A common feature of planktonic plants and animals. Behavior of planktonic organisms. Wanderers of the sky

The plant life of the ocean is concentrated only in the uppermost, illuminated layer of water. It would seem that herbivorous planktonic organisms should be here all the time in order to be closer to the source of food. However, in reality, the behavior of marine zooplankton is much more complex than one might think. The vast majority of its representatives feed on algae only at night, and during the day they hide in the dark depths.

During the whole daylight hours, in the upper hundred-meter layer, where photosynthesis takes place, there are almost only algae. No matter how much you filter sea water with a special device - a plankton net, in its glass invariably there is only a greenish-brown algal suspension. As soon as the sun goes below the horizon and darkness sets in in the upper layers of sea water, the crustaceans begin to work hard with their antennae and limbs and rush upward. Together with them, salps, fish fry rise, and the whole company in complete darkness attacks the algae. Plant-eating plankton are followed by small and large planktonic predators, followed by larger fish. As dawn approaches, all zooplankton sinks into the depths, and by the beginning of the day, the illuminated zone of the ocean is empty again.

In the early days of marine biology, when the plankton net was invented, people immediately noticed good catches at night and bad catches during the day. At first, it was assumed that planktonic organisms see the net in daylight and flee from it. At night, the net is not visible and therefore brings a rich catch. Of course, fish, squids, large crustaceans capable of active swimming, as a rule, do not fall into the plankton net, as they are really frightened of it. But this cannot in any way apply to planktonic animals that passively move from place to place, carried away by currents.

When the reality of daily vertical movements of plankton became obvious, it was necessary to explain the reason for this strange phenomenon. At first, it was suggested that planktonic crustaceans, remaining in the dark depths during the day, more easily escape from predators, who easily detect them in the light. So, many terrestrial herbivores spend the day in the saving thick of the forest and go out to graze only under the cover of night darkness. This analogy may be figurative, but it is not based on anything.

A number of planktonic crustaceans are known that emit bright phosphorescent light. They seem to deliberately signal to predators about their location, and such crustaceans glow both during the day in the depths of the sea and at night near its surface. In addition, not all eaters of planktonic animals find their prey with the help of vision. Baleen whales are known to detect accumulations of food items through echolocation. For them, it is completely indifferent whether the crustaceans are lit by the sun or stay in the dark. Then a hypothesis was put forward, according to which plants, during photosynthesis, release some substances that are harmful to zooplankton. However, after careful experiments, this assumption was not confirmed.

It also turned out that daily up and down movements do not necessarily end at the sea surface. There are many planktonic organisms that spend the night at a depth of 500 - 200 meters, and during the day they go down a kilometer or more. They do not penetrate into the layer where photosynthesis takes place at all, and never see light, but nevertheless make significant vertical movements every day.

Thus, it seems that each species of planktonic and pelagic (also living in the water column, but capable of active movement) animals lives within certain depth limits. At night they stay near the lower, and during the day near the upper border, moving up and down during the day. It is quite obvious that the degree of illumination plays a dominant role in the movements of all these animals.

Zooplankton also begin to rise during total solar eclipses. The light seems to scare away planktonic animals, and the darkness attracts. But then why do the masses of planktonic organisms, having risen to the surface of the ocean at night, accumulate in the rays of bright lamps lowered overboard? Why do schools of fish and squid rush to this stream of light? The expediency of such actions could not be explained in any way.

Some experts, for example, the English hydrobiologist Alec Laurie, tried to connect the movements of planktonic animals not with light, but with temperature. The idea is as follows. At low temperatures, life processes are slower, energy consumption is reduced. Therefore, plankton stays in the cold depths, economically consuming nutrients, and at night quickly penetrates to the food fields, eats up and goes back into the coolness. Among other things, the viscosity of cold water is higher than that of warm water. This means that plankton organisms living in the cold zone have to expend less energy to maintain their position in space than if they lived in warm surface waters.

Perhaps A. Laurie is right to a certain extent, although changes in the viscosity of water are so insignificant that they can hardly play a significant role in the adaptations of plankters. The fact is that this theory does not explain in any way why the ascent and descent are timed to a change in light intensity and occur at a certain time of the day, and not as the planktonic organisms feel hungry. The harmonious picture of general ideas about the daily vertical movement of plankton was completely broken by the discovery of species that spend the day near the surface and descend into the depths at night.

In the end, the English researcher D. Harris, not finding an explanation for the massive daily movements of plankton, came to the conclusion that they have no adaptive value, that this is a side manifestation of the internal biological rhythm of plankton. It's just that planktonic organisms, like all other plants and animals, have their own biological clock, and their pendulum swings once a day a hundred meters up and a hundred meters down (for others and more).

Of course, in a number of cases, the actions of animals lead to clearly inappropriate, but clearly visible results. Here, during the flight, a huge flock of birds rose from the lake and for a moment eclipsed the sun, this is not a device for protection from predators, but only a shadow from the flock. But after all, regular, strictly regulated in time and distance movements of huge masses of plankton are not a shadow! Organisms move! Even a trace is not indifferent to the one who left it. In the footsteps, the predator trails the prey. Even a shadow can be dangerous. According to it, the enemy detects the one who discards it. It is all the more impossible to imagine that such serious actions as the transition from cold to heat, from depth to surface and back, would be an indifferent by-product of the internal rhythm of the organism. There is no doubt that these movements are necessary, only we do not know why they are necessary. While this is one of the mysteries of the ocean. Maybe some of the readers of this book will be able to solve it.

If the significance of the vertical movement of plankton: and for the life of the plankters themselves is not yet entirely clear, then the role of this phenomenon in the balance of the ocean, according to one of our leading planktonologists, Professor Mikhail Vinogradov, is obvious. The regular movement of plankton up and down leads to the contact of inhabitants of different depths, accelerates the process of transition of organic substances from the place of their synthesis (near the surface of the ocean) to the place of main consumption (in the depths and at the bottom), unites the inhabitants of the water column and the bottom into a single community.

When swimming in the sea or ocean, you can accidentally step on fish or algae in the water, which does not cause very unpleasant sensations, but fortunately this happens very rarely. In fact, most of us do not suspect that being in salt water, every second a person comes into contact with hundreds and thousands of organisms that he does not see or feel, but at the same time they experience his presence. These invisible inhabitants of the seas and oceans make up plankton - a huge number of animal and plant organisms that drift with the flow and are not able to choose the direction of their movement in space. Rarely among them there are also larger representatives, but there are very few of them among the whole plankton.

History of study

Though this group living organisms is mostly invisible to human eyes without the use of special equipment; biologists have long guessed about its existence. Officially, the term "plankton" was introduced by the German oceanographer Viktor Gensen, who devoted his whole life to studying the diversity of the nature of the ocean. The word was introduced into the official dictionary of terms almost 130 years ago - in 1887.

The word is borrowed from the Greek language, from which it is translated as “wandering” or “wandering”. This aptly reflects the way the smallest marine life exists, so the term has taken root perfectly and has never been disputed.

At the moment, planktonic organisms are the group in which scientists annually make the largest number of discoveries of new species that have not been previously described by anyone.

Now, out of more than a million different species, only 250 thousand have been described, the rest will have to be described by future generations of oceanographers.

What does it consist of

The composition of plankton is very diverse, here you can find many types of bacteria, algae, coelenterates, protozoa, crustaceans and crustaceans, mollusks, fish eggs and larvae, invertebrate larvae, etc.

Under the microscope, the microcosm of the seas and oceans looks fantastic: most microorganisms resemble smaller representatives of films about the future or aliens. Many of them have a bright color, an interesting shape and an unusual geometry of covers. Some of them are quite complex, possess the circulatory and nervous systems of higher animals, so it would be wrong to call them a faceless mass.

All representatives are divided into two large groups:

Plant organisms that need sunlight in order to photosynthesize. This includes diatoms, green and blue-green algae. It is phytoplankton that produces a huge amount of organic matter, which provides food for the vast majority of aquatic life. The abundance of phytoplankton depends on how much of the substances it needs in the water: nitrogen, phosphorus or silicon. When looking at a drop of sea water through a microscope, scientists can appearance planktonic organisms to infer the presence of each of these substances. With the active development of phytoplankton, the water masses can change their color, it is this phenomenon that causes the summer "bloom" of water.

- living organisms that cannot move or are very limited in this respect. The species diversity is quite large, here you can find crustaceans and rotifers, crustaceans, protozoa, coelenterates, pteropods, fish fry, insect larvae, etc. only in the surface layers, but also closer to the bottom.

Planktonic organisms are also distinguished depending on how long they have been in this group:

holoplankton- these representatives from birth to death are planktonic and lead a corresponding existence.
meroplankton- spend only part of their life in the form of planktonic organisms, most often - its first period, later turning into creatures that increase their weight and move freely in the water. These representatives include fish, sea worms, etc.

Dimensions

It is generally accepted that plankton are exclusively microscopic microorganisms that cannot be seen with the naked eye. This is what students report in the lessons of geography and biology, making reports and reading out essays. Actually this is not true. The vast majority of representatives of this group are indeed very small, but there are those that significantly exceed the size of the human body.

femtoplankton- represented by the smallest viruses, up to 0.2 microns in size;
picoplankton- it includes large bacteria and unicellular algae ranging in size from 0.2 to 2 microns;
nanoplankton- large unicellular algae and bacterial colonies 2-20 microns in size;
microplankton- this includes rotifers, protozoa and most algae ranging in size from 0.02 to 0.2 cm;
mesoplankton- this group includes crustaceans and marine animals up to 2 cm;
macroplankton- jellyfish, shrimps and other animals from 2 to 20 cm;
megaplankton- the largest representatives with a size of 20 cm to 2 m.

The largest in plankton are the cyanide jellyfish with a body 2 m in diameter and tentacles extending 30 m around, as well as colonies of pyrosomes that form a ribbon 1 m wide and 30 m long.

The most numerous group is represented by organisms in the range of 0.2-2 microns, it is they who significantly exceed the rest, even the largest representatives, in terms of biomass.

An interesting picture of the dependence of the weight and size of these microorganisms. Not always large specimens weigh a lot. In order to drift faster, in the process of evolution, many adaptations have been developed that do not increase body weight, but increase the ability to float on water: inclusions of gas or drops of light fat, internal chambers with sea water, outgrowths, a thin and flat body, pores inside skeleton, etc.

biological seasons

Like most species of wildlife, plankton have seasonal fluctuations in abundance, which are determined by the temperature of the habitat and the length of daylight hours. During good weather conditions, warmth and a sufficient amount of light, a surge in reproduction is observed, and under adverse factors, development slows down. During each season, the composition, number and age of phytoplankton and zooplankton representatives change.

The annual cycle looks like this:

In spring, with significant warming, algae begin to multiply rapidly, so phytoplankton develops rapidly, often causing water blooms. Since phytoplankton serves as food for many species of zooplankton, the increase in algae invariably entails a rapid burst of active reproduction of the smallest living planktonic organisms.
By the summer, the population growth stops and freezes at the same level.
In autumn, the number of phytoplankton and zooplankton begins to decrease, this process starts especially early in the northern waters. In southern latitudes, autumn again provokes an outbreak of reproduction, as in spring.
In winter, the number decreases, most specimens go dormant.

The duration of each season is related to the geographical location, so for representatives drifting in the north, the dormant period can take nine months a year, while in the southern regions it can be reduced to several weeks. In the tropics, the state and quantity of phytoplankton and zooplankton are in a balanced state throughout the year.

Where does it live

The ideal conditions for this group are the same as for all other living beings: the warmth and light of the sun. Such conditions exist in the upper layer of water, which warms up well and passes the sun's rays through it in sufficient quantities. This is especially important for phytoplankton, whose life processes directly depend on sunlight. Most of all, it can be found in the surface layer of the seas and oceans, which is called the euphotic layer. At a depth of 50 m, the population density begins to decrease, and after 100 m one can meet a planktonic representative only occasionally.

A paradise for plankton is the tropical waters of the ocean, so a huge species diversity and abundance are concentrated in the warm waves of the Indian Ocean. Most often, the composition is diverse and mixed, but some specimens live without neighbors. These include brine shrimp, living in waters with such high salinity that no other planktonic organisms can tolerate it.

But most often the species diversity in the sea is very extensive. Average population data show that in one glass of sea water there are 200 million viruses that infect 20 million bacteria that are also in the same glass. Therefore, one can only imagine how much plankton we “push” with our bodies when entering sea water.

Previously, plankton did not survive in the northern parts of the Atlantic Ocean due to low temperatures, but now, after 800 thousand years, they have returned to these areas again. The reason for this was the melting of polar glaciers, which is happening more intensively due to global warming. The presence of food in these waters attracted gray whales here. What other changes in nature can cause the resettlement of these marine microorganisms, one can only speculate.

It is possible to meet plankton not only in exotic places: it lives in any reservoirs, even in a small bucket of water that has stood at home for several days. In the aquarium, fish eat it with pleasure, diversifying their diet and bringing it closer to the natural one. You can also meet zooplankton in the supermarket, here it will be sold under the name "krill", which is a rather tasty delicacy, highly valued not only by whales, but also by people.

Ecological role

The importance of phytoplankton and zooplankton in the life of the planet is difficult to overestimate. It was these microorganisms that were the first on Earth to start producing oxygen. Even now, 50% of oxygen is produced by plankton, and due to rapid deforestation, this percentage is increasing annually, so the title "lungs of the planet" can be safely transferred to ocean inhabitants.

Plankton consumes organic matter that enters the world's oceans, and if it were not for these tireless "purifiers", the water would have become uninhabitable long ago. They are the initial element of the food chain, saturating marine life and birds all year round. An interesting fact is that the largest mammals on the planet - blue whales - feed on the smallest representatives of the ocean depths - plankton. Many whales follow currents that have a large concentration of planktonic microorganisms in order to always stay near the feeder.

Scientists use this group to indirectly assess the purity of water bodies, since its representatives quickly die out in polluted water.

Glowing wonder

Everyone knows the wonderful phenomenon of the glow of the sea, which can be observed at night, due to the presence of planktonic photosynthetic bacteria in it. This process is most actively observed in the warm season, at the moments of active reproduction of phytoplankton. Tourists can observe a bright glow in the coastal zones of the Black Sea waters, in the Sea of \u200b\u200bAzov, oversaturated with fertilizers, and in the Maldives.

The main source of luminescence are cyanobacteria and dinoflagellates. They are capable of producing so much light that even astronauts can see it in the form of a blue veil while in orbit. A huge number of photographers tend to the coast at such a time to take their best pictures.

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The smallest organisms of the water column are combined into the concept of "plankton" (from the Greek " planktos"- soaring, wandering). The world of plankton is huge and diverse. This includes organisms that inhabit the thickness of the seas, oceans, lakes and rivers. They live wherever there is the slightest amount of water. It can be even the most ordinary puddles, a vase of flowers with stagnant water, fountains, etc.

The plankton community is the most ancient and important from many points of view. Plankton has existed for about 2 billion years. They were the first organisms that once inhabited our planet. Plankton organisms were the first to supply our planet with oxygen. And now about 40% of oxygen is produced by aquatic plants and primarily by planktonic plants. Plankton is of great importance in the food balance of aquatic ecosystems, as many species of fish, whales and some birds feed on them. It is the main source of life for the seas and oceans, large lakes and rivers. The impact of plankton on water resources is so great that it can even affect the chemical composition of waters.

Plankton includes phytoplankton, bacterioplankton and zooplankton. Basically, these are small organisms, the size of which most often does not exceed tens of micrometers for algae and a few centimeters for zooplankton. However, most of the animals are much smaller. For example, the size of the largest freshwater daphnia reaches only 5 mm.

However, most people know very little about plankton, although the number of organisms in water bodies is extremely high. For example, the number of bacteria in one cubic centimeter of water reaches 5-10 million cells, algae - in the same volume - tens to hundreds of thousands, and zooplankton organisms - hundreds of specimens. It's an almost invisible world. This is due to the fact that most plankton organisms are very small, and in order to see them, you need a microscope with a sufficiently high magnification. The organisms that make up the plankton are in the water column in a state of soaring. They cannot resist being carried by the currents. However, this can only be said in general terms, since in calm water many planktonic organisms can move (albeit slowly) in a certain direction. Algae, changing buoyancy, move vertically within a few meters. During the day they are in the upper well-lit layer of water, and at night they sink three to four meters deeper, where there are more minerals. Zooplankton in the seas and oceans rises to the upper layers at night, where it filters out microscopic algae, and in the morning descends to a depth of 300 meters or more.

Who is part of the plankton? Most planktonic organisms spend their entire lives in the water column and are not associated with a solid substrate. Although the resting stages of many of them in winter settle to the bottom of the reservoir, where they wait out unfavorable conditions. At the same time, among them there are those who spend only part of their lives in the water column. This is meroplankton (from the Greek. “ meros» - part). It turns out that the larvae of many bottom organisms - sea urchins, stars, brittle stars, worms, mollusks, crabs, corals and others lead a planktonic lifestyle, are carried by currents and, ultimately, find places for further habitat, settle to the bottom and already to the end life does not leave him. This is due to the fact that benthic organisms are at a disadvantage compared to plankton, because move relatively slowly from place to place. Thanks to planktonic larvae, they are carried by currents over long distances, in the same way that the seeds of land plants are carried by the wind. The eggs of some fish and their larvae also lead a planktonic lifestyle.

As we have already noted, most planktonic organisms are true plankters. They are born in the water column, and they die there. It consists of bacteria, microscopic algae, various animals (protozoa, rotifers, crustaceans, molluscs, coelenterates, etc.).

Planktonic organisms have developed adaptations that make it easier for them to soar in the water column. These are all kinds of outgrowths, flattening of the body, gas and fat inclusions, a porous skeleton. In planktonic mollusks, shell reduction has occurred. They have it, unlike benthic organisms, very thin, and sometimes barely visible. Many planktonic organisms (such as jellyfish) have gelatinous tissues. All this allows them to support the body in the water column without any significant energy costs.

Many planktonic crustaceans make vertical migrations. At night, they rise to the surface, where they eat algae, and closer to dawn, they descend to a depth of several hundred meters. There, in the darkness, they hide from the fish, who eat them with pleasure. In addition, low temperature reduces metabolism, and, accordingly, energy expenditure to maintain life. At greater depths, the density of water is higher than at the surface, and organisms are in a state of neutral buoyancy. This allows them to be in the water column at no cost. Phytoplankton inhabits mainly the surface layers of water, where sunlight penetrates. After all, algae, like land plants, need light for development. In the seas they live to a depth of 50-100 m, and in fresh water - up to 10-20 meters, which is associated with different transparency of these reservoirs.

In the oceans, the depths of algae habitat are the thinnest film of a huge water column. However, despite this, microscopic algae are the first food for all aquatic organisms. As already noted, their size does not exceed several tens of micrometers. Only the size of the colonies reaches hundreds of micrometers. These algae feed on crustaceans. Among them, krill is the best known to us, which mainly includes euphausiid crayfish up to 1.5 cm in size. Crustaceans are eaten by planktophage fish, and they, in turn, are larger and predatory fish. Whales feed on krill, which filter them out in large quantities. So, in the stomach of a blue whale 26 m long, 5 million of these crustaceans were found.

Marine phytoplankton plankton is mainly composed of diatoms and pyridine. Diatoms dominate in polar and subpolar marine (ocean) waters. They are so large that silicon skeletons after their death form bottom sediments. Most of the bottom of the cold seas is covered with diatomaceous silts. They occur at depths of about 4000 m or more and consist mainly of valves of large diatoms. Small shells usually dissolve before reaching the bottom. The mineral diatomite is a product of diatoms. The number of valves in diatoms in some areas of the ocean reaches 100-400 million in 1 gram of silt. Diatomaceous silt eventually transforms into sedimentary rocks, from which "diatomaceous earth" or the mineral diatomite is formed. It consists of tiny porous flint shells and is used as a filter material or sorbent. This mineral is used to make dynamite.

In 1866-1876. Swedish chemist and entrepreneur Alfred Nobel was looking for ways and means of producing a powerful explosive. Nitroglycerin is a very effective explosive, but it explodes spontaneously with small shocks. Having established that to prevent explosions it is enough to impregnate diatomaceous earth with liquid nitroglycerin, Nobel created a safe explosive - dynamite. Thus, the enrichment of Nobel and the famous “Nobel Prizes” established by his will owe their existence to the smallest diatoms.

The warm waters of the tropics are characterized by a higher species diversity than the phytoplankton of the Arctic seas. Peridinea algae are the most diverse here. Calcareous flagellar coccolithophores and silicoflagellates are widespread in marine plankton. Coccolithophores mainly inhabit tropical waters. Lime silts, including coccolithophorid skeletons, are widespread in the World Ocean. Most often they are found in the Atlantic Ocean, where they cover more than 2/3 of the bottom surface. However, in the silts, shells of foraminifers belonging to zooplankton are presented in large quantities.

Visual observations of sea or ocean waters make it easy to determine the distribution of plankton by the color of the water. The blueness and transparency of the waters testify to the poverty of life; in such water there is practically no one to reflect light, except for the water itself. Blue is the color of sea deserts, where swimming organisms are very rare. Green is an unmistakable indicator of vegetation. Therefore, when fishermen encounter green water, they know that the surface layers are rich in vegetation, and where there is a lot of algae, animals that feed on them always abound. Phytoplankton is rightly called the pasture of the sea. Microscopic algae are the main food of a large number of ocean inhabitants.

The dark green color of the water indicates the presence of a large mass of plankton. Shades of water indicate the presence of certain planktonic organisms. This is very important for fishermen, since the nature of the plankton determines the type of fish that live in the area. An experienced fisherman can catch the subtlest shades of the color of sea water. Depending on whether he fishes in "green", "yellow" or "red" water, an "experienced eye" can predict the nature and size of the catch with a reasonable degree of probability.

Blue-green, green, diatom and dinophyte algae predominate in fresh water bodies. Abundant development of phytoplankton (the so-called "bloom" of water) changes the color and transparency of the water. In fresh water bodies, blue-green blooms are most often observed, and in the seas - peridine. The toxic substances released by them reduce the quality of water, which leads to the poisoning of animals and humans, and in the seas causes the mass death of fish and other organisms.

The color of the water in certain areas or seas is sometimes so characteristic that the seas got their name from the color of the water. For example, the peculiar color of the Red Sea is caused by the presence in it of the blue-green algae Trichodesmium ( Trichodesmium egythraeum), which has a pigment that gives the water a reddish-brown hue; or the Crimson Sea - the former name of the Gulf of California.

Certain plant dinoflagellates (eg Gonyaulax and Gymnodinium) give the water its distinctive coloration. In tropical and temperate warm waters, these creatures sometimes multiply so rapidly that the sea turns red. Fishermen call this phenomenon "red tide". Huge accumulations of dinoflagellates (up to 6 million cells in 1 liter of water) are extremely poisonous, so many organisms die during the "red tide". These algae are not only poisonous in themselves; they release toxic substances, which then accumulate in organisms that eat dinoflagellates. Any creature, be it a fish, a bird or a person, after eating such an organism, receives a dangerous poisoning. Fortunately, the red tide phenomenon is local and does not happen often.

The waters of the seas are colored not only by the presence of algae, but also by zooplankton. Most euphausiids are transparent and colorless, but some are bright red. Such euphausiids inhabit the colder northern and southern hemispheres and sometimes accumulate in such numbers that the entire sea turns red.

The water is colored not only by microscopic planktonic algae, but also by various particles of organic and inorganic origin. After a heavy rain, the rivers bring a lot of mineral particles, which is why the water takes on different shades. Thus, the clay particles brought by the Yellow River give the Yellow Sea an appropriate shade. The Yellow River (from Chinese - Yellow River) got its name for its turbidity. Many rivers and lakes contain such an amount of humic compounds that their waters become dark - brown and even black. Hence the names of many of them: Rio Negro - in South America, Black Volta, Niger - in Africa. Many of our rivers and lakes (and the cities located on them) are called "black" because of the color of the water.

In fresh water bodies, water staining due to the development of algae appears more often and more intensely. The mass development of algae causes the phenomenon of "blooming" of reservoirs. Depending on the composition of phytoplankton, the water turns into different colors: from green algae Eudorina, Pandorina, Volvox - green; from diatoms Asterionella, Tabellaria, Fragilaria - yellowish-brown color; from flagellates Dinobryon to greenish, Euglena to green, Synura to brown, Trachelomonas to yellowish-brown; from dinophytes Ceratium - in yellow-brown color.

The total biomass of phytoplankton is small compared to the biomass of zooplankton feeding on them (respectively, 1.5 billion tons and more than 20 billion tons). However, due to the rapid reproduction of algae, their production (harvest) in the World Ocean is almost 10 times greater than the total production of the entire living population of the ocean. The development of phytoplankton largely depends on the content of mineral substances in surface waters, such as phosphates, nitrogen compounds, and others. Therefore, in the seas, algae develop most abundantly in areas of rising deep waters rich in minerals. In freshwater bodies, the influx of mineral fertilizers washed off from the fields, various domestic and agricultural effluents leads to the massive development of algae, which adversely affects the quality of the waters. Microscopic algae feed on small planktonic organisms, which in turn serve as food for larger organisms and fish. Therefore, in areas of the greatest development of phytoplankton, there are many zooplankton and fish.

The role of bacteria in plankton is great. They mineralize organic compounds (including various pollutants) of water bodies and re-include them in the biotic cycle. The bacteria themselves are food for many zooplanktonic organisms. The number of planktonic bacteria in the seas and clean fresh water bodies does not exceed 1 million cells in one milliliter of water (one cubic centimeter). In most fresh water bodies, their number varies within 3-10 million cells in one milliliter of water.

A.P. Sadchikov,
Professor of Moscow State University named after M.V. Lomonosov, Moscow Society of Naturalists
(http://www.moip.msu.ru)

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How to find out where the birds spent the winter? How to study the paths they flew?

For centuries, Europeans were unaware of long-distance flights of birds and were convinced that in winter they hide in secluded and inaccessible places, where they spend unfavorable days in a state of lethargy. This opinion existed until the 18th century.

Even the great Swedish naturalist Carl Linnaeus wrote: “Each autumn, when it starts to get colder, swallows seek refuge in the reeds of rivers and lakes.” Later, scientists discovered the migratory routes of various bird species and plotted them on a map of the globe. And the light rings worn on the paws of birds helped the scientists in this.

The idea of bird ringing was born in 1899. It rightfully belongs to the Danish teacher Martensen. He was the first to ring birds to find out how far they fly. Martensen put light aluminum rings on the legs of 102 different birds, on which he engraved his address. Each ring had its own serial number, by which it was possible to clarify which bird, what number and where it was worn.

Of the 102 birds ringed by Martensen, 9 were killed the following year in Western Europe. The idea of such a kind of "certification" of birds was adopted by scientists around the world. This was the beginning of the scientific ringing of birds. Now, in almost all countries of the world, special bird ringing centers have been established.

However, back in 1740, the Italian scientist Spalanzani “ringed” birds, tying red ribbons on their legs. Now ringing of birds in different institutes is carried out according to the Martensen method. On the leg of a caught live bird, a ring is put on with the name and address of the institution conducting the ringing, and the serial number of the bird. Most often, rings are put on the legs of chicks that have not yet learned to fly. In a special diary, the type of bird, the number of the ring, the date and place where it was worn are recorded.

Ringed birds often fall into the hands of people, most often hunters and nature lovers. A shiny ring immediately catches the eye, and the ringing center receives reports of the capture of a bird, and if it is killed, then a ring is sent there, which is placed in a regular envelope along with information about the time, place and circumstances of the bird's capture.

Let's say the ring was put on a stork chick in Bulgaria and then received from Egypt. It means that the stork flew to Africa for the winter. Then we get a second letter with a similar ring from another stork, also ringed in Bulgaria. This time the ring was filmed in Equatorial Africa. It turns out that he flew over Egypt and continued on his way.

This method helped to undeniably establish that storks winter thousands of kilometers from their native places, in southern Africa. In the same way, we learned that our swallows winter south of the equator, from Tanzania to Guinea. And our cuckoo, it turns out, is a great traveler - she visits the Arab countries and oases of the Sahara in winter, the jungles of Sudan and even reaches Mozambique.

Thanks to ringing, we know that some species of birds return in the spring to the nests in which they spent the previous year, for example, storks, swallows, starlings and other birds. It turns out that every small ring worn on a bird's leg is a valuable scientific document. He tells us about the path along which our winged friends move. In many countries, hundreds and thousands of birds are ringed every year.

The nature, timing and paths of fish schools are studied by observations from land, from ships and aircraft. But the main method of studying fish migration is marking. It provides the best results. The study of the biology of fish, including their migration, is carried out by scientists from many special scientific institutes around the world. According to the International Council for the Study of the Sea, in Copenhagen for the period 1925-1951, scientists from various countries labeled more than 5 million fish, mostly migratory species.

Typically, the stamp is attached to the body of the fish with a needle with nylon thread and special clips. It is fixed near the dorsal fin. On the stamp, as well as on the ring, the address of the scientific institute that marked the fish and the corresponding number are indicated. Data on marking fish are recorded in the appropriate diary.

Recently, marking of fish with hydrostatic marks, which are transparent plastic tubes, has been practiced. A note is enclosed in them, where the following data is marked: the address to which the found note should be sent, and information about the date and place of fishing. In the light on transparent celluloid, you can read: "Cut off the edges, the letter is inside." The text of the note is written in several languages.

With the help of marking, data on the migration of whales were collected. The very first ideas about their ways were obtained by direct observation from the shore and from whaling ships. The success of whaling is directly dependent on the migration of whales, on knowing where and at what time they are. Migration travel affects the amount of subcutaneous fat.

When whales migrate from feeding areas to breeding areas in autumn, the fat layer in animals reaches its greatest thickness, while in the spring, when returning, it becomes very thin. The most complete information about the biology of whales has been obtained by marking. Whales are marked with various types of tags by shooting them into the subcutaneous fat layer from a harpoon weapon.

Scientists from a number of countries devoted a lot of effort to the study of butterfly migrations. At the beginning of the 20th century, American entomologists began to study the flight of the monarch butterfly, a classic traveler. Soon the flights of butterflies began to be studied in Europe. In some countries, special entomological stations have been set up to study their routes.

The main means of study was marking: a very thin and light aluminum plate is attached to the wing of a butterfly, which glitters in the sun, attracting attention. She does not interfere with the flight. The microscopic label shows the address of the station or researcher. West German entomologist Herbert Rehr marked 60,000 cabbages in this way. From the released butterflies, Rer received back about 20 records, one of which was found at a distance of 80 km from the place of release.

Today, the latest methods of marking are also used. For example, fish are labeled with radioactive isotopes. Modern technology provides more and more means with which to trace the paths of migrating animals. To establish the migratory path of sea turtles, which swim thousands of kilometers to their breeding grounds, scientists resorted to an original method of marking.

A special radio transmitter was placed on the back of a huge turtle weighing 150 kg, the signals of which made it possible to trace its route across the ocean. Today, even at some ornithological stations, instead of a ring, a miniature radio transmitter is mounted on the bird's back, with the help of which they determine where it is.

Recently, in some countries, the routes of migratory birds are being studied using radar. Observations of migratory birds are carried out in the same way as for aircraft. The radar screen marks flying birds, the space in which they are located, and the direction of flight. Large birds appear on the screen as small bright dots, and small ones are visible only when there are a lot of them.

With the help of radar, observations can cover fairly large areas and huge numbers of migratory birds. The study of radar images showed that birds fly over vast spaces and not along a fixed path, but along a very wide front. The exceptions are white storks and some birds of prey, which fly over strictly defined places and, as scientists suggest, use ascending air currents to facilitate the flight.

With the help of radar, a lot of valuable data has been collected, indicating that migratory birds navigate by the sun during the day, and by the stars at night. With heavy cloud cover, they often begin to rush about, circle, change direction, sometimes they go astray, but as soon as the stars become visible again, the ability of birds to orientate is immediately restored, and they again take right course. So the devices that serve military and civil aviation on the ground also help ornithologists.

The study of random and periodic wanderings of winged animals is not only of theoretical, cognitive interest for specialists, but is also of great national economic importance. For example, the study of the mass flight of locusts and other pests has long been placed on a strictly scientific basis. A special research institute has been set up in London to study issues related to locust flights.

UNDERWATER TRAVELERS

The lower representatives of the animal world are also subject to regular movements, which are similar to the migrations of higher organisms. There are two types of plankton movements: horizontal and vertical.

The horizontal movement of zoo- and phytoplankton organisms is also called passive migration. Due to the limited possibility of movement, planktonic organisms often travel not of their own free will, but are carried away by various water currents.

Planktonic organisms form accumulations with a total weight of up to several million tons. Sometimes they move hundreds and thousands of kilometers; the speed with which the current carries aquatic organisms is sometimes very high. For example, some equatorial currents have an average speed of about 100 km per day, and the speed of the Atlantic Gulf Stream is about 250 km per day! The pedestrian would not have kept up with him.

Vertical movements of planktonic organisms are active and sometimes reach distances of up to 500 m. If we compare this distance with the miniature size of the organisms themselves, then vertical migrations are truly long journeys. At different stages of their development, planktonic organisms live at different depths of the water basin. Adult forms inhabit mainly the depths of the sea, and eggs and organisms in the early stage of development inhabit the surface layers. Some males and females of the same species also live at different depths.

The movements of the simplest organisms are closely related to their reproduction cycles. Very interesting in this regard are the palolo marine worms from the Nereid group, which at certain times, associated with the phases of the moon, enter the upper layers of the water for reproduction. These worms are found in myriads in the Pacific Ocean near the islands of Samoa, Fiji and Tonga of the Polynesian group. They usually live in cracks in coral reefs, making moves in coral formations.

In autumn (October - November), a week after the full moon, they swim to the surface of the sea. At this time, at the rear end of the body of females, you can see a bag full of brown eggs; male sexual products are green. Ripe eggs, breaking away from the mother's body, swim freely. Their fertilization takes place passively, at the behest of the waves. The front parts of the body of the worms (red) remain in the water, they have the ability to regenerate - restore the lost parts of the body.

The inhabitants of the islands value palolo as a special delicacy. The islanders know the time at which palolos will appear on the surface of the ocean, to within one day. At this time, they go out to sea in their wooden boats, even before sunrise they stop near the reefs and patiently wait with nets in their hands for the appearance of sea worms.

Usually the exit of worms to the surface lasts two hours, then the bags of the females burst, and the reproductive products swim out. During the hours when palolos appear, the sea in vast areas is literally teeming with countless of them, turning dark green. For a whole week, the natives feast: they eat worms raw or prepare tasty and nutritious dishes from them.

Of great importance for the vertical distribution of life in water bodies is light, as well as water temperature and pressure. Nevertheless, the distribution of zooplankton at different depths of the sea is not a constant phenomenon; in different parts of the day it changes due to the vertical migration of organisms. The amplitude of movement in various organisms usually varies from 200 to 300 m.

Scientific research has established that the main reason that makes them make such long trips within one day is related to food. The surface layers of water, especially down to a depth of 25 cm, are densely populated by various types of bacteria, phytoplankton and other microscopic organisms - the main food of zooplankton.

Most planktonic animals rise to the surface of the water at night, and during the day they go to the depths of the water layers, although there is enough food at the top. The reason for this is not well understood, most likely, in the deep and dark layers, animals escape from enemies.

In addition to diurnal, part of the plankton also performs seasonal migrations. For example, the marine crustacean Calanus finmarchicus spends several months at depth, and the rest of the time it rises and lives in the upper layers of the sea. It is believed that this is due to changes in light intensity and temperature. Studies have established that some marine organisms that cannot tolerate high temperatures in the surface layers of the sea regularly make seasonal migrations, adhering to cool deep layers in summer, and surface layers in autumn and winter. Some species can make both diurnal and seasonal migrations.

From marine invertebrates, some soft-bodied and echinoderms also migrate, which, with the approach of spring, come to the coastal strip, where they lay their eggs. Chasing migrating fish stocks for 4 months, the Pacific squid, for example, travels up to 8000 km.

One of the amazing phenomena of wildlife is, no doubt, the massive wandering of fish. Indeed, it is hard to imagine how, at a strictly defined, as if "appointed" hour, hundreds of thousands and even millions of fish of the same species leave the vast ocean expanses in countless herds and set off on a long and disastrous path.

More than 2,000 km must be traveled against the flow of the river, overcoming countless dangerous rapids and waterfalls, in order to reach a place where you can spawn. No one will show them the path they must walk once in their lives. And yet the fish unmistakably reach their native places, where they spawn and die. Of course, not all fish travel. It turns out that there are some species that never leave their native waters, no matter how small they may be.

In fish, as in planktonic animals, two types of migration are distinguished: passive and active. Fish fry, for example, never move against the current, as they are too weak to overcome it. Therefore, eggs, fry and juveniles are carried over short or long distances by various water currents. Passive migration is observed in juvenile ocean herring.

Every spring, adult fish living in the northern regions of the Atlantic Ocean head to the coast of Norway, where they spawn. The sea current carries the hatched fry to the shores of the Scandinavian Peninsula, at a distance of 800-1000 km from the birthplace. Similar migrations are also made by herring fry hatched in the area of the Murmansk coast.

Eel larvae - leptocephali, insignificant in size and almost devoid of organs of active movement, make one of the most grandiose passive migrations. They travel 7-8 thousand km from the Sargasso Sea, where they are brought out, to the shores of Europe, carried away by the powerful movement of the Gulf Stream. There are many and actively migrating. They wander independently, but adhering to a certain direction associated with reproduction, feeding and wintering. Fish undertake random migrations, for example, under suddenly changing conditions.

In some cases, migratory fish travel more than 2000 km, and sockeye salmon, for example, undertakes wandering in the upper reaches of the Yukon River in Alaska, overcoming 3600 km, and at a speed of 30-40 km per day. Sometimes such trips last for whole months. The Caspian Beluga runs from the Caspian Sea to the upper reaches of the Ufimka River for 2950 km. Caspian sturgeon, whose spawning place is located in the upper reaches of the Kama, swim 2500 km.

Some fish, especially Pacific salmon, are so exhausted by long wandering that after spawning they are almost unable to actively move. The question arises, what is rational in these distant migrations of fish? Science has not yet given a complete and exhaustive answer to this question. And yet it can be said that, when dying, the fish provide good conditions for the fry that hatch from the fertilized caviar. Parents die for the life of their offspring.

Among the numerous species of fish, the European river eel undertakes the farthest travels. This fish is born in the depths of the ocean and soon leaves for freshwater pools - in rivers and lakes. When puberty sets in (at about 8-12 years of age), she again begins to irresistibly strive into the sea, overcoming from 7 to 8 thousand km; it goes first to the Atlantic Ocean, then to the Sargasso Sea, where it spawns at a depth of about 1000 m and dies of exhaustion in the same place where it was born.

Amphibians and reptiles roam

Interesting observations of the migration of some amphibians were made by the biologist V. Beshkov, a researcher at the Zoological Institute of the Bulgarian Academy of Sciences. With the help of marking, he established that the common frog traveled 120 km in search of a suitable wintering place. He observed how frogs of this species made migrations for the purpose of reproduction at a distance of 60-70 m from the banks of the Isker River, since they did not have enough places convenient for breeding.

In the process of studying the biology and behavior of various amphibian species, Beshkov found that the common toad also makes long migrations to breeding sites. He observed the movements of toads from the lowlands of Vitosha (the forest near Bayan) almost to the Vazov region in Sofia.

Toads go to the spawning grounds from April 1 to April 15 and stay there for 15 days, after which they return to the forests of Bayan. Beshkov also observed toads of this species, leaving in the spring from the elevated places of Nakatnik (rocky and waterless terrain) to the Priboynitsa River to lay their eggs there. Gray toads undertake vertical migrations up to 300 m. After breeding, they again return to where they came from.

Hatched in the forest near Priboynitsa, Beshkov found toads near the rocks in early October. But toads move not only over short distances. There are cases when they traveled for a week to reach a swamp or puddle in which they laid their eggs. These amphibians travel only at night and sleep during the day. An unerring instinct always tells them the right path, they never stray from the path to the reservoir to which they are heading.

Among amphibians, some species of newts also make short-distance migrations. Finding newts at a distance of a kilometer from a reservoir is not uncommon.

Reptiles also make trips to wintering grounds. For example, some vipers crawl more than a kilometer to get to a convenient place in the roots of a dry tree or in some quarry where they accumulate in multitudes. And crocodiles wander from one reservoir to another. There are cases when a densely populated swamp in India was abandoned in one night by dangerous inhabitants, as it became shallow.

Crocodiles crawled, not making out the road, through thickets and fields, even ended up in one village, where they dispersed through the streets, crawled into courtyards, and some climbed into wells, terrifying the population: in the morning people stumbled upon terrible aliens at every step. The next night, the crocodiles left the village, continuing their journey.

Long-distance migrations are undertaken by giant sea turtles, which, for thousands of years, lay their eggs in the coastal sand on certain islets. The Brazilian green tortoise, for example, needs to travel about 2,500 km to reach the island of Asuncion, where it lays its eggs. Long wanderings to breeding grounds are undertaken by other sea turtles - ridleys, common in the Atlantic Ocean from Canada to the Caribbean.

The migrations of these turtles have long been a mystery to scientists. Only in 1947 it was established that every year in April - May and until early June, about 40 thousand turtles of this species swim from different sides of the boundless ocean to their favorite beach to lay their eggs.

Wanderers of the Sky

Everyone knows the accuracy with which migratory birds leave their native places in the fall, heading south, and return home in the spring to lay eggs and breed. This rhythm is so strictly observed by various species of birds that in India, for example, in ancient times, some months of the year were even named after certain species of migratory birds.

Birds, undoubtedly, are champions in the animal kingdom, as they make the farthest travels. The absolute record belongs to the arctic tern, which every year overcomes the path from the Arctic to Antarctica and back!

The famous American ornithologist J. Audubon described in detail his observations of a flock of passenger pigeons that flew through Ohio in the autumn of 1813. He calculated that the flock numbered more than 1.1 billion pigeons. It would be hard to believe this if there were no other evidence. Alexander Wheelen, who observed a flock of passenger pigeons in Kentucky in 1832, claimed that its number was determined at 2,230,270,000 individuals.

Let's leave such an exact figure on the conscience of an eyewitness, but this is not the main thing. Unfortunately, human greed has caused these birds, whose flocks reached such astronomical numbers, no longer exist. They were barbarously exterminated in the 19th century because of their delicious meat. The last bird of this species died in 1914 at the Zoological Gardens in Cincinnati.

How fast do migratory birds fly? Wild ducks, for example, with an average speed of 70-80 km / h, swallows - 55-60 km / h; there is also an unlikely report that a redstart ringed in England was caught in the USA after 24 hours, flying 3500 km in a day. It should be noted that the direction of the wind has a great influence on the flight speed.

A bird that flies at a speed of 40 km/h in calm time, and with a tailwind of 50 km/h, significantly reduces its speed in a headwind. A gusty wind especially reduces the flight speed. The height at which migrating flocks of birds fly is also different. For example, small songbirds usually fly no more than 100 m from the surface of the earth; starlings, crows, thrushes prefer a height of 150-500 m, and storks 900-1300 m.

Many birds reach such a height where a person could not be without an oxygen apparatus. This applies to those species of birds that, during migration, are forced to overcome high mountain ranges. Over the Himalayas, small birds were observed and photographed flying from India to Siberia. And the English observer Harisen photographed from an airplane a flock of wild geese flying over the Himalayas at an altitude of 9500 m. Most of the migratory birds bypass the mountain ranges, adhering to river valleys and gorges.

Migrations are also observed in some species of flightless birds. Penguins, for example, sometimes cover distances of up to 2000 km, moving "on foot", sliding on their stomachs over icy hilly areas or swimming in the ocean. With the onset of winter, they move north from all parts of Antarctica, sometimes reaching the southern coast of Africa and South America.

Some representatives of the group of running birds, such as ostriches, cover a distance of 1000 km “on foot”, moving in a precisely defined direction.

It should be noted that different birds fly at different times of the day. Diurnal predators and a number of other birds fly exclusively during the day, some marsh and waterfowl fly at any time of the day. Many migratory birds follow a certain “formation” during the flight, for example, cranes fly in a wedge, geese in a line, and small birds widely spread flock. Some birds fly in complete silence, others (cranes, swans, wild ducks, and many others) make characteristic sounds that apparently serve to convey various information.

The wandering of birds is a phenomenon that people paid attention to many years ago. It is known that various legends of Ancient Hellas and Rome are associated with birds and their flights, this is also mentioned in ancient Egyptian legends. In the ancient hymn “Glorification of the Nile” that has come down to us, there are such words: “Above you, birds fly south, they protect you from the sultry wind ...”

The same is said in the book of the biblical prophets Job and Jeremiah. Aristotle, the greatest scientist-encyclopedist and philosopher-naturalist Ancient Greece, in his multi-volume "History of Animals" also devoted a large place to birds. In it, along with naive and erroneous ideas, there is a lot of accurate information about their flights. For thousands of years, people have collected data on bird migration, but so far this phenomenon has not been fully studied.

By the time of departure, the birds are divided into three main groups. The first is birds that begin to prepare for departure long before the onset of an unfavorable period. The cuckoo, for example, leaves our country at the end of July or the beginning of August, when there is still plenty of food and warmth.

Storks and swallows fly relatively early. Birds that are included in the second group fly away after the first signs of a change in the weather, i.e. with a decrease in air temperature and a decrease in the amount of food. Among these birds there are many insectivores: starlings, warblers, etc. The third group includes birds that fly away in late autumn, when living conditions become unbearable for them, for example, wild ducks and geese.

However, sometimes not all birds of the same species and even not all individuals of the same population migrate. Some fly away, while others remain within the nesting ranges. The "almighty" migratory instinct does not work on them. In cities in warm winters, near garbage containers, you can see rooks left to winter.

The issues related to navigation and orientation of birds during flight are still not fully resolved. Nevertheless, observational and experimental data suggest that the main role in the orientation of birds is played by their vision, which is well developed in all birds.

Of great importance for the orientation of birds are not only terrestrial, but also celestial landmarks: during the daytime flight - the sun, during the night - the moon and stars. It has also been established that during night flights, birds are guided mainly by the North Star. Some researchers are of the opinion that during long-distance flights, birds are guided by the Earth's magnetic field.

TRAVELING MAMMALS

Migration observed in mammals is of two types: non-periodic and periodic. Non-periodic migrations are often associated with a lack of food or with overcrowding in their area of occupation. The behavior of animals changes, and, in the end, they leave the inhabited area, i.e. migrate.

A typical example of mass migration is the grand migration of mouse-like rodents in the so-called "mouse years". For example, in 1727 countless hordes of rats from the Kazakh steppes crossed the Volga. In subsequent years, the animals populated all of Europe, spreading diseases and causing damage to the population. Other mouse-like rodents undertake similar wanderings from time to time - different kinds field mice, water rats, lemmings and many others.

Lemming movements are a classic example of spontaneous and irregular migrations. These animals reach a length of 15 cm, and they live in Asia, Europe, America. On the European continent, they are found mainly on the Scandinavian and Kola Peninsulas. Periodically gathering in myriad numbers, lemmings leave their habitats and move in a huge living stream across the tundra, as if trying to reach the horizon line.

Sometimes they go hundreds of kilometers from their native places. Lemmings are followed by wolves and foxes, which during this period feed exclusively on them. Animals can become prey for lynxes, bears, wolverines, arctic foxes, as well as domestic cats and dogs. While moving, owls, crows, buzzards, gulls and other birds are circling above them, which are attracted by easy and tasty prey. But nothing can stop the lemmings in their spontaneous movement forward: neither the enemies that destroy them in large numbers, nor the rivers and mountains.

Going on this campaign, the animals doom themselves to a kind of suicide. Having reached the seashore, they do not stop, do not return back, but in some inexplicable blindness they rush into the foaming waters of the surf. The chance of salvation is negligible. Only an insignificant part of the lemmings shows "prudence" and, finding themselves on the shore, moves further along its edge until they find a suitable place for themselves.

The main reason for the mass wandering of lemmings is a strong increase in their numbers. It has been established that in some years these animals breed especially intensively: instead of twice a year, females bring offspring three, and sometimes four times. At the same time, the number of babies in the litter is more than usual. As a consequence of this, "wanderlust" arises. It is known that migratory groups of lemmings consist mainly of young animals, but only 20% of them reach sexual maturity.

Wildebeest migration in Africa

Some zoologists believe that lemmings have an innate migratory instinct, but it only appears in years when the conditions mentioned above are present.

Squirrels also undertake massive non-periodic wanderings. Usually these cute animals are not prone to long-distance movements, but with a lack of food, they massively, in thousands of individuals, leave the areas of their permanent habitat and move hundreds and thousands of kilometers away. During these spontaneous migrations, usually shy squirrels do not stop at any obstacles.

Singly or in groups, the animals headlong fly from tree to tree, moving from one forest to another, cross rivers and lakes, bypass villages and cities. Without stopping, they move forward and forward until they reach the forest, where there is a lot of food, without being afraid of either people or enemy animals.

Squirrels move at a speed of 3-4 km / h, however, the overall speed of movement also depends on the number of migrating animals. The more numerous the accumulation of migrating squirrels, the faster they move forward, since in a short time they destroy food supplies along the way and they need to find new food as soon as possible. When migrating, squirrels do not adhere to a common group, like some other mammals (reindeer, bison, lemmings, etc.). And although they travel in the same direction, they often do not see each other. Brem described the case when in 1896 a huge accumulation of squirrels moved through Nizhny Tagil (Urals).

The bulk of the migrating animals passed 8 km from the city, and the flank detachments of this "army" defended 16 km from each other. One part of the squirrels passed through the city; the animals fearlessly galloped through the streets, ran into yards, jumped through windows into rooms, climbed trees and roofs. Dogs furiously crushed the animals, people killed them, but the squirrels did not deviate from the chosen path, moving uncontrollably forward.

The procession continued for three days. Even the stormy and wide Chusovaya River did not stop the migrant animals. Squirrels fearlessly entered the cold and stormy waters and swam to the opposite shore. “There is no more beautiful sight,” wrote the famous Siberian explorer Midendorf, “than a flotilla of squirrels swimming across a wide river.

Their tails sticking out of the water are like the sails of a ship. The journey of the squirrels, which Brem wrote about, continued until the animals got into the forest, where there was enough food for everyone. Sometimes the front along which squirrels move reaches 300 km, and the number of migrating animals is determined by tens, and sometimes hundreds of thousands of individuals.

Mass migrations are also undertaken by arctic foxes. The instinct awakens in them in the autumn - also in connection with the increase in the number of animals in the area and the increasing lack of food. By marking individual individuals, it was found that some arctic foxes migrate up to 2000 km from the place of marking. Often, during these wanderings, animals fall on the drifting ice of the Arctic Ocean and reach the islands most remote from the continent.

Well-known ground squirrels also make non-periodic migrations. In the area where these animals appear, they become dangerous pests of field crops. In the middle of the XIX century in the region of Schleswig (Germany) there were no these rodents at all. They appeared there from nowhere, in large numbers, and quickly became the scourge of agriculture in the region.

Of all representatives of the animal world, the most significant horizontal migrations are made by marine mammals, mainly whales, seals and fur seals. The movements of whales and pinnipeds are determined by the characteristics of their diet, and in some species they are associated with the characteristics of reproduction.

Migrations of different species of whales have a different character. In whales living in the northern seas, they are very limited. Species adhering to bays and coastal zones migrate mainly in northern and southern directions, and animals rarely go to the open sea.

Whales that live in open seas move during migrations in a strictly defined circular direction, adhering mainly to the ocean current. At the beginning of summer, these animals keep their way to the north, and at the beginning of winter (during the onset of great cold and ice accumulation in the northern seas), they move in the opposite direction, to the south, bypassing the equator.

However, not all whales migrate during strictly defined seasons of the year. Only humpback whales observe the greatest accuracy. Representatives of baleen whales of the Southern Hemisphere go south in summer to the cold waters of Antarctica, which are rich in food at that time, and in winter they return north to warm tropical and subtropical waters. Here they eat little or no food at all.

Long-distance travels are made by the blue whale, the largest animal on Earth. There is a known case when blue whales swam about 500 km in 32 days, in another case, about 800 km in 88 days. The recorded record distance from the marking site for the blue whale is 1600 km.

The minke whale makes regular seasonal migrations. He spends the winter in the northern waters of the Atlantic Ocean, and in the spring he goes on distant wanderings, reaching Svalbard and the Barents Sea.

Females of some species of whales penetrate the Strait of Gibraltar into the Mediterranean Sea. According to the zoologist P.U. Puzanov, in 1880 one of the whales penetrated through the Bosphorus and the Dardanelles into the Black Sea, running aground in shallow water near Batumi. His skeleton is still kept in the Tbilisi Museum.

Regular seasonal migrations, sometimes for thousands of kilometers, are also characteristic of many pinnipeds. One of the wanderers of this order of animals is the harp seal. In summer, these animals move to the areas of floating ice in the western regions of the Arctic Ocean, where they feed heavily, and in winter they go far south - to the throat of the White Sea.

Here seals appear in huge numbers, forming three separate herds - Newfoundland, Yanmayen and White Sea, numbering hundreds of thousands and even millions of animals. Here, seals give birth and nurse babies, molt. Later, they return together to the Arctic waters of the ocean.

It is interesting to note that the mentioned herds of harp seals not only stay in different areas, but also do not mix during migrations. Over many years of observing and marking a large number of individuals, Norwegian researchers have established that there is a partial exchange of individuals only in the Jan Mayen and White Sea herds.

Fur seals make distant seasonal migrations. In summer, they gather in thousands of individuals in the northern part of the Pacific Ocean, mainly on Pribylov Island and the Commander Islands. Old males arrive here in early May, a few weeks earlier than females.

Here the seals breed and keep until the end of August. In autumn, the herd from the Commander Islands swims to the Sea of Japan, while the Privilovo one winters off the coast of Southern California. Females, unlike males, winter in more southern regions and swim huge distances during migration - up to 5000 km.

Planktonic crustaceans and rotifers living in fresh waters are eaten by fish, as well as a number of relatively small invertebrate predators (cladocerans Leptodora Kindti, many copepods, mosquito larvae Chaoborus and etc.). Fish and invertebrate predators attacking “peaceful” zooplankton have different hunting strategies and different preferred prey.

In the process of hunting, fish usually rely on sight, trying to choose prey of the maximum size for them: for grown-up fish, these are, as a rule, the largest planktonic animals found in fresh waters, including invertebrate plankton-eating predators. Invertebrate predators mainly attack small or medium-sized planktonic animals, since they simply cannot cope with large ones. In the process of hunting, invertebrate predators usually orient themselves with the help of mechanoreceptors, and therefore many of them, unlike fish, can attack their prey even in complete darkness. Obviously, invertebrate predators themselves, being the largest representatives of plankton, can easily become victims of fish. Apparently, therefore, it is “not profitable” for them to be especially large, although this would allow expanding the size range of their potential victims.

In order to protect themselves from invertebrate predators, it is more advantageous for planktonic animals to have larger sizes, but at the same time, the danger of becoming clearly visible, and therefore easily accessible prey for fish, immediately increases. A compromise solution to these seemingly incompatible requirements would be an increase in real size, but at the expense of any transparent outgrowths that do not make their owners particularly noticeable. Indeed, in the evolution of different groups of planktonic animals, the emergence of such "mechanical" means of protection against invertebrate predators is observed. Yes, branched crustacean Holopedium gibberum forms a spherical gelatinous membrane around its body (Fig. 51), which, being completely colorless, does not make it particularly noticeable to fish, but at the same time protects it from invertebrate predators (for example, from larvae Chaoborus) because it is simply difficult for them to grasp such a victim. Various outgrowths of the shell of daphnia and rotifers can also perform a protective function, and, as it turned out, some of these formations develop in victims under the influence of certain substances secreted by nearby predators. At first, a similar phenomenon was discovered (Beauchamp, 1952; Gilbert, 1967) in rotifers: female prey - Brachyonus rotifers (Brachionus calyciflorus), grown in water, which previously contained predatory rotifers of the genus Asplanchna (Asplanchna spp.), produced juveniles with especially long lateral carapace spines (see Fig. 51). These spikes greatly prevented the asplanchns from swallowing the brachionus, as they literally stood across their throats.

Later, various body outgrowths induced by predators were also found in crustaceans. So, in the presence of predatory larvae Chaoborus in young individuals Daphnia pulex a “tooth-like” outgrowth grew on the dorsal side, which significantly reduces the likelihood of successful eating by these predators (Krueger and Dodson, 1981; Havel and Dodson, 1984), and in some Australian Daphnia carinata in the presence of predatory bugs Anisops calcareus(family Notonectidae), a transparent ridge formed on the dorsal side, apparently also strongly preventing the predator from grasping and eating prey (see Fig. 51).

Such outgrowths cannot protect against most fish, and therefore it is extremely important for planktonic crustaceans in the presence of fish in the reservoir to remain invisible and (or) avoid direct encounters with them, especially in good light conditions. Since the food concentration of planktonic crustaceans is maximum just at the surface, it is not surprising how often we find in them the existence of vertical diurnal migrations, which are expressed as rising at night to the surface layers rich in food and descending during the day to deeper layers, where there is poor illumination, as well as the possibility reduce local density by scattering in a larger volume prevent them from being eaten by fish.

Vertical migrations themselves require certain energy costs. In addition, a small amount of food and low temperature at great depths lead to a decrease in the intensity of reproduction and a slowdown in the development of crustaceans, and therefore, ultimately, to a decrease in the rate of their population growth. This negative consequence of vertical migrations for the population is usually considered as a “payment” for protection from predators. The question of whether it is worth "paying" for protection from predators in this way can be resolved in different ways in evolution. So, for example, in the deep Lake Constance in the south of Germany, two outwardly similar species of daphnia live: Daphnia galeata And Daphnia hyalina, moreover, the first species constantly keeps in the upper, heated layers of the water column (epilimnion), and the second - in summer and autumn, migrates, rising to the epilimnion at night and descending to great depths (into the hypolimnion) during the day. The food concentration of both Daphnia species (mainly small planktonic algae) is quite high in the epilimnion and very low in the hypolimnion. The temperature in the middle of summer in the epilimnion reaches 20°C, while in the hypolimnion it barely reaches 5°C. Researchers from Germany X. Stich and W. Lampert (Stich, Lampert, 1981, 1984), who studied in detail the daphnia of Lake Constance, suggested that migrations D. hyalina allow her to largely avoid the pressure of fish (whitefish and perch), and D. galeata, remaining all the time in the epilimnion, under conditions of strong fish pressure, it is able to withstand it with a very high birth rate. X. Shtikh n V. Lampert tested their hypothesis about the different survival strategies of these daphnia under laboratory conditions, when, in the absence of a predator, both species imitated the conditions of constant stay in the epilimnion (constantly maintained high temperature and a large amount of food) and the conditions of vertical migrations (changing in during the day, the temperature regime and the changing amount of food). It turned out that in such artificially created epilimnion conditions, both species felt great and had a high birth rate. In the case of imitation of the conditions of vertical migrations, the survival and intensity of reproduction of both species were significantly lower, but it is interesting that D. hyalina characterized by much better rates of survival and reproduction than D. galeata. When simulating the conditions of the epilimnion, a certain advantage (albeit insignificant) turned out to be D. galeata. Thus, the differences in the spatial and temporal distribution of these Daphnia species corresponded to the differences in their physiological characteristics.

The data obtained by the Polish hydrobiologist M. Gliwicz (Gliwicz, 1986) testify in favor of the assumption that it is the pressure of plankton-eating fish that is the factor responsible for the occurrence of vertical migrations in planktonic animals. After examining a number of small lakes in the Tatras, Gliwich discovered that a representative of the copepods cyclops, which is often found in them, Cyclops abyssorum makes daily vertical migrations in those lakes where there are fish, but does not make where there are no fish. Interestingly, the degree of severity of vertical migrations of cyclops in a particular water body depended on how long the permanent fish population existed in it. In particular, weak migrations were noted in one lake, where the fish were introduced only 5 years before the survey, and much stronger ones where the fish appeared 25 years ago. But the migrations of the Cyclopes were most clearly expressed in the lake where the fish, as far as is known, existed for a very long time, apparently for several millennia. Another additional argument in favor of the hypothesis under discussion can be the fact established by M. Gliwich that there was no migration of cyclops in one lake in 1962, just a few years after the introduction of fish there, and the presence of their clear migrations in 1985 after 25 years coexistence with fish.

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