Sharks are a group of cartilaginous bony fish that have five to seven gill slits on both sides of the head.
It is estimated that the order of sharks has existed for over 420 million years and now we know about more than 360 species that have inhabited our oceans for about 100 million years.
Sharks vary greatly in size, from the dwarf lanternshark with just 17 centimeters to the whale shark, the world's largest fish, which reaches up to 12 meters long. They live in very different habitats, from the surface of the ocean to the depths, being sharks able to live up to 3,700 meters deep. Life expectancy of sharks varies by species, most live 20 to 30 years, being the spiny dogfish and whale shark the sharks that live longer, as they can live up to 100 years.
Sharks are found in all seas and oceans in the planet and usually do not live in freshwater, although there are some exceptions such as the bull shark that sometimes goes into rivers. Sharks play a key role in the ocean, especially large ones such as the great white shark, the tiger shark and the blue shark, at the top of the ocean food chain, because they provide a fundamental balance between the different species of the ocean.
Sharks have different skeletons from the bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish like rays have skeletons made of cartilage and connective tissue, which gives them great flexibility and durability. Being about half the normal density of the skeleton bone tissue, the cartilage is lighter and requires less energy to move. Their skin is covered by dermal denticles (like many insects) which protects their skin from wounds and parasites while improving its hydrodynamics.
Sharks can change their teeth up to 30,000 times in a lifetime. Lemon sharks, image by WIlly Volk
One of their most striking (and feared) features are their jaws and teeth. The sharks' teeth are embedded in the gums rather than being attached to the jaw and are replaced all throughout their life. It is estimatied that from birth to death, up to 30,000 teeth are replaced. The teeth shape varies with the shark's diet. It can be wide and flat in those shark species that need crushing shellfish or pointy and sharp in those sharks that feed on larger prey such as marine mammals. They may even be smaller without any specific functionality for those that feed on plankton such as the basking shark.
The shark fins are elongated and elastic allowing them great mobility and speed. Most shark species have 8 fins and differ in shape, size and length. For example the caudal fin, that provide thrust, speed and acceleration, is different in the bull shark (robust and short) than that of the carpet shark, long and slender.
Unlike bony fish, sharks do not have gas-filled swim bladders that provide buoyancy. Instead they have a large liver filled with oil which together with the low weight of the skeleton provides great buoyancy. Most sharks need to constantly swim to breathe and are "cold blooded" meaning that their internal body temperature matches that of their environment.
Port Jackson shark teeth are adapted to feed from molluscs. Image by richard ling
Most sharks are carnivorous predators except the basking shark and the whale shark, that have evolved to feed by filtering plankton.
Hunting techniques of sharks differ between species. Some of the sharks are very effective at ambushes like wobbegongs or angel sharks, using their camouflage to stalk prey and suck. Many other sharks feed solely on crustaceans crushing them with their flat teeth, as the Port Jackson shark.
In other cases, when the diet is based on fish or squid, sharks attack and capture prey swallowing the whole animal. Large predators such as the great white shark can swallow the whole prey or parts of them tearing his flesh. Others have very refined techniques like the thresher shark, which use its long tail fin to hit fish schools, stun them and catch them more easily.
Several shark species such as the white tip reef shark hunt in packs and cooperate among themselves to capture food. These social sharks are often migratory, traveling huge distances around major ocean basins.
In general, sharks swim at an average speed of 8 kilometers per hour but when they attack they can reach speeds of 20 kilometers per hour. The shortfin mako shark, the fastest shark and one of the fastest ocean predators, can exceed 50 kilometers per hour. White sharks also reaches high speeds when attacking their prey, being able to jump several meters out of the surface due to the power of these attacks, swimming vertically from the bottom to the surface.
White shark hunting captured in slow motion
Sharks have a powerful sense of smell and are attracted by the chemicals found in the guts of many species. Sharks have the ability to determine the direction of a smell based on detection of the odor in each nostril. This capability is similar to the method mammals use to determine direction of sound.
Shark eyes are similar to the eyes of other vertebrates, including pupils and retinas. The sharks can contract and dilate their pupils, like humans, something no teleost fish can do. To protect their eyes some species have developed a membrane that covers the eyes while hunting and when the shark is being attacked. However, other species such as the great white shark do not have this membrane, but can direct their eyes backwards to protect them.
Bahamas Reef Shark. Picture of Joi Ito
Although it is difficult to assess the hearing of sharks, they may have an acute sense of hearing and can possibly hear prey many miles away.
Sharks have developed, over millions of years of evolution, some electromagnetic receptors called the ampullae of Lorenzini that detect the electromagnetic fields that all living things produce, helping them to detect prey. With these blisters sharks can, for example, find prey hidden in the sand by detecting the electric fields they produce and help orient themselves and travel the oceans guided by Earth's magnetic field.
Shark fertility varies from 2 to over 100 offspring per reproductive cycle. Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity around 13-15 years. Females in many of the larger species of sharks have bite marks that appear to be the result of one or several males grasping to maintain position during mating and courtship. In some species, females have evolved thicker skin that serves as a defense against those "love bites".
Shark embryo. Image Sander van der Wel
Sharks have three ways to breed, varying by species. Most sharks are ovoviviparous, the eggs hatch inside the mother's body and the young are born independent from their mothers. Other sharks are oviparous like most other fish, and lay their eggs in water. These eggs have a cover similar in consistency to leather which protects the embryo. Finally some sharks maintain a placental link to the developing offspring, are viviparous.
Although we have the idea of sharks as lone hunters who roam the ocean in search of food, most of the 360 species of sharks are sedentary and live in benthic areas. In fact sharks can be highly social, remaining in large groups that can exceed 100 specimens as in the case of hammerhead sharks.
In areas like Coco Island or Galapagos Island is possible to spot schools of hammerhead sharks over 100 specimens. Image by zlatkarp
Every year around 100 sharks attack humans worldwide, having recorded seventeen deaths in 2011 of the 118 total registered attacks. Despite the rarity of these attacks almost everyone is afraid of sharks, fear that began with a series of shark attacks off the coast of New Jersey in 1916 and by the movie "Jaws" by Steven Spielberg. Almost all shark experts consider that the danger presented by sharks is exaggerated, contrary to popular belief, only a few sharks are dangerous to humans. Only four species of shark have participated in a significant number of fatal attacks in humans: the great white shark, oceanic whitetip shark, the tiger shark and the bull shark. These sharks are large, powerful predators which can attack and kill people but you need to keep in mind that all of them are photographed without using a protective cage with no attacks.
The shark catch has increased rapidly over the last 50 years in correlation to the increased speed at which the Chinese middle class can afford a bowl of shark fin soup, which is priced at around $ 100. Fishermen capture live sharks, slice off the fins and throw the rest of the shark to the sea. Unable to swim the shark dies of suffocation on terrible pain. This practice called shark finning is still permitted in most countries.
Shark fin market in Hong Kong. Image Wikipedia
An estimated 100 million sharks are caught each year being Spain one of the leading exporters of shark fins and meat worldwide. Sharks are a common food in many places, including Japan, Australia, India, China, Iceland, UK and Spain. Although it is generally considered that shark meat has high nutritional value numerous studies confirm that eating the fins can accelerate the onset of Alzheimer's and its meat has high levels of mercury and is very dangerous for pregnant women and children.
The main problem of overfishing sharks over other fish is that sharks reach sexual maturity after many years and produce few offspring in comparison to other fish. Fishing sharks that have not yet mated seriously affect the balance of the sharks population. Many governments and the UN have acknowledged the need for management of shark fisheries, but progress has been limited largely by the poor public image of sharks.
In 2009, the International Union for Conservation of Nature has placed on the Red List of Threatened Species up to 64 species of sharks.
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