Mount Desert Island Biological Laboratory in Salsbury Cove, Maine found a salt gland at the end of the intestine. It is called the "rectal gland," and it removes chlorides.

Digestion can take a long time. Food moves from the mouth to a J-shaped stomach. Unwanted items may be vomited or ejected when the stomach is turned inside out. Sharks have much shorter intestines than mammals. This is because a spiral valve with many turns fits into a short section. Waste passes into the cloaca.

A few sharks, such as the swell shark and the chain catshark, appear fluorescent under blue light. This is due to a fluorophore that comes from a metabolite of kynurenic acid.

Senses

Sharks have good olfactory senses, which means they have a strong sense of smell. The nose sits in a short tube between their two nostrils. Some sharks can notice as little as one part per million of blood in seawater. Sharks that live in murky water usually have larger olfactory bulbs. Sharks of the family Carcharhinidae that live on clear reefs have smaller ones. Sharks can tell which way a smell is coming from by noticing when it reaches each nostril. Many sharks are drawn to chemicals from the intestines of other animals, so they often gather near sewage outfalls. Some species, such as nurse sharks, have barbels on the outside of their bodies that help them sense prey.

Shark eyes work much like the eyes of other vertebrates. They have lenses, corneas, and retinas. A tissue called the tapetum lucidum reflects light back to the retina, which helps them see better in dark water. Many sharks can change the size of their pupils, as humans do. No teleost fish can do this. Sharks do not blink, because the water keeps their eyes clean. Some species have nictitating membranes to protect their eyes, but the great white shark rolls its eyes backwards instead. Some sharks have only rod photoreceptors and no cone cells. This helps them see well at night, but it also makes them colorblind.

Scientists find it hard to test, but sharks may have a good sense of hearing. They may be able to hear prey from many miles away. A small opening on each side of the head leads straight to the inner ear. The lateral line connects to the water outside through a row of pores.

Main article: Electroreception

The ampullae of Lorenzini are organs that sense electricity. Sharks use them to detect the electromagnetic fields that all living things give off. This helps sharks, especially the hammerhead shark, find food. Sharks can also find prey hidden in sand by sensing the electric fields the animals produce.

Main article: Lateral line

The lateral line is found in most fish, including sharks. It helps sharks feel changes in water speed and pressure nearby. Its main part is the neuromast, a cell that is similar to hair cells in the vertebrate ear. It helps sharks sense currents, obstacles, and struggling prey that they cannot see. Sharks can sense frequencies from 25 to 50 Hz.

Life history

Most sharks live 20 to 30 years, but lifespans vary by species. The spiny dogfish has one of the longest lifespans, at more than 100 years, and Whale sharks may also live over 100 years. Earlier estimates suggested the Greenland shark could reach about 200 years. A recent study found that a 5.02-metre-long (16.5 ft) Greenland shark was about 392 years old, or at least 272 years old. This made it the longest-lived vertebrate known. Unlike most bony fish, sharks are K-selected reproducers. This means they have a small number of well-developed young instead of many poorly developed ones. Fecundity in sharks ranges from 2 to over 100 young per reproductive cycle. Sharks mature slowly. Lemon sharks reach maturity at around age 13–15.

Sharks fertilize eggs inside the body through internal fertilization. In males, the back part of the pelvic fins has changed into a pair of intromittent organs called claspers. These work like a mammalian penis to pass sperm into the female. Mating has rarely been observed in sharks. In a few documented cases, a female shark that had no contact with a male had a pup on her own through parthenogenesis. Genetic tests showed the pups had no father's genetic material, which ruled out sperm storage. The genetic fingerprinting method was used for this. Scientists say this kind of asexual reproduction is rare in the wild. It is probably a last try when no mate is present. It lowers genetic diversity, which helps species defend against threats, so species that rely only on it risk extinction. Mammals are now the only major vertebrate group in which asexual reproduction has not been observed. Asexual reproduction may have contributed to the decline of blue sharks off the Irish coast.

Sharks give birth in three ways, depending on the species: oviparity, viviparity, and ovoviviparity. Most sharks are ovoviviparous. Their eggs hatch inside the mother. The yolk and fluids from the oviduct feed the babies, which are born alive. Lamniforme sharks practice oophagy, where the first embryos to hatch eat the remaining eggs. Sand tiger shark pups eat neighboring embryos. These sharks brood their young until they are fairly large. The whale shark is now classified as ovoviviparous rather than oviparous. Most of these sharks give birth in sheltered places, such as bays, river mouths, and shallow reefs. These places offer protection from predators and plenty of food. Dogfish have the longest known gestation period of any shark, at 18 to 24 months. Basking sharks and frilled sharks may have even longer periods, but accurate data are lacking. Some species are oviparous and lay fertilized eggs in the water. Most of these eggs have an egg case with the feel of leather that protects the developing young. The case is often called a mermaid's purse. Oviparous sharks include the horn shark, catshark, Port Jackson shark, and swellshark. In viviparity, young develop without a traditional egg. They are born alive, fully formed, and able to care for themselves.

Behavior

For a long time, people thought sharks were loners that roamed the oceans looking for food. This is true of only a few kinds of sharks. Most sharks live more social, benthic lives and seem to have their own personalities. Even sharks that usually live alone meet up to breed or at places with lots of food. They may travel thousands of miles in one year. Some sharks gather in large groups. Sometimes more than 100 scalloped hammerheads come together around seamounts and islands, such as in the Gulf of California. When someone gets too close, some sharks do a threat display, which usually involves big, exaggerated swimming movements.

Sharks usually swim slowly, or "cruise," at about 8 kilometres per hour (5.0 mph). When they feed or attack, the average shark can swim up to 19 kilometres per hour (12 mph). The shortfin mako shark is the fastest shark. It can burst at up to 50 kilometres per hour (31 mph). The great white shark can also make fast bursts. Compared to their body size, sharks have brains similar in size to those of mammals and birds. They have shown curiosity and play-like behavior in the wild. All sharks need water flowing over their gills to breathe, but not all of them have to keep moving to do this. Some sharks that can rest use their spiracles to push water over their gills, though their eyes stay open. Species that must swim to breathe go through a kind of "sleep swimming." Experiments on the spiny dogfish showed that its spinal cord, rather than its brain, coordinates swimming. In 2016 a great white shark was filmed for the first time in a state that researchers believed was sleep swimming.

Ecology

Most sharks are carnivorous, which means they eat other animals. Some sharks filter tiny floating food called plankton. Basking sharks use ram feeding, and whale sharks use suction to take in plankton and small fish. Megamouth sharks use suction feeding and luminescent tissue inside their mouths to attract prey in the deep ocean. These sharks have gill rakers, which are long thin filaments that work like a sieve. This is similar to the baleen plates of the great whales. Their teeth are small because they are not needed for feeding. Cookiecutter sharks cut flesh from larger fish and marine mammals. Their teeth are huge compared to their size. Angel sharks and wobbegongs hide in their surroundings to surprise their prey. Many benthic sharks eat only crustaceans, which they crush with flat molariform teeth. Others eat squid or fish. The viper dogfish can point its teeth outward to catch prey. The great white swallows small prey whole or bites big pieces from large animals. Thresher sharks use their long tails to stun fish. Sawsharks stir up or slash prey with their toothed rostra. The bonnethead is the only known omnivore, eating crustaceans, mollusks, and seagrass. Many sharks, including the whitetip reef shark, hunt together in groups. These social sharks often travel long distances across ocean basins.

Sharks are found in all seas. Most do not live in fresh water, but the bull shark and the river shark can swim in both seawater and freshwater. Sharks are common down to 2,000 metres (7,000 ft), and some live even deeper. They are almost entirely absent below 3,000 metres (10,000 ft). The deepest confirmed shark was a Portuguese dogfish found at 3,700 metres (12,100 ft).

Relationship with humans

Only a few of the more than 470 shark species have been involved in some attacks on humans. These include the great white, oceanic whitetip, tiger, and bull sharks. In 2006, the International Shark Attack File looked into many reported attacks. Sharks have a reputation as dangerous animals, partly because of a few attacks, such as the Jersey Shore shark attacks of 1916, and fiction like the Jaws films. Later, Peter Benchley, the author of Jaws, and Steven Spielberg, its director, tried to change the idea of sharks as monsters. To lower the chance of an attack, people should avoid shiny jewelry and too much splashing. Sharks do not often attack people, since they prefer fish and other common prey. A recent study by researchers at the California State University's Shark Lab suggested that a shark may mistake a human for its normal prey, such as seals, and that humans and sharks can share the water safely.

Until recently, only a few benthic sharks, such as hornsharks, leopard sharks, and catsharks, lived for a year or more in aquariums. Now more species, including large pelagic sharks, live much longer in captivity. In 2004, the Monterey Bay Aquarium kept a young female great white shark for 198 days before releasing her. Most sharks are not suited to home aquariums, and not every species sold at pet stores is right for a home. Some young sharks, like the nurse shark, grow too large for home tanks, and some owners have been tempted to release them. Sharks also play a large role in culture. In Hawaiian mythology, shark gods and shark aumakua, or ancestor guardians, are common, and the kahuna helped families see their loved ones in a shark. The best-known shark god, Kamohoali'i, was the brother of Pele, and he has a sacred cliff on the crater of Kilauea. Fijian mythology includes Dakuwaqa, a shark god. On Tutuila in American Samoa, a U.S. territory, the place called Turtle and Shark is important in Samoan culture. The U.S. National Park Service notes that villagers from Vaitogi still sing to call the animals to the surface. Sharks in popular culture are often shown as eating machines in Jaws and its sequels. They also appear as threats in films like Deep Blue Sea and The Reef and others, and as funny characters in Finding Nemo and the Austin Powers series. Cartoons such as Tom and Jerry and Jabberjaw also feature them, and a character is sometimes shown on a plank above shark-filled water. A popular myth says sharks are immune to cancer and disease, but scientists have not proven this. Sharks can get cancer, and parasites affect them too. Claims that shark fins prevent cancer or treat osteoarthritis have no scientific support.

Threats to sharks

Many sharks are caught by people every year. Fishing, both for business and for fun, takes very large numbers of sharks. In 2021, scientists estimated that the number of oceanic sharks and rays had dropped by a large amount over the previous 50 years.

Sharks are eaten in many places, including Japan and Australia, where shark is used in fish and chips and is called flake. In India, small sharks are sold in local markets. Icelanders make a food called hákarl from Greenland sharks.

Sharks are often caught for shark fin soup. Fishermen cut the fins off live sharks and throw the rest back into the water. This is called shark finning. Sharks usually do not survive it. Shark fins are traded in secret markets around the world. In 2010, Hawaii became the first U.S. state to ban the sale of shark fins. Shark fin soup is a status symbol in Asian countries, and many people wrongly believe it is healthy. Research has found high levels of BMAA in shark fins. BMAA is a neurotoxin, and scientists are studying it for its possible role in diseases such as ALS, Alzheimer's disease, and Parkinson's disease.

Sharks are also caught for meat. The U.S. FDA lists sharks, along with swordfish, king mackerel, and tilefish, as fish with high mercury content. This can be harmful to children and pregnant women. Sharks take many years to grow up and have only a few young. If many are caught before they breed, future populations are harmed. The International Union for Conservation of Nature suggests that one quarter of all known sharks and rays are at risk of dying out.

Governments have also killed sharks in some places. In 2014, a shark cull in Western Australia killed many sharks, mostly tiger sharks, using drum lines. The cull was cancelled after people protested. The government of Queensland has used drum lines for shark control since 1962. This has also caught dolphins and endangered hammerhead sharks, including in the Great Barrier Reef. In New South Wales, nets have caught many sharks, along with dolphins, whales, turtles, and critically endangered grey nurse sharks. The number of sharks in eastern Australia has dropped greatly. In Kwazulu-Natal, an area of South Africa, many thousands of sharks were killed over 30 years, along with many turtles, rays, and dolphins.

Experts disagree with culling. Humane Society International says sharks are important top predators that help keep oceans healthy. George H. Burgess, a former director of the International Shark Attack File, called culling a form of revenge. Other threats include habitat change, coastal development, pollution, and fishing that damages the seabed and the animals sharks eat. The 2007 documentary Sharkwater showed how sharks are being hunted toward extinction.

Conservation

People have worked to protect sharks for many years. In 1991, South Africa became the first country in the world to protect great white sharks by law. However, the KwaZulu-Natal Sharks Board can still kill them in its "shark control" program in eastern South Africa. In the United States, Congress passed the Shark Finning Prohibition Act in 2000 to stop shark finning at sea. Later, a Federal Appeals Court ruled in 2008 that a loophole let boats that do not fish buy shark fins from fishing vessels on the high seas. To close it, Congress passed the Shark Conservation Act in December 2010. The European Union banned shark finning in 2003. In June 2013, it changed the rules to close the remaining loopholes. In 2009, the International Union for Conservation of Nature listed 64 shark species as being at risk of extinction on its IUCN Red List. Fishing and shark finning were among the reasons. In 2010, the Convention on International Trade in Endangered Species (CITES) did not approve stricter rules for several shark species. The countries needed two-thirds of the votes, and they did not get them. China and Japan led the opposition. In March 2013, three endangered sharks were added to CITES rules. These were the porbeagle, hammerheads and oceanic whitetip. Their fishing and trade now require licenses.

Other groups have also worked to protect sharks. In 2010, Greenpeace added several shark species to a list of common supermarket fish. These fish are often caught in ways that cannot continue for long. The Shark Trust campaigns to limit shark fishing. Seafood Watch tells American shoppers not to eat sharks. The Memorandum of Understanding on the Conservation of Migratory Sharks began in March 2010 under the Convention on the Conservation of Migratory Species of Wild Animals. It was the first global agreement made under that convention. In July 2013, New York state, a major market for shark fins, banned the shark fin trade. Many U.S. territories and states now protect sharks by law. These include California, Hawaii and Texas. Their laws stop the sale or possession of shark fins. Several regions, including the Bahamas, the Maldives and Palau, have shark sanctuaries or have banned shark fishing. In April 2020, researchers used DNA analysis to trace shark fins of endangered hammerhead sharks sold in Hong Kong. They found where the sharks were first caught. In July 2020, a survey of 371 reefs in 58 nations found that no sharks were seen on almost 1 in 5 of them. Sharks are an important part of the ocean ecosystem. A 2021 study in Nature found that overfishing caused a large drop in oceanic sharks and rays over the previous 50 years. The oceanic whitetip and the scalloped and great hammerheads are now critically endangered. Sharks in tropical waters have declined faster than those in temperate zones. A 2021 study in Current Biology found that overfishing is pushing more than one-third of sharks and rays toward extinction.