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When a person looks into the water from his familiar world, filled with light and air, the world in which fish live seems cold, dark, mysterious, populated by many strange, unusual creatures. He himself can move in this environment only with great difficulty and in a very limited space. The need to wear heavy, cumbersome equipment in order to see, breathe, stay warm and move at a speed that should seem like a snail's to a fish hides from humans some of the undoubted advantages of fish over land dwellers.

The advantages come from the very existence in the aquatic environment, which played an important role in the formation of fish. Water is not subject to sudden temperature changes and therefore can serve as an excellent habitat for cold-blooded animals. Changes in the water occur slowly and provide an opportunity to move to more suitable places or adapt to changed conditions. The problem of maintaining one's own body weight in water is also much simpler than on land, because protoplasm has approximately the same density as water, and therefore fish are almost weightless in their environment. This means that they can get by with a simple and light skeleton and at the same time sometimes reach significant sizes. A fish as huge as a whale shark moves with the same freedom and ease as a small guppy.

But there is one essential difficulty which is connected with life in water, and which, more than anything else, has shaped fish: the incompressibility of water. Anyone who has ever waded through water just above the ankle depth has felt the difficulty that fish constantly have to overcome: when moving, the water must be moved apart, literally pushed to the side, and it immediately closes behind you again.

Flat and angular bodies have difficulty moving through such a medium (if you push a board lying on water straight down, it will inevitably wobble from side to side), so the body shape of fish is remarkably consistent with this property of water. We call this shape streamlined: sharply pointed from the head, most voluminous closer to the middle and gradually tapering towards the tail, so that the water can flow smoothly on both sides with the least turbulence and, when approaching the tail, even impart some additional push to the fast-swimming fish. Of course, there is a certain variety of shapes, but in general this is the original form for all free-swimming fish, no matter what shape they acquire in the process of evolution.

The body of a fish, like that of any vertebrate animal, has bilateral mirror symmetry and is built according to the same simple scheme: a hollow cylinder, open on both sides, with a food tract that stretches inside from one end to the other. The oral opening is located at the anterior end, and the anal opening is located at the opposite end. Along the upper half of the cylinder there is a spinal column, a series of bone or cartilaginous discs that give rigidity to the entire structure. In the canal formed by the vertebrae is the spinal cord, which, expanding at the anterior end, forms the focal point, or brain. The walls of the cylinder along its entire length from head to tail are divided into numerous identical segments; strong motor muscles of these segments act on the bony or cartilaginous skeleton and enable the entire body to make wave-like movements from side to side.

Since fish are cold-blooded animals, life in the aquatic environment, as already mentioned, is especially favorable for them, but still it has its limitations. When the temperature drops below what fish can tolerate, they have to leave these places - which is why many fish in the temperate zone make seasonal migrations. With a strong and sudden change in temperature, the fish become too lethargic and do not have time to leave, and if conditions do not improve, they die. Some freshwater fish, which cannot migrate when the seasons change, circumvent this danger by plunging into winter or summer hibernation - they stop eating and winter time They lie inertly at the bottom, and in the summer they bury themselves in the mud until the temperature becomes favorable again.

The circulatory system of fish is the simplest of all vertebrates. Blood travels one circuit - from the heart through the gills, where it is saturated with oxygen, to various organs and parts of the body that take up oxygen, and back to the heart. The heart itself consists of only two chambers, the atrium and the ventricle (unlike the three-chambered heart of amphibians and the four-chambered heart of mammals), and works, so to speak, in line with the entire system.

A characteristic feature of fish is their fins, large or small wing-like structures that give them stability in the water and help them move and control their movements. Most fish have two types of paired fins - pectoral fins, on the sides of the head immediately behind the gills, and ventral fins, which are usually pushed back. At the top, the dorsal fin passes through the middle of the back; it can be divided into two parts, the anterior spiny one and the posterior soft one. On the ventral side of the body behind the anus there is an anal fin, and at the very end there is a caudal fin.

All fins have their own special purpose; they are all mobile and driven by muscles located inside the fish’s body. The dorsal and pectoral fins, acting together, play a major role in creating stability. The dorsal fin, pointing straight up, acts as a stabilizer to keep the fish upright; The pectoral fins, spread to the sides, help maintain balance and make turns. The pelvic fins are also used as stabilizers. The tail serves for control and in the fastest fish it also plays the role of a stabilizer and engine. The fish hits it forcefully from side to side, and the entire back part of its body makes wave-like swimming movements. In fast swimmers, the dorsal and anal fins are pressed against the body or even retracted into special recesses, which increases streamlining.

The location and structure of fins in fish can be very diverse. In most benthic species, the paired fins are very close together and the abdominal pair, strongly shifted towards the head, is sometimes even located in front of the pectoral fins, directly under the lower jaw. This arrangement allows you to keep the head and gills above the bottom surface. In other fish, the pelvic fins are greatly reduced or even completely disappeared, for example in eels. In triggerfish and other more or less disc-shaped fish, the pectoral fins take on the full or partial role of motors. In the sea cock, which leads a bottom-dwelling lifestyle, the lower rays of the pectoral fins are separated and act like the legs of an insect. And the pectoral fins of the striped lionfish serve it mainly for camouflage: their long and widely spread rays resemble a bunch of algae among the coral reefs where this fish lives.

The body shape of the fish also differs markedly from each other. The most amazing changes occurred with those of them that almost always lie at the bottom: they became flat. Some fish lie on their belly and are flattened on top, while others lie on their sides and are flattened on the sides. Flattening in such fish occurs during the growth of juveniles and ends with the unusual process of moving the eyes to one, upper, side of the head. Winter flounder ( Pseudopleuro-nectus americanus), for example, lies on its left side, and its eyes are on the right side, while its close relative, the summer flounder ( Paralichthys dentatus), on the contrary, the eyes are on the left side, since she lies on the right side.

Among the fish that are flattened on top is the monkfish. This fish rarely moves and catches its prey using its own fishing rod with bait - a fleshy lump on a thin flexible rod hanging from its head. His close relative, the sea clown, is more active: his pectoral fins have turned into a special kind of limbs, and with their help he moves in leaps.

The various rays are essentially sharks that have transitioned to a sedentary bottom life and become flat. While swimming, their wide pectoral fins make wave-like movements and the fish seem to float in the water. In many stingrays, the tail is extended like a whip and has no motor power.

Even in water, there are other modes of movement besides swimming, and fish use them all to varying degrees. They crawl along the bottom like gurnards and dolgoopers, and can even emerge from the water onto the shore, like the mudskipper does. The Malayan slider and Chinese snakehead easily walk along the ground from pond to pond, crawling in exactly the same movements as most fish swim. In order not to capsize, the creeper supports its narrow, nimble body with its pectoral fins, like props.

Some fish can also move through the air, albeit over short distances. The Mississippi pike skims the surface of the water using its tail like the propeller of an outboard motor. But flying fish really fly - they can fly through the air for almost a full minute and, if a strong wind blows, they rise to a height of three to six meters and glide over the waves on large front fins extended like wings. There are biplane flying fish, those that use their pectoral and pelvic fins to fly, there are monoplane flying fish, which fly only on their pectoral fins, and there is even a freshwater type of fish that flies like birds, flapping their pectoral fins over the water surface.

One remarkable feature of fish immediately attracts attention: from head to tail, fish are covered with a flexible, usually, shell made of round overlapping bone plates, or scales. These scales are strengthened in the inner layer of the skin and form the necessary protective cover. In addition to the armor of scales, the fish is also protected by a layer of mucus, which is secreted by numerous glands scattered throughout the body. Mucus, which has antiseptic properties, protects fish from fungi and bacteria, and also lubricates the surface of the body. The differences in the size and thickness of the scales can be very significant - from the microscopic scales of the common eel to the very large, palm-sized scales of the three-meter long barbel that lives in Indian rivers. Only a few species of fish, such as lampreys, have no scales at all. In some fish, the scales merged into a solid, motionless shell like a box, like in boxfishes, or formed rows of closely connected bony plates, like in seahorses and pipefish.

The scales grow as the fish grows, and some fish have distinct annual and seasonal markings on their scales. The substance necessary for growth is secreted by a layer of skin covering the scales on the outside and grows along its entire edge. Since in temperate zones scales grow fastest in the summer, when there is more food, the age of the fish can sometimes be determined by the number of growth rings on the scales.

The mouth of a fish is the only instrument for capturing food, and in all species of fish it is perfectly adapted for its task. The parrot fish, as already mentioned, has developed a real beak for pinching off plants and corals; The small American gerbil is equipped with a digging tool - a hard, sharp protrusion on the lower jaw, with which it digs through the sand in search of small crustaceans and worms.

In fish that feed near the surface, the mouth is usually directed upward, and the lower jaw is sometimes strongly elongated, as, for example, in half-snouts. Bottom-dwelling fish such as stargazers and monkfish, which grab prey floating above them, also have their mouths pointing upward. And in those fish that look for food at the bottom, for example, stingrays, haddock and the common chukuch, the mouth is located on the underside of the head.

Well, how does a fish breathe? To maintain life, she, like all animals, of course, needs oxygen - in fact, her respiratory process is not so different from the breathing of terrestrial animals. To extract oxygen dissolved in water, fish force water through their mouths, pass it through the gill cavity, and push it out through openings located on the sides of their heads. The gills act in much the same way as the lungs. Their surface is permeated with blood vessels and covered with a thin layer of skin that forms folds and plates, the so-called gill filaments, which increase the absorption surface. The entire gill apparatus is enclosed in a special cavity, covered by a bony shield, the operculum.

The gill apparatus is characterized by high functional adaptability, so that some fish can even obtain the oxygen they need not only from water, but also from atmospheric air. Common carp, for example, during the hot summer months, when the pond is dry or lacking oxygen, captures air bubbles and holds them in the mouth next to the wet gills. The slider, snakehead and Indian catfish have special air cavities with folded walls near the gills. Lungfish, if necessary, use fully developed lungs with the same network of blood vessels as those of frogs and newts. In some ancient fish, the rudimentary lung, which later turned into a swim bladder, is still connected to the esophagus, and in essence these fish - mud fish, armored pike - have spare lungs.

However, the swim bladder modern fish, if present, no longer performs respiratory functions, but acts as an improved lifting balloon. The bladder is located in the abdominal cavity below the spine and is an airtight sac equipped with glands that can, if necessary, extract gas directly from the bloodstream of the fish and fill the bladder with it. The amount of gas is adjusted with great precision, and the fish receives just the lift it needs to stay at its usual horizon, whether near the surface or at a depth of four hundred meters. Many fish that live at great depths or lead a benthic lifestyle do not need a swim bladder and do not have one. The swim bladder limits the ability of the fish to move arbitrarily to any depth, since adaptation to depth and pressure occurs gradually. Most fish living at significant depths cannot rise to the surface because their swim bladder would swell to an unbearable size for the fish - if such a fish is caught with a fishing rod and pulled out of the water, the swollen bladder can squeeze out its stomach through the mouth. There are fish, such as the mackerel family, with very little or no bladder. For them there is no such limitation, and they can forage at different depths. However, they pay a high price for this: in order not to drown, they need to be in continuous motion.

There are fish that live alternately in fresh and salt water; they have special difficulties - salt barriers that they need to overcome. Because fish live in water, they need to maintain a balance between the salts dissolved in their blood and lymph and the salts that may (or may not) be in the water around them. In freshwater fish, the concentration of salts in the blood is higher than in the surrounding waters, and therefore water constantly strives to penetrate the fish’s body through the skin, gill membranes, mouth and other open areas of the body. Under such unrelenting pressure, the fish must constantly secrete water in order to maintain proper balance. Marine fish have just the opposite problem: they are constantly releasing water into a saltier environment and therefore must constantly absorb it so as not to shrink like a baked apple. And to release excess salts that come with the water, marine fish have special cells on their gill filaments.

Since the aquatic environment is so different from the air, we have the right to ask ourselves how the fish's senses operate to notify it of where it is and what is happening around it. What does the fish see? How does she hear? Does she have a sense of smell like ours, a sense of taste, a sense of touch?

We can answer that fish have all these five senses, and in addition they have one more, truly sixth sense, which allows them to very subtly perceive the slightest change in the movement of water around them. This sixth sense is unique to fish (This system of organs is also characteristic of amphibians living in water), and its organs are located in a system of canals under the skin.

Let's start, however, with the organ of vision - it functions in fish in the same way as in humans, with the difference that fish that feed themselves above the surface of the water have to deal with the phenomenon of refraction. Due to the refraction of light rays when they pass from air to water (or vice versa), objects observed in water appear displaced if you do not look at them directly from above. A man who wants to hit a fish with an arrow from a bow must aim much lower than where he sees it, otherwise he will miss, and long practice has taught him to do this. In the same way, a trout, an eared perch or a salmon, preparing to grab an insect fluttering over their pond, must jump out of the water somewhat ahead of the intended target - and long ago in the process of evolution this skill turned into a reliable skill based on instinct.

Fish that feed in water do not have to overcome this difficulty, because light travels in a straight line in the same way as in the air. There are, however, other factors that influence the mechanism of visual perception in their underwater world, and therefore the structure of their eyes. Chief among these factors are the amount of light available underwater and the limit of visibility due to the fact that even the clearest water cannot compare with air.

The absence of bright light in the underwater world has contributed to a significant simplification in the structure of the eyes of most fish in comparison with the eyes of land animals: they can do with little or no contraction of the iris, they also do not need eyelids, because water constantly washes away foreign particles from their eyes . They have an iris - a metallic ring around the dark pupil, but to regulate the amount of light rays entering the eye, it does not need to expand and contract to the same extent as our iris, so in most fish it is motionless.

Since visibility underwater does not exceed thirty meters at best (and often much less), fish do not need to adapt their eyes to too much big difference in distances. Almost all the time they have to look at objects only in close proximity, and the structure of their eyes corresponds to this. Their lens is not a lens with adjustable curvature, like the human eye, but an incompressible ball. In the normal position of the fish’s eye, it sees only close objects, but if it is necessary to look at an object located at a far distance, a special muscle tightens the lens.

There is another, more important reason for the spherical shape of the fish's lens, and this again has to do with refraction.

Since the lens contains a substance almost the same density as that of water, light, penetrating from the surrounding aquatic environment into the lens, is not refracted - according to the laws of optics, this means that for a clear image of an object on the retina, the curvature of the lens must be significant, and the greatest curvature has ball. But, as some scientists believe, even with such curvature the image is not truly clear, and it is possible that the fish, even under the most favorable conditions, does not see objects under water clearly enough.

But fish have an advantage that land animals do not: they can see in more than one direction at the same time. Their eyes are not located in front, but usually on the sides of the head, and what each eye sees is recorded in the brain on the opposite side, that is, objects on the right are recorded by the visual center located on the left side of the brain, and vice versa.

This monocular vision of the fish has its limitations, especially in judging distance. Nevertheless, it is not at all impossible that there is a relatively narrow space directly in front of the fish that both eyes can see at the same time, therefore, fish have to some extent binocular vision (and therefore a sense of perspective), such as we have. Indeed, when something to the side attracts the fish’s attention, it seems as if it is really trying to make up for its monocular vision: it quickly turns so that the object is in the field of vision of both eyes and it would be possible to better estimate the distance to it.

DOUBLE VISION. The eyeball of the four-eyed fish living in the rivers of Central and South America is designed so that the fish can simultaneously and equally clearly see both in the water and above its surface. Both eyes of the four-eyed fish are located at the top of the head, and it can swim with them half out of the water. True, from time to time she has to dive to wet the upper, “above-water” part of the eye.

The extent to which fish can distinguish colors is unknown. Base tone underwater world fish - greenish-blue, since all other colors are absorbed and disappear at a short distance from the surface. The perception of color is therefore not particularly important for fish; The only exceptions are those fish that swim close to the surface. However, we do know that all fish except sharks can perceive some colors. Microscopic examination of the fish's retina showed that it contains cones, color-detecting nerve cells, and rods, which function primarily at night and are insensitive to color.

But what does color mean? Everyday life fish remains a mystery. Some fish prefer one color to another: trout, for example, distinguish artificial flies by color. If a darkened aquarium is illuminated with all the colors of the spectrum, the fish will swim to the green and yellow stripes and stop there, but if only red is left, they will behave as if in the dark.

Bright and sharply contrasting colors, of course, can be a certain means for fish to identify each other, but here again we are not sure that this is what actually happens. The bright, colorful outfit of some tropical fish makes one naturally think that it must have some significance for other inhabitants of the underwater world. For example, does a shark recognize a pilot fish by the contrasting transverse stripes on its dark back and sides? This would explain to us why such a small fish, a little more than twenty centimeters long, can fearlessly swim next to its huge and voracious companion and he will never swallow it by mistake.

It is also possible that bright colors serve as an identification mark, warning that fish are inedible or poisonous. There are fish that probably do not represent tasty prey for other fish, and in the shallow waters of tropical coral reefs, where visibility under water is relatively high, the bright coloring, which so clearly distinguishes them from their underwater counterparts, can serve as protection.

In any case, it seems likely that some species of fish recognize each other by their colors. In their greenish-blue world, a bright color catches the eye faster than a gray, barely noticeable shadow flashing somewhere nearby. This guess is supported by the fact that most species of fish that usually swim in dense schools are rarely brightly colored, while fish that live separately, among a rather monotonous color environment, usually have a noticeable appearance, and other individuals of this species can identify them.

The colors themselves are produced by a layer of cells in the skin under transparent scales. These cells are called chromatophores, or flower carriers, and contain various grains of pigment.

Primarily these are orange, yellow and red pigments, very similar to the pigments in a red or yellow flower. Then the black pigment, which is essentially unnecessary waste from the body and can be found not only in the skin (the internal organs of fish with black skin also usually have a black shell), and finally, the substance guanine, contained in the form of crystals, which, depending on their numbers and arrangements can produce white, silver, or rainbow colors. When combined with black pigment, guanine produces blue and green metallic tints.

Of course, the main thing in the coloring of most fish is its protective properties. The protective coloring of fish that live in the upper layers of the sea - a dark back and a white or silvery bottom - makes them inconspicuous, no matter where you look at them. The camouflage of bottom-dwelling fish is very skillful - their color matches the color of the bottom or, like the zigzag pattern of camouflaged warships, breaks up the contours of the fish’s body. To this “breaking” coloring is added the so-called “deceptive” coloring, which completely changes the appearance of the fish.

Sometimes surrounding objects are imitated not only in color, but also in shape. The Amazonian leaf fish surprisingly resembles a leaf floating in the water. Fish can even change their disguise different periods life - in the tropical waters off the coast of Florida there are, for example, fish that at a young age take the shape and color of a mangrove pod lying on a white sandy bottom, but when they outgrow the pod, so to speak, this camouflage becomes useless, the fish then leave into deeper waters, becoming striped. One of the most skilled masters of camouflage is the common flounder; it can imitate stones, sand, and dark silt with the ease of a chameleon.

Camouflage can even affect the structure of the fish. The Sargassum clown sea is covered with skin projections like threads and patches that imitate the algae where it hides, and the rag seahorse has long shoots similar to sea grass leaves that it clings to.

Most fish retain the same basic color throughout their lives, but for some it changes with age. Young salmon and trout are streaked with dark stripes, but in adult fish the stripes disappear. Male salmon, trout, stickleback and many other fish change color during the breeding season. One day, Dr. William Beebe discovered coral fish that changed color combinations seven times in one day.

Even males and females can differ in their coloring. The male minnows, or lyre fish, and the European wrasse look like exotic birds with brilliant plumage, while the females of both species are completely inconspicuous. There are fish that become darker at night or, like barracuda, take on a completely different color. Many fish change color when scared or caught on a hook.

After death, the color of the fish usually changes immediately and often becomes completely different from what it was during life. The most amazing changes are probably happening with the bright green and gold sea bream, or sea bream. During the death throes, the green and gold colors turn into blue and pure white, and then gradually, when the last convulsions stop, the whole body acquires a dull brownish-olive hue.

For a long time, scientists have been studying the hearing of fish, trying to find out whether they can perceive sounds. It was believed that they could not, and that what we call the ear serves simply as an organ of balance in fish. But since some fish do make sounds underwater (these can be call and response signals during the mating season or identification signals), it is logical to conclude that they still perceive them. Most likely, when sound waves are perceived, the swim bladder serves as a resonator. Since they do not have an eardrum and auditory ossicles of the inner ear, which represent the true hearing apparatus of higher animals, it is believed that the role of the hearing organ, which perceives sound in the form of wave vibrations, in some fish is performed by the swim bladder and the so-called Weberian apparatus - a series of small bones connecting the swim bladder with the area of ​​the inner ear. Some fish are, of course, very sensitive to vibrations, including simple water movement. They can hear the sound of a propeller at a great distance, and the steps of a person on the shore, which very slightly shake the earth and thus the water, are quite enough to scare away the trout in the pond. The tactile sensitivity in fish is carried out by nerve endings distributed throughout the skin. Most of them are on the head and around the lips, and in many fish they are also located on special antennae. Cod and mullet examine the bottom with rather short antennae sitting on the chin; Catfish have very long mustaches.

Almost all fish are characterized by a finely developed sense of smell. They have nostrils somewhat similar to ours - a pair of small indentations that open outward and are located directly on the snout, lined inside with folded tissue, which greatly increases their surface. This tissue contains nerve cells that perceive odor.

The sense of smell in most fish is so developed that when looking for food, it means much more to them than vision. Sharks can smell blood from afar and appear near a wounded fish or animal out of nowhere. Sports anglers have successfully used fish blood to attract bluefish and other predatory fish. If you pour just one glass of water into a pool with lampreys, in which another fish was swimming, the lampreys will immediately become wary and begin to look for the source of this suddenly pleasant aroma.

As for taste sensitivity, it probably doesn't matter. big role in the life of fish. First of all, none of them, with the exception of lungfish, have taste organs in the mouth. They have taste buds, but they are located on the head, body, tail, modified fins or antennae, and therefore if the fish taste food, it happens before it gets into their mouth. Many fish simply swallow food, it goes directly into the stomach and is digested there.

The most remarkable feature of the fish is its unique “sixth sense”, which allows it to subtly perceive all movements and currents of water. In the most perfect way arranged system The channels under the skin are quite clearly marked on the sides of the fish as a series of scales of a different shape from the rest. This is the side line. In the main channel, specialized sensory organs are located at a certain distance from each other. The same channels spread throughout the head.

Scientists have yet to reveal all the secrets of the lateral line, but it is already clear that its main function is related to capturing the movement of water. If the base of the nerve running from the lateral line to the brain is cut, the fish clearly loses the ability to respond to disturbances in the water or changes in the direction of the flow. Apparently, it is this special sensory organ that allows the coral fish to shoot like an arrow through a narrow crevice, which it probably does not see properly, or allows fish to bypass obstacles invisible in muddy water during floods. And, probably, it is the lateral line that allows huge schools of fish of many thousands of individuals to swim in such a coordinated formation.

Anyone who has ever fished or seen others fish has probably wondered whether fish feel pain. This question is too difficult to give a clear answer to. Pain is not only a physical, but also a mental reaction, and we cannot find out from the fish exactly what it feels. But we can be almost sure that fish do not feel pain mentally.

Well, do they experience physical pain? In humans, pain is generated in the cerebral cortex as a result of information sent by sensory nerves, but fish do not have a formation comparable to the human cerebral cortex, or any other part of the brain that would perform its functions.

The strength of stimulation of certain sensory organs necessary to cause the sensation of pain is called the pain threshold. In some animal species, as well as in individual individuals, it is much higher than in others. The lower we go down the evolutionary ladder, the higher the pain threshold becomes, the more irritation is needed to cause a pain reaction. We can be quite sure that it is high in fish. In response to too much irritation, they simply leave or try to get away.

This is why a fish can calmly swim away with a hook in its mouth or a harpoon in its back, but a wounded shark will continue to attack even if its fellows are tearing out its entrails.

FISH
(Pisces),
a large group of jawed vertebrates that spend all or most of their lives in water and breathe using gills. This definition immediately excludes from the list of fish vertebrates that breathe with lungs, i.e. whales, seals, dolphins and others aquatic mammals. All of them also feed their offspring with milk, and fish have neither mammary glands nor the hair characteristic of mammals. Frogs, toads, newts and salamanders breathe in the early stages of development using external gills and then lungs. These amphibians (amphibians) also differ from fish in the presence of paired limbs in adult individuals, which are homologous to the fins of fish.
Anatomy. External structure fish is complex and varied. In principle, each structure of an organism ensures its adaptation to specific living conditions. However, some features are common to most fish, such as dorsal, anal, caudal, pectoral and ventral fins.

Digestive system. In terms of their internal structure, fish are similar to other vertebrates. The body is bilaterally (two-sided) symmetrical, except for the digestive tract. The latter consists of the mouth, jaws, usually covered with teeth, tongue, pharynx, esophagus, stomach, intestines, pyloric appendages, liver, pancreas, spleen, rectum or colon and anus or anus. In the intestines of sharks and some other primitive fish there is a spiral valve, a unique organ that increases the “working” surface of the digestive tract without increasing its length. In predatory fish, the intestine is usually short, forming one or two loops, while in herbivorous species it is long, convoluted, with many loops. The respiratory system consists of gill arches covered with delicate fleshy gill filaments, abundantly supplied with blood through capillaries and larger vessels. At the front of the mouth there are special oral valves that prevent water from flowing back out. When the mouth is closed, it enters the pharynx, flows between the gill arches, washes the gill filaments and exits through the gill slits (in cartilaginous fish) or the opening under the operculum (in bony fish). The nervous system - the brain, nerves and sensory organs - coordinates the functions of the body and connects it with outside world. Like other vertebrates, the nervous system of fish includes the brain and spinal cord. The brain consists of the olfactory lobes, the forebrain hemispheres, the diencephalon with the pituitary gland, the optic lobes (midbrain), the cerebellum and the medulla oblongata. Ten cranial nerves depart from these sections. The eye consists of the cornea, lens, iris, retina, and sharks also have an eyelid - a nictitating membrane that can slide over the cornea from below. Fish have no external ear. The inner ear consists of three semicircular canals with ampoules, an oval sac, and a round sac with a projection (lagena). Fish are the only vertebrates with two or three pairs of otoliths, or ear stones, which help maintain a certain position in space. In some groups, the swim bladder communicates with the inner ear by a very thin tube, and in minnows, carps, catfish, characins and electric eels, it is connected to it by a complex bone mechanism - the Weberian apparatus. This allows you to better perceive (“hear”) vibrations in the environment. The lateral line system is a unique sensory organ in fish. It usually consists of a network of depressions or channels in the scalp and torso with nerve endings in the depths. These channels in bony fishes usually open on the surface with pores. The entire system is connected by nerves to the inner ear. It serves to perceive low-frequency vibrations, which allows you to detect moving objects.
Anatomical adaptations. Fish are extremely diverse in structure and adaptations. They walk, swim and fly (glide). Some are able to see in both water and air, make various sounds, emit light and even generate a strong electrical charge. Each structure fulfills its purpose - serves for protection, obtaining food or reproduction.
Mouth, jaws and teeth. The jaws of fish are varied - from toothless to those equipped with chisel-shaped incisors and long sharp fangs. Some herbivorous forms, such as surgeonfish and South American catfish, have teeth on long, thin stalks with a cup-shaped top. Parrotfish are remarkable for their beak-forming teeth, which give them a bird-like appearance, hence the name of the family. The mouth can point downward, like a shark's, forward, like a salmon's, or upward, like a stargazer's. The lips are covered with long hair-like projections, like those of the hairtooth (Trichodon), which, burrowing into the ground, uses this filter to cleanse the inhaled water from sand. There are two types of gill openings. Sharks and rays typically have five external gill slits, while bony fish have four or five openings covered by an operculum, which directs the water pushed through the gills into one common outward-opening slit.
Eyes. In general, the eyes of fish are designed in the same way as those of other vertebrates. On the outside they are covered with the cornea. Light passes through the pupil, an opening in the iris, and is focused by the spherical lens onto the retina, which occupies the back wall of the eye. Visual stimuli are transmitted from the retina along the optic nerve to the brain. Since fish have both rods and cones in their retina, we can conclude that they distinguish colors. The four-eyed fish (Anableps), which lives in Central and South America, has eyes divided into two parts: the upper one is adapted to see in the air, and the lower one is adapted to see under water. The lens here is oval and is located at such an angle as to focus light rays from both sources onto the retina. Since bony fish lack eyelids to moisten the eyes while in the air, the four-eyed fish solves this problem by periodically immersing its head in water.
Luminescence. The ability to emit cold light is widespread among different, unrelated groups of marine fish. The glow is usually provided by special glands located in the skin or on certain scales. The glands consist of luminous cells, behind which there may be a reflector, and in front - a lens. Pisces are able to arbitrarily “turn on” and “turn off” their glow. The location of the luminous organs varies. Most deep sea fish they are collected in groups and rows on the sides, belly and head, reminiscent of pearl buttons or modern road markings that reflect light at night. The purpose of this cold glow is not entirely clear. In the absolute darkness of the ocean depths, where some anglerfish live, it is probably used to attract small prey and members of the opposite sex.
Sounds. The sounds made by some fish can be clearly heard by the human ear many meters away. They vary in height and intensity. Among the many "vocal" fish, the most famous are croakers, drummers, hornfish, triggerfish, toadfish and catfish. Their sounds are reminiscent of grunting, squealing, creaking, barking and, in general, barnyard noise. The origin of the sounds produced is different. In some catfish, the back and forth movement of gas in the swim bladder causes the taut membranes to vibrate. Ronki rubs his pharyngeal teeth against each other. Croakers and drummers produce particularly loud noises by oscillating their swim bladders, sounding something like the muffled sound of a jackhammer hitting the pavement. Some triggerfish make sounds by rotating their fin rays. Typically, fish are used most frequently and intensively sound signals during the breeding season.
I. Some fish can sting no less dangerously than poisonous snakes. The effect of their venom is similar to the bite of cobras, rattlesnakes or bees. The most famous of these fish are stingrays (Dasyatidae), scorpionfishes (Scorpaenidae), toadfishes (Batrachoididae) and dragonfishes (Trachinidae). Less poisonous are catfish, tropical perches from the Pacific Ocean belonging to the family Siganidae, some sharks (Squalus, Heterodontus) and chimeras. In stingrays, the sting is located on the upper side of the tail, approximately a third or half of its length from the end. It reaches 30 cm in length, is serrated on the sides and surrounded at the base by poisonous glands. Stingrays are found in shallow waters, near sandy and muddy beaches warm seas, in river mouths and quiet bays, and some species even in the rivers of Asia and South America 1600 km from the sea. Stingrays hide in soft ground. If you step on them, they swing a powerful tail, on which a poisonous sting rises, and it sticks deep into the victim, causing piercing pain. This device serves both defense and attack. Stingrays feed on invertebrates living in mud and sand. In most other poisonous fish, such glands lie along the dorsal and pectoral fin spines and at their base. When a thorn pierces the victim’s body, poison is squeezed out of the surrounding tissues and enters the wound through a special groove. Siganus in everyone pectoral fin two grooved poisonous spines. The most developed stinging organs are in sea dragons and toad fish. The spines on the gill covers and the first two dorsal rays are hollow, like the teeth of poisonous snakes. The base of such a spine is surrounded by a poisonous gland.
Electricity. Fish of five groups are capable of generating an electric charge: stargazers (Astroscopus), freshwater gymnarchs (Gymnarchus) and electric catfish (Malapterurus) living in Africa, electric sea rays (Tetronarce) and the famous South American electric eel(Electrophorus electricus). The latter lives in the slow-moving waters of the Amazon and Orinoco, reaching a length of 180 cm. Experiments carried out at the New York Aquarium showed that this remarkable creature generates a voltage of 600 volts and can, at will, release electricity in volleys at intervals of two to three seconds, after which the discharge power drops for several hours. The voltage generated by electric catfish and stingrays is much lower, and in stargazers and gymnarchs it is even weaker.
Coloring. Modern aquariums give a good idea of ​​the magnificent colors of a variety of freshwater and saltwater fish. Some freshwater species acquire a dazzling shine with crimson, bright yellow and blue spots during the breeding season, but the rest of the time they are much more modestly colored. Among the coral reefs in tropical seas live several hundred species of fish, competing with butterflies and birds in their colors. Here you can find almost every conceivable type of color: from gray and silver to contrasting black with yellow, blue, red lines, rings, stripes, streaks or green, yellow and purple specks, spots, blots and circles encircling the body. Pigments corresponding to black and brown shades, are called melanins. Vivid colors are provided by fat-soluble lipoids. Both types of pigments are found in special cells, chromatophores, deep in the skin. In addition, special reflective granules - iridocytes - give the fish a milky white and silver color. The ability of chromatophores to expand and contract allows fish to change patterns on their bodies, which helps in camouflage. The nature of the environment is perceived by vision and purely reflexively changes the state of the chromatophores. As a result, many fish become almost invisible. Famous examples species with such patronizing coloring- clown fish living in thickets of Sargassum algae, pipefish among the green eelgrass grass, poisonous warts (Synanceja) at the bottom of holes in coral reefs and ragworms (Phyllopteryx), reminiscent of branched algae thalli.
Number and size. Fish are the most numerous vertebrates. Known approx. There are 40,000 different species, more than double the total number of species of mammals, birds, amphibians and reptiles combined. As for the number of individuals, there are truly countless of them in the waters. For many years, the smallest fish was considered to be the 19 mm long Heterandria formosa, from the southeastern United States. However, a species of Pandaka pygmaea was discovered in the Philippines, the name of which is much longer than the animal itself (9-11 mm). It is the smallest known vertebrate. The largest species of catfish is Pangasius sanitwongsei from Siam with a length of 3 m, and the largest freshwater fish is the North American white sturgeon from the Columbia and Fraser rivers in the northwestern United States, which reaches a length of 3.8 m and a record weight of 583 kg. However, the beluga sturgeon (Acipenser huso), caught in the Volga near Astrakhan, turned out to be even larger: its length was 4.4 m and its weight was 1022 kg. However, even these giant sturgeon are pygmies compared to the champions among sea fish. Man-eating sharks 9-12 m long give way to two harmless species. One of them, the basking shark (Cetorhinus maximus) from Arctic waters, reaches more than 12 m in length. But the most enormous of the fish is the whale shark (Rincodon typus), broad-headed, blackish, with white spots on its back the size of silver dollars. This giant feeds on plankton - small animals and algae that drift with ocean currents. The maximum accurately recorded length of such a shark is approx. 13.5 m, but according to rough estimates, it could be more than 21 m with a mass of approx. 68 t.
Ecology. Fish occupy almost all aquatic habitats. They are found in polar and tropical seas, in cold mountain lakes and streams and in hot springs with temperatures up to 43 ° C. Many species live in the open sea, far from the coast, some - at enormous ocean depths, in complete darkness. Fish live in thickets of aquatic vegetation, rock crevices and among stones; they can burrow into mud, sand and pebbles. Some lead night image life, but most hunt during the day. Several species live in dark caves: they are almost or completely blind.
Spreading. Pisces are found in all large rivers, in almost all large lakes and are absent only in a few reservoirs. Sea fish are divided into coastal, oceanic and deep-sea forms. The former live in shallow waters off the coast, among them are herring (Clupea), mackerel (Scomber), sea bass (Sebastodes), pomacentridae (Pomacentridae), flounder (Pleuronectes), borracites (Salarias), etc. On continental shelf There are halibut (Hippoglossus) and cod (Gadus). Oceanic fish live in the open seas to depths of 90-150 m. They are called pelagic. Among them are such large sport fish species as tuna (Thunnus), swordfish (Xiphias), marlin (Makaira), and small lanternfish (Myctophidae) and scomberfish (Scomberesocidae). At depths from 135 to 540 m, there are many small fish with huge eyes and a silvery color. Even deeper live bathypelagic species with small eyes and luminous organs, such as stomiids (Stomiatidae) and deep-sea anglerfish (Ceratiidae). The color of these fish is mostly black. Abyssal fish, in particular long-tailed fish (Macrouidae), spend their entire lives in the ocean depths near the bottom. Freshwater fish are distributed across all continents and large islands. They are often divided according to their belonging to seven zoogeographical regions: 1) Nearctic - Canada, USA and most of Mexico; 2) Neotropical - Central and South America; 3) Palaearctic - Europe and Asia north of the Himalayas and the Yangtze River; 4) Indo-Malay - India, Southeast Asia, islands of Java, Sumatra, Borneo; 5) Ethiopian - Africa; 6) Australian - Australia, New Guinea and the islands of the Malay Archipelago east of the Wallace line passing between islands of Borneo and Sulawesi, Bali and Lombok; 7) Madagascar. Certain regions, for example the Nearctic and Palaearctic, are very similar in their ichthyofauna - in both regions there are cyprinids (Cyprinidae), chukuchans (Catostomidae), percids (Percidae) and eupods (Umbridae). Similarly, characins (Characinidae), nematognathoid catfishes (Nematognathoidea) and cichlids (Cichlidae) live in both the Neotropical and Ethiopian regions. In terms of the composition of freshwater ichthyofauna, Europe, Northern Asia and North America are closer to each other than North and South America, and there are more similarities between South America and Africa than between Africa and Eurasia.
Reproduction. The methods of fish reproduction are different. Some are viviparous - active young emerge from the mother's body. The rest are oviparous, i.e. lay eggs that are fertilized in the external environment. The reproductive behavior of some fish is very peculiar. It is difficult to see a clear evolutionary sequence in their methods of reproduction. Primitive in their anatomy, sharks and rays are mainly viviparous or lay horny egg capsules. In more highly developed fish, both viviparous and oviparous species can be found in the same group.
Atherina grunion. Sandwiches (Leuresthes) can be seen in the spring and summer on the southern coast of California, where on the second, third and fourth nights after high tide they splash in the moonlight on wide sandy beaches. As soon as a surf wave hits the shore, whipping the water into white foam, and then spreading over the sand, silversides rush to land. For some time, these fish, 15-20 cm long, find themselves out of the water. Females seem to “stand” on their tail, plunging it into the sand and leaving 2/3 of their body outside. Males hover around them. At this moment in the sand to a depth of approx. Fertilized eggs are deposited 5 cm. The next wave captures the spawned silverside grunions and carries them back to the ocean. Under the influence of the surf, the clutch of eggs sinks even deeper into the sand, and over the next few days the tide recedes and it ends up on the shore. Here, under a sandy blanket, the eggs of the silverside grunion are not afraid of the hot rays of the sun and predators. Two weeks later, spring tide comes again, waves flood the beach and free it from sand. At this moment, the juveniles hatch from the eggs and go into the ocean.
Salmon and trout. All species of salmon spawn in nests on the pebble bottom of cold rivers or spring lakes. Most of these fish migrate from the sea to fresh water to spawn: they are called anadromous, or anadromous. The female, sometimes with the participation of the male, digs the nest. To do this, she lies on her side and begins to bend her tail up and down, moving slightly upstream. So she “irons” the same place several times in a row. With each stroke of the tail, pebbles and sand rise from the bottom and are carried downstream until the saucer-shaped hole is ready. During nest construction, the male and female protect their territory from encroachment by other fish. When a male of the same species and similar size approaches, the rightful owner of the territory swims out to meet him, may attack, or simply sends the uninvited guest away. In the latter case, the fish swim some distance parallel to each other before moving apart. The male devotes the rest of his time to courting the female, which consists of lightly nudging her with his nose and simultaneously shaking her whole body. Fertilization occurs when both fish lie down on the bottom of the nest, side by side, with their heads upstream. At the same time, shuddering, the male and female spawn eggs and milk and immediately cover the clutch with soil raised from the bottom a little higher upstream. At all stages of spawning, the actions of the parents are strictly synchronized. If male and female reproductive cells do not appear in the water at the same time, fertilization will not occur. The eggs will swell due to the entry of water into it, and after a few minutes the micropyle, i.e. the pore through which sperm can penetrate will close. Trout can spawn several times in their lives, but Pacific salmon die shortly after spawning.
River eel. Well-coordinated and specialized reproductive behavior is characteristic of many fish, including the river eel (Anguilla). The European eel migrates a distance of approx. 3220 km across the North Atlantic to spawn northwest of Bermuda in the Sargasso Sea. The American eel spawns in approximately the same place. The juveniles of the European species develop within two years, drifting back to the shores of Europe, where they enter fresh waters. American eel fry reach rivers in the coming spring.
Origin of fish. The oldest fossilized remains of true fish were found in Ordovician deposits. The next four periods (Silurian, Devonian, Mississippian and Pennsylvanian) are called the “age of fish” - these were the largest and most diverse animals on Earth. In later geological eras, their species richness and numbers remained high, but more evolutionarily advanced groups appeared - amphibians and reptiles, then birds, mammals and, finally, humans. The most primitive of modern fish are sharks, rays and chimeras with a cartilaginous skeleton. It partially ossifies in sturgeon, mudfish and some other fish. Finally, species with a completely ossified skeleton appear; they are called teleostei.
See also COMPARATIVE ANATOMY.
Classification of fish. Fish are members of the phylum Chordata, which also includes amphibians, reptiles, birds and mammals. This phylum is variously subdivided into lower-ranking taxa. The system given below distinguishes two of its subtypes: skullless (Acrania), lacking a real head (lancelets), and cranial (Craniata), or vertebrates, which include fish. Among the latter there are several subclasses and orders. Chordata type(chordates)

Subphylum Acrania (skullless)

Class Cephalochordata (cephalochordates)

Order Branchiostomoidea (lancelets)

Subphylum Craniata (cranial)

Superclass Agnatha (jawless)

Class Marsupobranchii (sacbranchii)

Order Petromyzonoidea (lamreys)

Myxini class (mixins)

Order Myxinoidea (hagfish)

Superclass Gnathostomata (gnathostomes)

Class Elasmobranchii (elasmobranchs)

Subclass Selachii (sharks and rays)

Superorder Selachoidea (sharks)

Order Heterodontoidea (heterodontoids)

Order Hexanchoidea (polybranchiaceae)

Order Lamnoidea (lamniformes)

Order Squaloidea (katraniformes)

Superorder Hypotremata (rays)

Order Batoidea (stingrays)

Class Holocephali (whole-headed)

Order Chimaeroidea (chimaeras)

Class Osteichthyes (bony fishes)

Subclass Choanichthyes (choanaceae)

Order Dipnoidea (lungfishes)

Order Crossopterygoidea (lobe-finned)

Subclass Actinopterygii (ray-finned)

Superorder Chondrosteoidea (osteocartilaginous)

Order Cladistioidea (polyfeathers)

Order Acipenceroidea (sturgeons)

Superorder Holostei (bony ganoids)

Order Semionotoidea (carapaceans)

Order Amioidea

Superorder Teleostei (bony fishes)

Order Isospondyloidea (herrings, or softfins)
Order Esociformes (pike-like creatures)
Order Bathyclupeoidea (deep-sea herrings)
Order Mormyroidea (beaked whales) Order Ateleopoidea (false-tailed whales) Order Gyanturoidea (gigantuformes) Order Lyomeroidea (sac-like whales) Order Ostariophysoidea (cyprinids, or bone-vesicals) Order Apodoidea (eels) Order Heteromoidea (spinociformes) Order Synbranchioidea (merged) knifebranchiformes) Order Synentognathoidea (garfish) Order Cyprinodontoidea (carp-toothed) Order Salmopercoidea (percopsiformes) Order Berycomorphoidea (beryxiformes) Order Zeomorphoidea (sunfishes) Order Anacanthoidea (gadlikes) Order Thoracostoidea (sticklebacks) Order Solenichthyoidea (acicularids) Order Allotriognathoidea (odoriformes) Order Percomorphoidea (percoids) shaped) Order Scleropareioidea (scorpion-shaped) Order Cephalacanthoidea (long-feathered) Order Hypostomosoidea (pegasiformes) Order Pleuronectoidea (flounder-like) Order Icosteoidea (rag-toothed) Order Chaudhurioidea (chaudhuriformes) Order Mastocembeloidea (proboscis-like) Order Discocephalioidea (adherent-like) Order Plectognathoidea (cliff-toothed) shaped) Order Gobiesociformes (sucker-shaped) Order Bathrachoidea (toad-like) Order Pediculatiformes (anglerfish)

Beluga, crucian carp, herring, trout, carp, silver carp, carp are well-known fish. This list can be continued endlessly. And their commercial importance is difficult to overestimate. And indeed, very diverse. Modern taxonomy includes more than 20 thousand species of these aquatic animals. Thanks to what structural features did they manage to master this habitat and occupy a dominant position in it? What class do fish that differ in their structure belong to? You will find the answer to these and other questions in our article.

Signs of fish

It’s not for nothing that they say about self-confident people: “They feel like a fish in water.” Scientists know that the first fish lived in the Silurian period. Outwardly, they were similar to modern sharks with movable jaws on which sharp teeth were located. Millions of years have passed, and in the process they have changed and acquired a number of new adaptive characteristics.

As aquatic animals, they all have a streamlined body shape, fully or partially covered with scales, various types of fins located on the body, and gills as respiratory organs. This general signs for all representatives of a given systematic unit. But what class fish belong to can be answered by considering their significant differences. At the moment there are two of them: Bone and Cartilaginous.

Features of the external structure

The body of absolutely all fish is covered with scales. It protects the skin of aquatic inhabitants from excessive water friction. After all, most of them spend most of their lives on the move. Additional protection Friction also produces a large amount of mucus, which the skin is rich in. This helps many species survive in unfavorable conditions of temporary drought. Not all fish species have a completely covered body with scales. For example, in sharks it is located in one row along the surface of the body, resembling their teeth in appearance. The same can be said about numerous representatives of the Sturgeon order. Most bony fish are protected by scales, like a durable shell. It also performs additional functions: camouflage from predators, warning coloring in predatory and poisonous species, sexual identification in water.

Fin structure

The next characteristic feature of fish is the presence of fins. These formations serve as limbs for movement in water, and some ancient species are even able to crawl with their help. The fins are divided into two groups. The first are the paired ones: abdominal and thoracic. They help maintain the balance of fish in the water column. The caudal, anal and dorsal are unpaired. They work like a rudder, guiding the body of aquatic animals in the desired direction. As a result of evolution, the limbs of reptiles were formed from the fins of fish.

You can easily see the lateral line on the body of the fish. This is a unique organ of balance and touch, characteristic only of fish.

Internal structure of fish

The organ systems of these animals also have their own characteristics associated with the aquatic habitat. The musculoskeletal system is represented by the skeleton. Depending on the class, it is formed by cartilage or bone tissue. All bones of the head skeleton are connected motionlessly, except for the lower jaw. This allows fish to easily capture prey. This section of the skeleton also includes gill covers and arches, the latter of which are attached to the respiratory organs of fish - gills. consists of individual vertebrae connected to each other and to the skull motionlessly. The ribs are attached to the trunk of the spine. The skeleton of the fins is represented by rays. They are also formed by bone tissue. But the paired fins also have belts. Muscles are attached to them, causing them to move.

Through type. It begins with the oropharyngeal cavity. Most fish have sharp teeth on their jaws, which are used to capture and tear food. Enzymes from the liver and pancreas also take part in the digestion process. In the processes of excretion and salt metabolism, the main role in the fish body is played by paired kidneys. They open to the outside with the help of the ureters.

Fish are cold-blooded animals. This means that their body temperature depends on changes occurring in the environment. This sign is determined by the circulatory system. It is represented by a two-chambered heart and a closed structure of blood vessels. During its movement, venous and arterial blood mix.

The nervous system is represented by the brain and spinal cord and nerves. And its peripheral part is made up of nerve fibers. In the brain, the cerebellum reaches special development. This part determines the fast and coordinated movements of fish. The sense organs are capable of perceiving any irritation possible in the aquatic environment. Since the lens of fish eyes does not change its shape and position, animals see well only at a short distance. But at the same time they are able to distinguish both the shape and color of various objects. The organ of sound perception is represented by the inner ear and is associated with the structure responsible for balance.

Fish reproduction also has its own characteristics. These animals are dioecious, with external fertilization.

What is spawning

The process of fish reproduction is also called spawning. It happens in water. The female lays eggs, and the male waters her with seminal fluid. As a result, a fertilized egg is formed. As a result of successive mitotic divisions, an adult individual develops from it.

Sometimes fish reproduction is associated with spawning migrations and significant changes in the behavior and structure of fish during this period. For example, pink salmon form large herds, in which they move from the seas to the upper reaches of rivers. During this journey, they have to overcome many obstacles, moving against the current. These fish develop a hump on their backs, and their jaws become twisted and twisted. Having lost a lot of strength, after the fertilization process, adult individuals die. Surprisingly, the young fry return independently to the same habitat.

Groups of fish

The enormous species diversity necessitated the classification of this fish. Currently, scientists have precisely identified the characteristics by which the class of Fish can be classified. Systematic affiliation determined by the presence of gill slits or covers and the type of scales. This way you can distinguish between bony and cartilaginous fish. There are other characteristics by which these animals are grouped. For example, fish that move to other habitats to spawn are called migratory. But, taking into account the scope of application, a distinction is made between commercial and ornamental representatives of these aquatic animals.

Cartilaginous fish

What class do fish that have a cartilaginous skeleton and gill slits that open outward belong to? It's not hard to guess. These are cartilaginous fish. They lack a swim bladder, so they either live on the bottom or are constantly on the move. Sawfish, white, giant, whale sharks, stingrays... You know such fish. The list of dangerous predators can be continued with the sea devil, electric ramp and These marine inhabitants pose a great danger to animal and human life. Although there are quite innocent specimens among cartilaginous fish. Thus, it feeds on fish and crustaceans. Apart from its terrifying appearance, it does not pose any danger to humans.

Bony fish

Perhaps every schoolchild will answer the question of which class the fish that are most numerous belong to. Their skeleton consists entirely of bone tissue. The swim bladder, located in the body cavity, allows its owners to stay in the water column. The gills are covered with gill covers, and do not open outward with separate openings. Bony fish have these characteristics.

The meaning of fish

Representatives of this superclass of vertebrate animals are primarily of commercial importance. People eat their nutritious meat and protein-rich caviar. And the number of recipes for preparing different types does not know the count. Fish oil has long been used as a treatment for bacterial and viral respiratory diseases. Man annually catches a huge number of individuals and breeds them on his own. Flour is also obtained from meat and bones. It is used as fertilizer and feed for many domestic animals.

IN Lately Sport fishing is becoming increasingly popular, attracting participants from different countries. And certainly every one of us dreams of catching a goldfish that makes all our wishes come true!

Thus, which class fish belong to can be determined by the characteristics of their structure, organization and way of life.

Fish classification(from Latin classis - category - class and ..., fication) - this is, simply put, the division of fish according to lifestyle, structural features, method of reproduction and appearance. There are a variety of classifications, and the aquarist needs to know the main ones.

Let's start with the fact that of all vertebrates, fish are the most numerous animals in terms of the number of species. If you combine all mammals, birds, amphibians and reptiles, then the number of their species will be less than fish, of which there are over 20 thousand species!

Fish inhabit almost all bodies of water on the globe. Through evolution, these animals have adapted to different conditions existence, which led to the emergence of many of their species. All of them are combined into one general class of “fish”.

According to this system, the class of “fish” is divided into subclasses, subclasses, in turn, into orders, orders into suborders, suborders include superfamilies, superfamilies - families, families - subfamilies, subfamilies - genera, which already include species.

The Latin name for fish usually has a specific ending. Thus, an order, as a rule, ends in -formes, a suborder in -oidei, the name of a superfamily is written with the ending -oidae, a family ends in -idae, and a subfamily in -ini.

Other not specified systematic units fish classification do not have a specific ending and can end in different ways.

Fish classification is carried out as follows. Very similar species fish according to their structure and way of life, as well as according to their kinship, are united into a genus. A genus, in turn, belongs to a subfamily, a subfamily belongs to a specific family, and so on. In some cases, species are also divided into subspecies.

The scientific name of the fish is indicated on the letter in two words. The first of these is the genus, and the second is the species name. In addition, the surname of the author who first described this species is indicated, as well as the year in which the description was created, if this year, of course, is known.

For example, the Latin name for fish zebrafish looks like this: Brachydanio rerio Hamilton-Buchanan, where Brachydanio is the name of the genus, rerio is the name of the species, and Hamilton-Buchanan, respectively, is the surname of the author.

In addition to the division described above, there are other fish classification. First of all, fish are always divided according to their habitat into marine and freshwater species.

Then, according to the method of reproduction, they are divided into viviparous and egg-bearing.

Next, and no less important, classify fish according to what is optimal for their life temperature conditions: Fish are warm-water, tropical and cold-water. Typically, aquariums contain tropical species, which are easiest to create suitable temperature conditions.

There is also fish classification according to their shape and structural features of the body. Usually, there is no separate Latin name in this case, and aquarists call different forms of fish species breeds.

For example, if the fish is called zebrafish veil, then such fish have elongated fins that look like a veil.

Besides, classify fish depending on the color form. In general, it might look something like this: black fork guppy, where guppy is the name of the fish species, black is the color of the body and fins, forked is the forked shape of the caudal fin.

Aquarists can call these same guppies, for example, “black prince,” although this species is exactly what scientists call it. fish classification not described or “patented”, but aquarists themselves came up with the name for the fish after they developed this form.

Everyone has heard the expression “roars like a beluga,” but not everyone has a clear idea of ​​what this animal looks like. What kind of beluga is this and what else besides its roar could it be famous for? Let's try to figure this out. Well, first of all, let’s say right away that the beluga cannot roar at all. If only because it belongs to the class of fish, and fish, as you know, are silent.

Description of beluga

Beluga is the largest freshwater fish living in the waters of our country.. It has lived on Earth for almost 200 million years and, like all other sturgeons, has learned to adapt to a wide variety of living conditions. These fish do not have a spine, and instead of a skeleton there is a flexible chord.

Appearance

Beluga is distinguished by its large size: its weight can be equal to one and a half tons, and its length can be more than four meters. Some eyewitnesses even saw belugas reaching a length of nine meters. If all this anecdotal evidence is true, then the beluga could be considered the largest freshwater fish in the world. She has a thick and massive body.

With its head and the shape of its muzzle, the beluga resembles a pig: its snout, somewhat like a snout, is short and blunt, and its huge, toothless mouth, which occupies almost the entire lower part of the head, surrounded by thick lips, has a sickle shape. Only beluga fry have teeth, and even those disappear after a short time. The antennae, hanging down from the upper lip and reaching the mouth, are slightly flattened downward. The eyes of this fish are small and blind, so it navigates mainly with the help of a well-developed sense of smell.

This is interesting! WITH Latin name Beluga (Huso huso) translates as “pig”. And, if you take a closer look, you can really notice that these two creatures are similar in some ways, both in appearance and in their omnivorousness.

Male and female belugas differ little in appearance and their bodies are both covered with equally large scales. The scales look like diamonds and do not overlap each other anywhere. This type of scale is called ganoid. The beluga's back is gray-brownish, its belly is lighter.

Behavior and lifestyle

Beluga is a migratory fish; it mainly leads a bottom-dwelling lifestyle. Myself appearance This amazing creature, reminiscent of the appearance of ancient armored fish, indicates that the beluga rarely appears on the surface: after all, with such a massive body, it is more convenient to swim in deep water than in the shallows.

It constantly changes habitats in the reservoir and often goes to the depths: there the current is faster, which allows the beluga to find food, and there are deep holes that this fish uses as resting places. In spring, when the upper layers of water begin to warm up, it can also be seen in shallow water. With the onset of autumn, the beluga again goes into the depths of the sea or river, where it changes its usual diet, eating mollusks and crustaceans.

Important! Beluga is a very large fish; it can only find enough food for itself in the seas. And the very presence of belugas in a reservoir is evidence of a healthy ecosystem.

Beluga travels vast distances in search of food and spawning grounds. Almost all beluga whales tolerate both salt and fresh water equally well, although some species can live exclusively in fresh water bodies.

How long does a beluga live?

Beluga is a real long-liver. Like all other sturgeon, it matures slowly: up to 10-15 years, but lives for a very long time. The age of this fish, if it lives in good conditions, can reach one hundred years, although now belugas live forty years.

Range, habitats

The beluga lives in the Black Sea, the Azov Sea and the Caspian Sea. Although less common, it is also found in the Adriatic. It goes to spawn in the Volga, Don, Danube, Dnieper and Dniester. Not often, but you can find it in the Urals, Kura or Terek. There is also a very small chance of seeing beluga in the Upper Bug and near the coast of Crimea.

There was a time when the beluga walked along the Volga to Tver, along the Dnieper to Kyiv, along the Ural River to Orenburg, and along the Kura to Tbilisi itself. But for some time now this fish has not climbed so far upstream in rivers. This is primarily due to the fact that the beluga cannot rise upstream due to hydroelectric power stations blocking its path. Previously, it also appeared in rivers such as the Oka, Sheksna, Kama and Sura.

Beluga diet

Newly born fry, weighing no more than seven grams, feed on river plankton, as well as larvae of mayflies, caddis flies, eggs and fry of other fish, including related sturgeon species. Grown-up beluga whales eat juvenile stellate sturgeon and sturgeon. Young belugas are generally characterized by cannibalism. As the young beluga grows, its diet also changes.

After the young of the year move from rivers to the sea, they feed on crustaceans, mollusks and small fish such as gobies or sprat, as well as fry of herring and cyprinids until they are two years old. Upon reaching two years of age, beluga whales become predators. Now approximately 98% of their total diet is fish. Beluga's food preferences vary depending on the season and feeding areas. In the sea, this fish feeds all year round, although with the onset of the cold season it eats less. Having remained in the rivers for the winter, it also continues to feed.

This is interesting! The food of many adult sturgeons is various small creatures that live on the bottom, and only the largest of them - beluga and kaluga - feed on fish. In addition to small fish, their victims may include other sturgeon and even small seal pups.

In the belly of one of the caught belugas there was found a fairly large sturgeon, several roach and bream. And another female of this species had a catch of two large carp, more than a dozen roach and three bream. Also, a large pike perch became her prey even earlier: its bones were found in the stomach of the same beluga.

Reproduction and offspring

Beluga begins to breed late. Thus, males are ready to breed at the age of at least 12 years, and females do not breed until they are 16-18 years old.

Females of the Caspian beluga are ready to continue their race at the age of 27: only by this age do they become fit for reproduction and accumulate sufficient weight for this. Most fish die after spawning ends. But the beluga spawns repeatedly, albeit with intervals of two to four years.

In total, 8-9 spawnings occur during its long life. She spawns on a sandy or pebble bottom, where there is a fast current, which is necessary for a constant flow of oxygen. After fertilization, the eggs become sticky and stick to the bottom.

This is interesting! A female beluga can lay several million eggs, while total weight caviar can reach up to a quarter of the weight of the fish itself.

In 1922, a five-meter beluga weighing more than 1200 kg was caught in the Volga. It contained approximately 240 kg of caviar. The hatched larvae, which later turn into fry, set off on a difficult journey - in search of the sea. “Spring” female belugas, entering the river from mid-winter until the end of spring, spawn in the same year. “Winter” beluga, in order to find and occupy a place convenient for spawning, comes to the rivers in August and remains there to winter. It spawns only the next year, and before that it lies in a kind of hibernation, sinking to the bottom and becoming covered with mucus.

In May or June, the “winter” beluga comes out of hibernation and spawns. Fertilization in these fish is external, like in all sturgeons. The eggs attached to the bottom of the reservoir mostly become prey for other fish, so the survival rate among juvenile beluga is very low. Beluga cubs live in shallow waters warmed by the sun's rays. And after they grow up enough, they leave their native rivers and go to sea. They quickly increase in size and by the age of one year their length becomes approximately equal to a meter.

Natural enemies

Adult belugas have practically no natural enemies. But their eggs, as well as larvae and fry that live in rivers, are eaten by freshwater predatory fish.

This is interesting! Paradoxical as it may seem, one of the main natural enemies beluga is the fish itself. The fact is that beluga calves, which have grown to 5-8 cm, happily eat the eggs of their relatives on the spawning grounds.

Population and species status

TO beginning of XXI century, the beluga population decreased significantly, and this species itself began to be considered endangered and was listed in Russia and in the International Red Book.

In the natural environment, due to the small population of its species, beluga can interbreed with other related sturgeon fish. And in 1952, through the efforts of scientists, an artificial hybrid of beluga and sterlet was bred, which was called bester. It is bred, as a rule, in artificial reservoirs, since bester is not released into natural ones, where other sturgeon fish are found, in order to preserve natural populations other types clean.