Sharks are a group of fish with cartilaginous skeleton that have five to seven gill slits on both sides of the head. It is considered that the order of sharks has existed for more than 420 million years and today we know more than 360 species that have been living in our oceans for around 100 million years. Sharks vary enormously in size, from the 17-centimeter dwarf lantern shark to the whale shark, the world’s largest fish, which reaches 12 meters. They also differ with respect to the depths that inhabit existing a great variety of species of shark both on the surface and especially in the depths, and can get to live up to 3,700 meters. The life expectancy of sharks varies according to the species, most live from 20 to 30 years, being the spiny dogfish and the whale shark the longest since they can reach 100 years of age.
Sharks are found in all the seas and oceans of the planet and usually do not live in fresh water, although there are some exceptions such as the bull shark that sometimes goes into rivers. Sharks have a fundamental role in the ocean, especially large predators such as the white shark, tiger shark or blue shark that are at
the top of the food chain providing a fundamental balance between different species of the ocean.
Sharks have skeletons different from those of bony fish or terrestrial vertebrates. Sharks and other cartilaginous fish such as rays have skeletons formed by cartilage and connective tissue, which gives them great flexibility and durability. By having about half the normal density of bone tissue your skeleton is less heavy and need less energy to move. Sharks have a covering of dermal denticles, like many insects, which protects their skin from wounds and parasites but also improves their hydrodynamism.
One of the most striking and even feared characteristics of sharks are their jaws and teeth. The teeth of the sharks are embedded in the gums instead of being attached to the jaw and are constantly replaced throughout life to the point of estimating that from birth to death 30,000 are the teeth replaced. Depending on the diet of each species of shark, this will be their teeth, and they can be broad and flattened in those shark species that need to crush mollusks and crustaceans or sharp and sharp in those that feed on large prey such as marine mammals. They may even be small and featureless for those who feed on plankton such as the basking shark.
The fins of the sharks are elongated and elastic allowing them great mobility and rapidity of movement. Most shark species have 8 fins and differ in shape, length and size. For example, the caudal fin, which provides thrust, speed and acceleration is different in the bull shark, robust and short than in the shark carpet, thin and elongated.
Unlike bony fish, sharks do not have swim bladders filled with gas that provide them with buoyancy. Sharks, on the other hand, have a large liver filled with oil that, together with the low weight of their skeleton, provides them with great buoyancy. Most sharks must swim constantly to breathe and are “cold blooded” which means that their internal body temperature matches that of their surrounding environment.
Most sharks are carnivorous predators except for basking sharks and whale sharks that have evolved into plankton filtrate feeding. The hunting techniques of sharks differ among species. Some of the sharks that inhabit the seabed are very effective in ambushing such as angel sharks or wobbegongs, which use their camouflage to stalk and suck the prey. Many other sharks feed exclusively on crustaceans that crush with their flat teeth like the shark of Port Jackson. In other cases, when the diet is based on fish or squid, the sharks attack and capture their prey by gobbling the whole prey. Large predators like the white shark can swallow whole prey or parts of these prey by tearing their flesh. Others have very refined techniques such as fox sharks that use their long caudal fins to hit the schools of fish, stun them and catch them more easily.
Several shark species, such as white tip reef sharks, hunt in herds and cooperate with each other to capture the food. These social sharks are often migratory, traveling great distances around large 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 mako shark, the fastest shark and one of the fastest inhabitants of the ocean, can exceed 50 kilometers per hour. The white shark also reaches great speeds when it attacks its prey, being able to stand several meters above the surface due to the power of these attacks that occur from the bottom to the surface.
Sharks have a powerful sense of smell and are more attracted to the chemicals found in the intestines of many species. Sharks have the ability to determine the direction of a given odor based on the time of detection of the odor in each nostril. This ability is similar to the method that mammals use to determine the direction of sound.
The eyes of sharks are similar to those of other vertebrates, including pupils and retinas. Sharks can contract and dilate their pupils, like humans, something that no teleost fish can do. To protect their eyes some species have developed membranes that cover the eyes during hunting and when the shark is being attacked. However, some species, such as the great white shark do not have this membrane, but can turn their eyes backwards to protect themselves
Although it is difficult to assess the hearing of sharks, they may have a keen sense of hearing and may possibly hear their prey many miles away.
The sharks have developed, after millions of years of evolution, electromagnetic receptors called Lorenzini ampoules with which they detect the electromagnetic fields that all living beings produce, helping them to detect their prey. With these blisters sharks can, for example, find prey hidden in the sand by detecting the electric fields they produce and help them orient and travel the oceans guiding themselves through the Earth’s magnetic field.
Shark fecundity ranges from 2 to more than 100 offspring per reproductive cycle. Sharks mature slowly in relation to many other fish. For example, lemon sharks reach sexual maturity around 13 to 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 grabbing them to maintain position during mating and courtship. In some species the females have developed a thicker skin that serves as a defense against those “bites of love”.
Sharks have three ways of reproducing, varying according to the species. Most sharks are ovoviviparous, the eggs hatch inside the mother’s body and the offspring are born independent of their mothers. There are other sharks that are oviparous, like most other fish, and lay their eggs in the water. These eggs have a covering similar to the consistency of the leather that protects the developing embryo. Finally, some sharks maintain a placental bond with the developing offspring, they are viviparous.
Although we have the idea of sharks as lone hunters that roam the ocean in search of food, most of the 360 species of sharks are sedentary and live in benthic zones. In fact sharks can be highly social, remaining in large groups that can exceed 100 specimens as in the case of hammerhead sharks.
Every year about 100 shark attacks occur around the world, with seventeen deaths in 2011 from the 118 recorded attacks. Despite the rarity of these attacks, almost everyone fears sharks, a fear that began with a series of shark attacks off the coast of New Jersey in 1916 and that the movie “Shark” by Steven Spielberg led to all the planet through the cinema. 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 deadly attacks, caused in humans: the white shark, oceanic whitetip, tiger shark and bull shark. These sharks are large and powerful predators that can attack and kill people but you have to keep in mind that all of them are photographed without using a protection cage and without causing any attack.
The catch of sharks has increased very rapidly in the last 50 years in correlation to the speed at which the Chinese middle class increases that can afford a bowl of shark fin soup, which has a price of around $ 100. Fishermen catch live sharks, cut their fins and throw the rest of the shark into the sea. Unable to swim the shark dies of asphyxia among terrible pains. This practice called shark finning is still allowed in most countries.
It is estimated that 100 million sharks are caught every year, with Spain being one of the main exporters of fins and shark meat worldwide. Sharks are a common food in many places, including Japan, Australia, India, China, Iceland or Spain, where the dogfish is consumed. Although it is generally considered that shark meat has a high nutritional value, numerous studies confirm that the consumption of fins can accelerate the appearance of Alzheimer’s and its meat has high levels of mercury, being very dangerous for pregnant women and children.
The difference of the overfishing of the sharks with respect to other fish is that the sharks reach sexual maturity after many years and produce few offspring in comparison with other fish. The fishing of sharks that have not yet reproduced seriously affects the balance of their population. Many governments and the United Nations have recognized the need for shark fisheries management, but progress has been limited largely due to the poor public image of sharks.
In 2009, the International Union for the Conservation of Nature has placed up to 64 species of sharks on the Red List of Endangered Species.