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Sharks (lat. Selachimorpha) is a superorder of cartilaginous fish comprising eight teams, twenty families and about 350 species. Members of the superorder widely distributed in all seas and oceans, there are also freshwater sharks. Most species belong to the so-called real predators, some species, particularly whale, and giant deep-sea sharks, eat plankton.

For many millions of years before the planet appeared the first man, ruler of the prehistoric seas was a shark. The power of evolution sowed on Earth, countless forms of life that could not stand the severe tests and disappeared one after another, as the shark continued to exist. One prehistoric era is succeeded by another, have any amphibians, reptiles, birds, and mammals – the shark continued to exist. Pterodactyls – the winged reptile for millions of years plied the skies, disappeared from the face of the Earth. Dinosaurs-Triceratops, Brontosaurus, allosaurus and many other “saurus”, it is important chestvovali on our planet died out, leaving no trace. As the shark continued to exist.

About 20 million years ago in the sea scoured a huge Stingray, weighing over 100 pounds, and more than 25 different species of sharks, including sharks, reaching 36 meters in length. The length of these monstrous sharks was Opredelenie their huge teeth. Some of the teeth found in California, weighing 340 grams each and reach 15 centimeters in length, and seven – and ten teeth are quite common. They are triangular in shape and closer to the teeth of modern great white sharks that do not exceed 5-6 inches now with a body length of shark of 12 meters (maximum length). These teeth belonged to the ancestor of the modern great white shark the carcharodon, in huge quantities inhabited the seas. When they were first found in the seventeenth-century scientists, naturalists believed that this fossil bird tongues or teeth of serpents. They could not imagine that this shark’s teeth, and ascribed them to the mythical animal.

Carcharodon – the biggest fish, the remains of which was found by a man, and, although it is considered extinct, sure. At the beginning of our century, during the dredging from the bottom of the Pacific ocean were raised, the teeth of carcharodon ten centimeters long. They are much more like “fresh” teeth, than fossils. The fact that the cars uglublennim the bottom, managed to capture them, indicates that they are not very long lay on the seabed; otherwise they would be covered with a thick layer of silt and would not have been able to detect.We can assume that some deemed extinct sharks still exist today. At the end of the nineteenth century off the coast of Japan was caught by a shark of unknown species. She was a bit more than a meter long, with a long snout, similar to a knife for cutting paper. The shark was an absolute mystery. To identify its helped teeth. These teeth are acute, with a subulate crowns – were exactly such as fossil teeth, found in layers with fossils in Europe, North and South America, Asia, Africa and New Zealand. It was believed that the shark that owned these teeth, went extinct around 100 million years ago. And here she was found alive.She was given the name of its fossil ancestor – scapanorhynchus. But the appearance of shark teeth its strange and ugly snout, which led to the memory of exotic animals or a house from fairy tales, and among the fishermen it is called the shark-Rhino or shark-house.

Physiology of sharks Sharks are moving from the first to the last day of his life and rest only on the bottom, since the lack of the swim bladder deprives them of the buoyancy possessed by bony fish. No swimming (or otherwise referred to as air) of the bladder prevents the shark still “hang” at any depth. Her body is denser than the displaced water, and to stay afloat a shark can only incessantly moving. The shark all the time we need to make an effort not to drown. If she even for a moment will stop a wavelike motion of its muscular tail and, to a lesser extent, fin, she will not be able to overcome the force of gravity that inexorably pulls it down. Unlike the typical bony fishes, which, after death float to the surface of the sea, the shark, when her body is no longer able to move, finds his last rest on the bottom. Only one species of shark-the sand tiger shark (Carcharias taurus) is thought to have found a substitute for swim bladder: she swallows the air and holds on to it and the stomach in the so-called “air pocket”. Thus, the stomach it is believed, has a second function – the function of a hydrostatic body, similar to a swim bladder of bony fish. Sharks never sleep, at least in our sense of the word. Those that live in coastal waters, rivers and lakes, have a rest, zaplyv in underwater caves located at a shallow depth, and lying on the bottom or on ledges of the walls.

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