Sharks are one of the most formidable inhabitants of the ocean. The absence of bone tissue and gill covers, structural features of scales and many other signs of the structure speak of their ancient origin, as evidenced by paleontological data - the age of the fossil remains of the first sharks is approximately 350 million years. Despite the primitiveness of the organization, sharks are among the most advanced predatory fish in the ocean.
Modern cladistic-systematic representation of the evolution of the jaw. The most primitive “fish of group 1” from our review is not shown here, because they are not Gnatostomy yet. With the exception of farm animals, we can designate all taxa listed here with a clear conscience and scientifically correct as “fish.” We see that there are many lines of "fish", some have become extinct.
Anyone who now thinks that he has passed a complex systematic lesson will be disappointed. Thus, they constitute a traditional, but not monophyletic, taxon bone fish and farm animal. The monophyletic lower taxa of the eutelostomy is the radial fins and the muscular thread, to which, in the sense of cladistics, the peasants must also be taken into account. General characteristics: for terrestrial vertebrates. In addition, we can refer to all taxa of the pine of mouths with a clear conscience and from a scientific point of view correctly, like “fish”, whether it is coelacant, armored fish or sharks.
For a long period of existence, they managed to perfectly adapt to life in the water column and now successfully compete with bony fishes and marine mammals. Unlike bony fish, sharks and stingrays do not spawn, but lay large eggs covered with a cornea or give birth to live young.
Whale sharks (up to 20 meters) and so-called giant sharks (up to 15 meters) reach the highest value. Both those and others, like baleen whales, feed on planktonic organisms. Having opened its mouth wide open, these sharks slowly swim in the thick of clusters of plankton and filter water through the gill openings, tightened by a network of special outgrowths of the surrounding tissue. A giant shark per hour filters up to one and a half thousand cubic meters of water and extracts from it all organisms larger than 1-2 millimeters.
It was a long way: pre-scientific classification and the problem of defining fish
Hardly any other group of animals is as difficult to define as “fish” because this word has undergone a change in meaning over time. For people of the Middle Ages, everything that lived in the water was fish - unlike animals on land and birds in the air. As a result, whales, seals, crabs, jellyfish, mussels, etc. there were all the fish - not because you didn’t know better, but because you defined it that way. The definition was not only in theory, but had quite pragmatic reasons for food and other pleasures, because during the liturgical year, there were more than 100 quick days on which sexual intercourse and meals were banned from warm-blooded animals.
About reproduction of plankton sharks there is very little information. Eggs and embryos of a giant shark are generally unknown. The smallest specimens of this species are 1.5 meters in length. Whale shark lays eggs. It is safe to say that this is the most large eggs in the world, their length reaches almost 70 centimeters, width - 40. Plankton-eating sharks are slow and not at all aggressive.
However, fish are known as “cold-blooded” and therefore do not fall under the ban. The more animals are declared “fish”, the easier it is for people to continue to starve. A curious idea was that land creatures in the water have an appropriate fish friend. There were not only sea animals, sea lions, seals, sea horses, seaweed, sea hares, sea water, sea snakes and others.
Long before humans, long before dinosaurs conquered the land, sharks crossed worlds. The first prototypes already existed almost 400 million years ago, and about 200 million years ago they already looked something like this. However, it is difficult for researchers to recreate the history of shark development. This is because sharks are cartilage fish. Their skeleton consists not of bone, but of cartilage. Because of its internal structure, cartilage is barely stained - unlike bone. The forerunners of bony fish, whose biodiversity today is much larger than that of sharks, are still often found as fossils.
Some shark species live near the bottom and feed on bottom-shellfish and crustaceans. These are small (no more than a meter in length) feline sharks. They live near the coast, often forming large shoals. The catch of a prickly shark in the Atlantic Ocean sometimes reaches 20 thousand pieces for one notice of the seine.
In the open ocean, there are sharks of other species, and they do not form shoals, and scour alone or in small groups. It happens that such sharks come to the shores, and most of the attacks on bathing people are made by them. Among these predators, the most dangerous are the white, gray-blue, tiger, blue, long-armed sharks and the hammerhead shark.
Among sharks, usually only fossil teeth indicate the past of this group of animals. Based on dental finds, such as Megalodon, which lived more than 25 million years ago and is the ancestor of the White Shark, researchers can determine the size and shape of the Ura shark's body. Since petrified teeth often resemble the comb of modern sharks, it is assumed that sharks already looked in antiquity like today.
Another difference between sharks and bone fish: sharks do not have a swim bladder that provides buoyancy in bone fish. Sharks, on the other hand, must constantly move, otherwise they will drown. Today, sharks are home in all seas, some species, such as the bull shark, can be found even in fresh water, for example, in the African Sambesi or in the South American Amazon. But sharks are not like sharks. There are hundreds of species. Most of them have a typical body shape in the form of a torpedo, but the angel sharks are more like skates.
The largest representative of this group of fish - the whale shark, does not threaten the life of people; it feeds on small fish and crustaceans. Although statistics show that there are significantly fewer deaths from sharks than is commonly believed, you should still be wary of any shark whose length exceeds 1–1.2 meters, especially when there is blood or food in the water. Sharks have the phenomenal ability to detect a wounded or helpless animal at a great distance by its convulsive movements or by blood trapped in water.
Some species live on the seabed, others in the middle of the water or near the surface. From a well of a 20-meter small lantern of a dwarf shark to a 14-meter whale shark, the largest fish in the world, differences in size. Most sharks eat fish, clams, crabs. Species such as whale and giant shark, feed on plankton, and large predators, such as the White Shark, also snatch seals, penguins or dolphins.
Perfect hunter: White shark. Typical sharks are one or two. dorsal fin, from five to seven gill slits and their revolving bite. Sharks have several rows of sharp sharp teeth, requiring only the first row for a bite. The rest are interchangeable teeth that are constantly growing. If the wear tooth is removed, the new tooth will move out of the replacement of the dental bearing.
Physiologists still have not precisely established what organs of senses the shark is guided in searching for food. It is believed that it detects the victim through a highly developed sense of smell, and a lateral line system that senses the slightest low frequency oscillations. A significant role, apparently, is also played by hearing and special taste organs. One way or another, but as soon as the water is stained with blood, there are sharks, which have never been seen before. An ominous shadow glides quickly in the clear water, led and accompanied by an escort of pilot fish.
Shark skin is no less inventive: a lot of small pennies that stand in the direction of the tail provide a significantly reduced swimming speed. Sharks can hunt faster and easier through the water. In the meantime, a person has also developed a shark skin plan and, in accordance with this principle, for example, films can be developed with which the aircraft can be used to reduce fuel consumption. In the case of ships, the shark must also ensure that a smaller amount of smallpox and mussels are detected.
One of the reasons why sharks have been successful for a long time, because the predators of the seas lie in their phenomenal sense organs. They can hear very well, in the dark better than cats or wolves, they see and smell fantasticly: on hundreds of meters sharks can detect the smell of their prey, often enough drops of blood, sharks already absorb the weather.
A shark hunting alone acts slowly and calmly. Usually she does not attack her victim right away and often shows a certain caution. When fishing for sharks on a special tackle from the ship’s side, it’s clear how the predator circles around the bait for a long time until it finally decides to grab it. It attacks quickly and suddenly. Special dangerous sharks become, when in water a lot of blood and food. Like mad, they float vertically from depth to surface, jump out of the water and violently grab everything that comes into their eyes, including their relatives. At such moments, they can not scare or drive away. Only when food runs dry will sharks disappear.
In addition, the so-called page organ ensures that they can experience pressure fluctuations. Hair sensory cells under the skin transmit pressure changes in nervous system. For example, a shark can recognize floats or currents. This is by far the most amazing person for us, as he completely eludes us, this is the electric meaning of a shark. Through the so-called Lorenzin ampoules, pores filled with nerve cells in the muzzle, the shark perceives a weak electric field, which - through heart rhythm or muscle movements - produces every living thing.
Different types of sharks lead different lifestyles and are quite different from each other in body structure and behavior. Together with the stingrays, sharks belong to the most primitive group of fish, which is called cartilage, since their skeleton consists only of cartilage and is completely devoid of bone tissue. If you “stroke” a shark or a stingray in the direction from the head to the tail, their skin will appear only slightly rough, but when you move your hand in the opposite direction you will feel sharp teeth like on coarse sandpaper. This is due to the fact that each scale of cartilaginous fish is supplied with a small spike, with its tip turned back. Outside, the pinch is covered with a layer of durable enamel, and its base in the form of an expanding plate is embedded in the skin of fish. Inside each scale are blood vessels and a nerve. At the edges of the mouth is located larger scales, and in oral cavity sharks spines scales reach significant size and serve no longer as covers, but as teeth.
Sharks are not only killed out of sports ambitions or fear, but also because of their flesh. For example, Schillers are made from meat spikes. Cod liver skin and oil in sharks are still exploited mainly in third world countries. At the same time, animals are cut off by living bodies. The fins can be dried more easily and, therefore, more durable than the rest of the meat. They are used, for example, for boiling soup, which finds fast sales in China and Japan. Sharks are thrown back into the sea, like debris after cutting off fins, where they can be smothered or eaten by other animals, since they can no longer swim.
Thus, shark teeth are nothing but modified scales. It is easy to see that the structure of the scales (and teeth) of sharks corresponds in general to the structure of the teeth of mammals. Maybe this discovery will not please everyone, but the fact remains: we inherited our teeth from very distant ancestors, who are similar in structure and way of life to modern sharks.
Sharks also often fall into size off the coast or into giant nets of the fishing industry, where they die from deadly death. The increasing use of drifting nets is so finely meshed that they cannot be detected by sharks and are a deadly trap for them, like for dolphins, seabirds and turtles.
Biology Greenland sharks 400 years
The Swiss Shark Foundation claims that three sharks die every second in the world. In the World Wide Fund for Nature conservation organization, it is estimated that one-year fishing alone saves millions of sharks and rays annually - as unwanted by-catch. Greenland shark on the surface of the water.
The teeth of sharks, like their scales, are staggered and sit in several rows. As one row of teeth abrades, new ones in the depth of the mouth are replaced by new ones. The shark does not chew food, but only holds, tears and torments it, swallowing pieces of such size, which only can pass through its wide throat.
Sharks live in the North Atlantic and the North Polar Sea and grow only about one centimeter per year.
- What is the secret of their old age is not yet known.
- Probably slow growth is not the only reason.
Thus, their maturity reaches only at the age of 150 years. It reaches a size of more than five meters. Since it grows very slowly - probably only about one centimeter per year - researchers have long known that it can become very old. The usual determination of the age of these cartilaginous fishes is impossible due to the absence of calcified tissue, write Nielsen and his colleagues.
Cartilaginous fish do not have gill covers; therefore, from 5 to 7 gill slits are seen behind the head on each side of the shark's body. By this outward sign, sharks can be easily and unmistakably distinguished from other fish. The gill slits of the slope are located on its ventral side and are hidden from the viewer's eye.
It should be noted that these animals, in spite of the aversion of humans to them, are of great commercial value. They use their meat, skin and liver fat, which contains several dozen times more vitamin A than cod liver oil. Salted, smoked and specially cooked fresh meat of many species of sharks has a high taste. One of these fish, the fins of which are used to make soup (the pride of Chinese cuisine), even got the name of the shark soup.
Instead, they are outdated by age, using the radiocarbon method using proteins in the lens of the eye, which are already formed in the mother's body. Researchers investigated a total of 28 female sharks that were caught during several expeditions. Animals were measured between 80 centimeters and a good five meters. Their average age was 272 years. The largest studied specimen was also the oldest with approximately 392 years. However, the uncertainty of measuring up and down is 120 years for an animal.
Thus, the Greenland shark still lives much longer than the Greenland whale, which may be over 200 years old and has until now been considered a record holder among the vertebrate species. Vertebrates are animals with a spine, including mammals, birds, reptiles, and amphibians. Among invertebrates, two sharks are still surpassed by Icelandic mussels, which are also found in the Baltic Sea. She may be over 500 years old. According to current findings, Greenland sharks are sexually mature with a length of four meters.
How do sharks breed
Sharks belong to the type of skeleton of cartilaginous fish and breed, like their relatives, with the help of the so-called internal fertilization, in which the male's sexual products are introduced into the body of the female and already there is conception of embryos.By the type of offspring of the world, sharks are divided into three main types - egg-laying, egg-breeding and viviparous. In egg-laying sharks, at the same time, 2 to 12 eggs are formed, which they hang in small groups on algae. The eggshell is very strong and reliably protects the young from mechanical damage and encroachments of other predators.
In determining the new age, this would mean that the animals must be only 150 years old before they can reproduce. “It was expected that the Greenland sharks would live a long life,” says Jürgen Krieu from the Institute of Paleontology of the University of Vienna, who did not participate in the study. "But they are getting so old, but this is amazing." It is unclear why sharks live so long. Durability is often associated with body size. “But the Greenland sharks are not the most big sharksand not the only ones who live in such cold waters, ”says Kriu.
Therefore, their amazing durability should have another reason. In the Red List of the World Nature Management, the bowhead shark is classified as “potentially endangered”. New results have emphasized Nielsen and his colleagues that he needs caution. Greenland sharks are common by-catch for cod or pollack. Especially their late maturity makes animals very vulnerable, says Kriu. That is why they are caught before they multiply.
In ovoviviparous sharks, egg development and shell rupture occurs in utero. After the “birth”, the offspring are still for some time inside the mother, and practically formed individuals capable of independent full-fledged existence come into the world.
In viviparous female sharks, the conception, growth and development of embryos occurs without the formation of a shell. This method of reproduction corresponds to the highest type of organisms that inhabit planet Earth. More than 10% of all their species belong to viviparous sharks, and they simultaneously give birth to from 3 to 15 cubs.
Egg sharks often have a very unusual shape and they are called "mermaid wallet". Oceanologists found a clutch in which all the eggs were placed in a shell resembling a bag, the cavity of which was filled with a collagen mass.
A single shark can contain several embryos at a time, but only one of them, the strongest, survives. And in the course of his development inside the egg, he eats his weaker counterparts.
The sizes of shark eggs vary from the size of a goose egg or a human palm to elongated spheres up to 2 meters long.
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