Small planktonic plants and animals have. Features, types, nutrition and reproduction of zooplankton

Zooplankton (animal plankton) are small organisms that are often at the mercy of ocean currents, but, unlike phytoplankton, are not capable of.

Peculiarities

The term zooplankton is not taxonomic, but characterizes the lifestyle of some animals that move thanks to the flow of water. Zooplankton are either too small to resist the current, or large (as is the case with many jellyfish) but do not have organs that allow them to swim freely. In addition, there are organisms that are plankton only at a certain stage of their life cycle.

The word plankton comes from the Greek word planktos, meaning "wandering" or "wandering". The word zooplankton includes the Greek word zoion, meaning “animal.”

Types of zooplankton

It is believed that there are more than 30,000 species of zooplankton. It can live in fresh or salt water all over the world, including oceans, seas, rivers, lakes, etc.

Types of Zooplankton

Zooplankton can be classified by size or body length. Some terms that are used to refer to zooplankton include:

• Microplankton are organisms 20-200 microns in size - this includes some copepods and other zooplankton.
• Mesoplankton are organisms 200 µm-2 mm in size, including crustacean larvae.
• Macroplankton are organisms 2–20 mm in size that include euphausians (for example, krill are an important food source for many organisms, including baleen whales).
• Micronekton are organisms 20-200 mm in size. Examples include some euphausians and cephalopods.
• Megaplankton - planktonic organisms larger than 200 mm, including salps.
• Holoplankton are organisms that are planktonic throughout their lives - such as copepods.
• Meroplankton are organisms that have a planktonic stage of their life cycle, but grow out of it at some point, for example, fish and.

What do zooplankton eat?

Zooplankton and food chains

Zooplankton are generally found in the second trophic level, which begins with phytoplankton. In turn, phytoplankton is eaten by zooplankton, which is eaten by small fish and even giant whales.

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 seas, oceans, lakes and rivers. They live wherever there is the slightest amount of water. These 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 have 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, primarily planktonic. Plankton is of great importance in the nutritional balance of aquatic ecosystems, as many species of fish, whales and some birds feed on them. It is the main source of life in 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. These are mainly small organisms, the size of which most often does not exceed tens of micrometers for algae and several centimeters for zooplankton. However, most animals are significantly smaller in size. 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 large. 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. This is an almost invisible world. This is due to the fact that most plankton organisms are very small in size, and to view them, you need a microscope with a fairly high magnification. Organisms that make up plankton are floating in the water column. They cannot resist being carried by currents. However, this can only be discussed 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 descend three to four meters deeper, where there are more minerals. Zooplankton in the seas and oceans rises to the upper layers at night, where they filter out microscopic algae, and in the morning they descend to a depth of 300 meters or more.

Who is part of plankton? Most planktonic organisms spend their entire lives in the water column and are not associated with solid substrate. Although the resting stages of many of them settle to the bottom of the reservoir in winter, where they wait out unfavorable conditions. At the same time, among them there are those who spend only part of their life 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 are completely life does not leave him. This is due to the fact that bottom organisms are at a disadvantage compared to plankton, because They move relatively slowly from place to place. Thanks to planktonic larvae, they are carried by currents over long distances, just as the seeds of terrestrial 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 there they die. It consists of bacteria, microscopic algae, various animals (protozoa, rotifers, crustaceans, mollusks, 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, and a porous skeleton. In planktonic mollusks, shell reduction occurred. Unlike benthic organisms, it is very thin and sometimes barely visible. Many planktonic organisms (such as jellyfish) have gelatinous tissue. All this allows them to maintain their body in the water column without any significant energy expenditure.

Many planktonic crustaceans undergo 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 vital functions. At great depths, the density of water is higher than at the surface, and organisms are in a state of neutral buoyancy. This allows them to stay in the water column without any costs. Phytoplankton inhabit mainly the surface layers of water where sunlight penetrates. After all, algae, like terrestrial plants, need light to develop. In the seas they live to a depth of 50-100 m, and in fresh water bodies - up to 10-20 meters, which is due to the different transparency of these water bodies.

In the oceans, the depths of algae habitat are the thinnest film of a huge thickness of water. However, despite this, microscopic algae are the primary food for all aquatic organisms. As already noted, their size does not exceed several tens of micrometers. The size of colonies alone reaches hundreds of micrometers. Crustaceans feed on these algae. Among them, we are most familiar with krill, which mainly includes euphausiid crustaceans up to 1.5 cm in size. The crustaceans are eaten by planktivorous fish, and they, in turn, are eaten by larger and more predatory fish. Whales feed on krill and filter them out in huge quantities. Thus, 5 million of these crustaceans were found in the stomach of a 26 m long blue whale.

Marine phytoplankton plankton mainly consists of diatoms and pyridiniums. Diatoms dominate in polar and subpolar sea (ocean) waters. There are so many of them that silicon skeletons form bottom sediments after they die. Diatomaceous ooze covers most of the bottom of cold seas. 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 oozes eventually transform 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 spontaneously explodes with small shocks. Having established that to prevent explosions it was enough to soak diatomaceous earth in liquid nitroglycerin, Nobel created a safe explosive - dynamite. Thus, Nobel’s enrichment 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 higher species diversity compared to the phytoplankton of the Arctic seas. The most diverse algae here are peridinea. Calcareous flagellated coccolithophores and silicoflagellates are widespread in marine plankton. Coccolithophores mainly inhabit tropical waters. Calcareous silts, including the skeletons of coccolithophores, 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, the silts contain large quantities of shells of foraminifera belonging to zooplankton.

Visual observations of sea or ocean waters make it possible to easily determine the distribution of plankton by the color of the water. The blueness and transparency of the waters indicate the poverty of life; in such water there is practically no one to reflect the light except the water itself. Blue is the color of sea deserts, where floating organisms are very rarely found. Green color 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, there is always an abundance of animals that feed on it. 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 living in the area. An experienced fisherman can detect the subtlest shades of color in sea water. Depending on whether he is fishing in “green”, “yellow” or “red” water, an “experienced eye” can predict with a reasonable degree of probability the nature and size of the catch.

Blue-green, green, diatom and dinophyte algae predominate in fresh water bodies. The abundant development of phytoplankton (the so-called “blooming” 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 blooms are observed. The toxic substances they release reduce the quality of water, which leads to poisoning of animals and humans, and in the seas causes mass deaths of fish and other organisms.

The color of 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 of blue-green algae Trichodesmium ( Trichodesmium egythraeum), which has a pigment that gives the water a reddish-brown tint; or Crimson Sea - the former name of the Gulf of California.

Some plant dinoflagellates (for example, Gonyaulax and Gymnodinium) give the water a peculiar color. In tropical and warm temperate waters, these creatures sometimes reproduce so quickly 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 during the “red tide” many organisms die. 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, who eats such an organism becomes dangerously poisoned. 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 live in the colder northern and southern hemispheres and sometimes accumulate in such numbers that the entire sea turns red.

Coloring the water is given not only by microscopic planktonic algae, but also by various particles of organic and inorganic origin. After heavy rain, rivers bring a lot of mineral particles, which is why the water takes on different shades. Thus, clay particles brought by the Yellow River give the Yellow Sea a corresponding shade. The Yellow River (from Chinese - Yellow River) got its name due to its turbidity. Many rivers and lakes contain so many 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 coloring due to the development of algae appears more often and more intensely. The massive development of algae causes the phenomenon of “blooming” of water bodies. Depending on the composition of phytoplankton, the water is colored in different colors: from green algae Eudorina, Pandorina, Volvox - green; from diatoms Asterionella, Tabellaria, Fragilaria – yellowish-brown color; from flagellates Dinobryon – greenish, Euglena – green, Synura – brown, Trachelomonas – yellowish-brown; from dinophyte Ceratium - yellow-brown.

The total biomass of phytoplankton is small compared to the biomass of the zooplankton that feed on it (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 fresh water bodies, the influx of mineral fertilizers washed off from fields and various household and agricultural wastewater leads to the massive development of algae, which negatively affects the quality of water. Microscopic algae feed on small planktonic organisms, which in turn serve as food for larger organisms and fish. Therefore, in areas of greatest phytoplankton development there is a lot of zooplankton and fish.

The role of bacteria in plankton is great. They mineralize organic compounds (including various pollutants) of water bodies and reintroduce them into the biotic cycle. The bacteria themselves are food for many zooplankton 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 numbers vary between 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 Natural Scientists
(http://www.moip.msu.ru)

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Plankton composition. The organisms that make up plankton are very diverse. Plant forms are represented here almost exclusively by microscopic lower unicellular algae. The most common among them are diatoms, enclosed in a kind of flint shell, similar to a box with a lid. These shells come in a variety of shapes and are very durable. Falling to the bottom after death, algae cover vast areas of the ocean floor in high latitudes with so-called diatomaceous silt. In the fossil state, such accumulations of diatom shells give rise to silica-rich rocks - tripoli, or ciliate earth.

Only slightly inferior to diatoms in their importance in plankton are peridinian algae, characterized by the presence of two flagella lying in grooves, one of which, transverse, encircles the body, and the other is directed backward. The body of the peridinium is covered either with a thin protoplasmic membrane or with a shell of many plates consisting of a substance similar to cellulose. The body shape is round, sometimes there are three processes. Also interesting are the extremely small coccolithines, which have a shell permeated with calcareous bodies. Silicon flagellates equipped with skeletons have the same small size.

Blue-green algae are of subordinate importance in the plankton of the seas, but in some desalinated seas, for example in the Azov Sea, they often multiply in such quantities that the water turns green.

Of the unicellular animals, the most characteristic of plankton are the globigerina rhizomes with multi-chambered calcareous shells covered with long thin spines. Falling to the bottom after death, they cover vast areas of the ocean floor with lime-rich globigerina silt.

Clusters of radiata or radiolarians with very beautiful, lace-thin silicon skeletons also cover large areas of the ocean floor.

Widespread bell-shaped ciliated ciliates are very characteristic of marine plankton, but their skeleton is less durable, and therefore they do not form such bottom sediments as diatoms, rhizopods and radiolarians. Their houses have the shape of bells, vases, pointed cylinders, tubes, etc.

Of the colorless flagellates, the most famous are undoubtedly the spherical nocturnal lights, or noctiluces, which have the ability to glow.

Very interesting are hydroid polyps - siphonophores, colonial coelenterates with complexly differentiated colonies, with a deep division of functions: feeding, protective, swimming, hunting and sexual. Jellyfish, shaped like umbrellas or discs, and ctenophores are very numerous and varied.

Worms are represented mainly by various larvae - trochophores and nectochaetes. Some types of worms lead a planktonic lifestyle during the breeding season, rising to the very surface. There are two families of purely planktonic annelids.

Crustaceans play a decisive role in plankton.

All orders of this class live in plankton either their entire lives (for example, copepods and cladocerans) or only during the larval period (shrimps, crabs). Copepods make up the main background of the animal plankton of the seas.

Of the mollusks, the purely planktonic groups are the completely transparent kelepods and pteropods. The shells of the latter, after the death of the mollusks, sink to the bottom, where, like rhizopods and radiolarians, they form pteropod silt, characterized by an abundance of lime. Gastropods and bivalves have planktonic larvae, which are characterized by the presence of a spirally curled or bivalve shell and a peculiar bilobed organ of locomotion, covered at the edges with cilia. During the breeding season, they fill masses of plankton.

Bryozoans and echinoderms are represented only by larvae. Holothurians lead a planktonic lifestyle. Of the lower chordates, salps, luminous pyrozomas, and appendicularia living in transparent hunting houses are very numerous. Numerous fish eggs and larvae also fill the plankton.

Finally, the thickness of sea water is populated by countless bacteria. The variety of external forms of bacteria is very small and is limited to only a few types: rods, balls, or cocci, more or less long spirals - spirochetes. Many of them have flagella and are actively motile. To distinguish them, mainly physiological characteristics and, to a lesser extent, external form are used. They play a vital role in the processes of transformation of substances in the sea - from the decomposition of complex remains of plant and animal organisms to their transformation into compounds of carbon, nitrogen, sulfur and phosphorus assimilated by plants.

Among bacteria there are autotrophic ones, which are capable, like plants, of building proteins and carbohydrates from inorganic substances. Some of them - photosynthetics - use solar energy for these processes; others - chemosynthetics - use the chemical energy of the oxidation of hydrogen sulfide, sulfur, ammonia, etc.

Mobile plants and attached animals. The presence of plankton in the sea led to the development of an exclusively unique category of animals that are not found at all on land, namely immobile, attached, or so-called sessile. Plants on land are attached to the soil and do not move. Herbivores must have the ability to approach food and move around to do this. Predators must catch their prey. In short, all sushi animals must actively move.

In water, due to the presence of plankton and suspended remains of dead organisms - detritus, the animal can remain motionless, food will be brought to it by water currents, therefore the attached lifestyle is widespread among marine animals. These include hydroid polyps and corals, many worms, crustaceans, or sea acorns, bryozoans, sea lilies, etc. Among mollusks, as an example, we will cite the well-known oysters, tightly cemented to rocks or generally to solid objects. All these animals either have unique apparatuses, not found in terrestrial animals, for straining food from water, or they strive to cover the space as widely as possible with numerous, near-oral tentacles, or they develop a tree-branched form.

It is not surprising that biologists for a long time did not know whether to classify these plant-like creatures as the plant world or the animal world, and called them animal plants.

Now we know that they cannot, like plants, absorb carbon dioxide and other inorganic substances, but feed, like all animals, on ready-made organic food created by other organisms, and therefore we consider them animals, although they cannot move. Thus, due to the high specific gravity of water and the salts dissolved in it, free-floating plants and attached animals can exist in the aquatic environment.

The population of the bottom, or benthos, in addition to these attached animals, collectively called sessile benthos, also includes freely moving animals - vaginal benthos: worms, crustaceans, mollusks - bivalves, gastropods and cephalopods, echinoderms, etc. Some of them feed on plankton itself, others are planktivores. Thus, benthos as a whole - both mobile and attached - in their nutrition is directly or indirectly related to plankton, since attached algae play a very insignificant role in the economy of the sea. Therefore, we can expect that where there is a lot of plankton, benthos will be abundant. However, this is not always the case. Conditions in the bottom layers may be unfavorable for the development of benthos (presence of hydrogen sulfide, lack of oxygen, etc.) and then, despite the richness of plankton, there may be little or no benthos. At significant depths in layers accessible to sunlight, nutrients are used in the water column and so little reaches the bottom that the benthos can be poor, despite the large production of plankton in the upper layers. But this ratio, when there is little plankton and a lot of benthos, can only be temporary.

Almost all benthonic animals have planktonic larvae. Plankton is like a kindergarten for benthos organisms. This means that in certain seasons benthos is not only a consumer of plankton, but also its producer.

Life and relationships of plankton organisms. Free-floating plant organisms - diatoms and flagellates - feed, grow, and reproduce due to carbon dioxide, nitrates, phosphates and other inorganic compounds dissolved in water, from which they build complex organic compounds of their body in sunlight. These are producers of organic substances. These microscopic plants feed on crustaceans, worms and other herbivorous animals, which can only feed on ready-made organic substances created by plants and cannot use inorganic compounds from the environment. These are first-order consumers. Predators - second-order consumers - feed at the expense of herbivores. They, in turn, are eaten by larger predators - third-order consumers, etc. These are the relationships within this community.

Ultimately, all organisms - both producers and consumers - die. Their corpses, as well as secretions and excrement, as a result of the activity of bacteria and other microorganisms, are transformed again into biogenic elements dissolved in water - the source material for the new construction of the bodies of plant organisms with the help of solar energy, and the cycle of transformations of matter is closed.

Thus, the chemical elements that make up organisms - nitrogen, carbon, hydrogen, oxygen, phosphorus, sulfur, etc. - are in constant motion in a circle: algae (producers) - animals (consumers) - bacteria and biogenic compounds dissolved in water.

This circular movement of elements occurs due to solar energy captured and accumulated by plant organisms in the form of chemical energy of complex organic substances. Animals consume only organic substances created by plants, using up the energy they accumulate. These are, in general terms, the relationships between the plant and animal parts of plankton. From this it is clear that the ratio of zooplankton and phytoplankton should be direct, that is, in places where there is little phytoplankton, there should be little zooplankton, and, conversely, with an increase in phytoplankton, the amount of zooplankton should also increase.

However, this ratio between the plant and animal parts of plankton cannot remain unchanged constantly. On the rich food of phytoplankton, zooplankton reproduces intensively and a moment may come when, for example, as a result of the depletion of the supply of biogenic compounds in the water, phytoplankton production begins to decrease. In the end, it may turn out that there will be a lot of zooplankton and little phytoplankton, that is, the ratio will become the opposite. Zooplankton will begin to die out from lack of food.

Thus, the quantitative ratios of zooplankton and phytoplankton cannot remain constant due to the biological nature of the relationship between the plant and animal parts of plankton, the basis of which is the struggle for existence.

The question of the numerical relationships between bacteria, phytoplankton and zooplankton has not yet been studied enough. However, based on the fact that bacteria mostly live off the decay of organisms, it can be assumed that the more phytoplankton and zooplankton there are, the more bacteria there will be. Due to the colossal rate of bacterial reproduction, their consumption by zooplankton is unlikely to significantly change these relationships.

In addition to purely biological internal reasons, these relationships can also be affected by external conditions, as will be discussed below.

Adaptations to a planktonic lifestyle. As was said, due to the fact that the specific gravity of protoplasm, although insignificant, is still greater than the specific gravity of pure water, planktonic organisms, in order to stay in the water column, must have some adaptations that prevent immersion or at least slow it down. To understand the essence of these devices, it is necessary to become familiar with the conditions of buoyancy. The relationship between these conditions is expressed as follows:

Let's look at what the individual components are.

Viscosity, or internal friction, is a property of fluid bodies that determines the resistance of particles when they are displaced relative to each other. When the water temperature increases from 0 to +30-40°C for each degree, the viscosity decreases by approximately 2-3%. As salinity increases, viscosity increases, but very little. The viscosity of air is 37 times less than the viscosity of water. Consequently, by virtue of this alone, a body in the air will fall 37 times faster than in water. In warm and fresh water, buoyancy conditions will be worse than in sea and cold water. In tropical waters, adaptations to the planktonic lifestyle should be more pronounced than in cold waters.

Form resistance is the ability of bodies to resist any influences or changes.

The residual weight is equal to the weight of the organism minus the weight of the water it displaced. Thus, the greater the weight of displaced water, the smaller the residual weight, and this value is directly dependent on the specific gravity of water. Therefore, as salinity increases, buoyancy will increase. The closer the water temperature is to the temperature of its highest density (+ 4°C for fresh water), the more buoyancy will increase.

If the viscosity of water and its specific gravity, as factors determining the speed of immersion (buoyancy), do not depend on the organism, then the weight of the organism itself and the resistance of the form are its signs and, as such, are subject to natural selection and, therefore, can be improved in the course of evolution, adapting to changing conditions.

Let us first consider the ways in which weight loss can be achieved. The average specific gravity of protoplasm is taken to be 1.025, that is, only slightly greater than the specific gravity of water; At the same time, on the one hand, in organisms we find heavier substances (bones, shells, crustacean shells and other skeletal formations), and on the other, lighter ones (fats, oils, gases, etc.). From here it is clear that adaptation to buoyancy should be aimed at: 1) reducing, or reducing, the mineral skeletons of shells and other heavy parts; 2) on the development of such light supporting formations as fatty and oily inclusions, gas bubbles; 3) finally, the specific residual weight of the organism will decrease when the tissues are saturated with water, the volume of the organism will be increased with a relatively small amount of dry matter.

All these ways of reducing residual weight in various combinations are observed in nature among planktonic organisms.

Reduction of severe formations. Due to the high specific gravity of water, organisms in the aquatic environment lose almost all their weight. Water, with its pressure, seems to support the body. Therefore, soft, skeletal, gelatinous forms can exist in water. Such, for example, are the ctenophores, as tender as semi-liquid jelly, of which the belt of Venus (Cestus veneris) is especially remarkable, reaching, despite the fragility of its tissues, over a meter in length. These are jellyfish, especially the Arctic blue jellyfish, which reaches two meters in diameter. When taken out of the water, such forms are flattened and die.

The reduction of skeletal formations in planktonic rhizomes is expressed in the fact that they have thin shells and have larger pores than the shells of rhizomes living on the bottom.

In keelfoot mollusks leading a planktonic lifestyle, we encounter all stages of shell reduction: 1) the body of the mollusk can be completely hidden in the shell; 2) the shell covers only the gonad; 3) the shell completely disappears.

In pteropods the shell is thin and transparent or, for the most part, completely absent.

The accumulation of substances with a lower specific gravity (fats, oils) is observed in diatoms, nocturnals, many radiolarians, and copepods. All fatty inclusions represent reserves of nutrients and at the same time reduce residual weight. The same functions are performed by fat droplets in pelagic eggs and fish eggs. In the shells of planktonic crustaceans, compared to the forms inhabiting the bottom, the amount of calcium in the ash decreases and at the same time the amount of fat increases: in the grass crab (Carcinus) crawling along the bottom, calcium in the ash is 41%, fat 2%. One of the large planktonic copepods, Anomalocera, has 6% calcium and 5% fat.

Gas accumulation is even more effective in reducing residual weight. Thus, blue-green algae have special gas vacuoles. Multicellular Sargassum algae, floating in the Atlantic Ocean, have gas bubbles that support them in the water. But the gas-filled hydrostatic devices of siphonophores, swallowtails, aquatic flowering plants of bladderwort, etc. are especially famous.

The impregnation of tissues with water and the formation of jellies are found in various unicellular and colonial algae, jellyfish, ctenophores, pteropods, and keelfoot mollusks. It has been established that in the Baltic Sea, where the water is fresher and, therefore, buoyancy conditions are worse, the body of the Aurelia jellyfish contains 97.9% water, and in the Adriatic, where salinity is over 35% and buoyancy conditions are better, it contains only 95. 3%. It is possible that this is due specifically to the buoyancy conditions in these seas.

Resistance shapes and dimensions of plankters. It is known that the resistance exerted by the medium to a moving body is associated with internal friction of the displaced water particles and in proportion to the displaced surface. Thus, the rate of immersion will be inversely proportional to the specific surface area, that is, the ratio of the surface of the body to its volume. As the size of a body decreases, its surface decreases in proportion to the square, and its volume decreases in proportion to the cube of linear dimensions. For a ball, the specific surface area is equal to 4r 2 π: 4 / 3 /r 3 π = 3/r, that is, a ball with a radius of 1 will have a specific surface area of ​​3; in 2 - 1 1/2; 3 - 1; 4 - 3/4; 5 - 3/5; 6 - 1/2; 7 - 3/7; 8 - 3/8, etc.

Thus, the small size of the organism gives it an advantage over large ones in terms of buoyancy. This makes it clear why small forms predominate in plankton. For algae, for example, their small size provides an advantage for greater adsorption of nutritional salts, which are found in very small quantities in the seas.

Plankters are classified by size.

Ultraplankton are organisms up to several microns in size.

Nannoplankton. Dimensions - less than 50 microns. Organisms of this size pass through the thickest mill gas with a mesh size of 65-50 microns. Therefore, to count nannoplankton, centrifugation or sedimentation in high vessels is used (centrifuge, or sedimentary plankton, contains bacteria of unshelled and silicon flagellates, coccolithophores).

Microplankton are already trapped in the thick numbers of mill gas. These include armored peridines, diatoms, protozoa, small crustaceans, etc. The sizes of microplanktonic organisms range from 50 to 1000 microns.

Mesoplankton is the bulk of animal plankton organisms: copepods, cladocerans, etc. Dimensions - from 1 to 15 mm.

Macroplankton is measured in centimeters. These include jellyfish, siphonophores, salps, pirozomes, kelnopods, pterygopods, etc.

Megaloplankton includes very few large forms measuring about one meter, among them the already mentioned Venus belt, the Arctic blue jellyfish and other exceptional giants. Note that both macroplankton and megaloplankton consist exclusively of forms with a highly developed gelatinous body soaked in water, which obviously compensates for their large size, which is disadvantageous in terms of buoyancy.

However, to overcome the resistance of the environment, not only the relative size of the surface of the immersed body, but also its shape is important. As is known, of all geometric bodies of the same volume, the sphere has the smallest surface area. Despite this, spherical forms are quite widespread among planktonic organisms (some green algae, a number of flagellates, including the well-known Noctiluca noctule, Thalassicola radiolaria, some ctenophores, etc.).

One must think that in this case such devices as reducing the specific gravity, soaking the body with water and the like so compensate for the disadvantage of the spherical shape that they completely eliminate the effect of gravity. For such an organism, the water column is homogeneous. No other environment and no other habitat, except the water column, presents such uniformity in all directions, and therefore spherical organisms are not found anywhere except the water column. It is possible that in conditions that exclude gravity, the spherical shape, with its minimal surface area, can provide some advantages.

To increase buoyancy, of particular importance is the increase in the so-called frontal surface, that is, the surface that, when moving, displaces particles of the medium (in this case, when diving).

Given the negligible weight of plankters, simply elongating the body in a direction perpendicular to the direction of gravity already gives the organism an advantage in terms of buoyancy. This form is especially beneficial for those organisms that have some mobility. Therefore, elongated, rod-shaped, thread-like or ribbon-like forms of both solitary and colonial organisms are very often found in plankton. Examples include a number of green algae, numerous diatoms, some radiolarians, sea arrows (Sagitta), the larva of the decapod crustacean Porcelain and other mobile plankters. It is clear that the friction surface is further increased by numerous spines and projections directed in different directions, which we also find in numerous representatives of a wide variety of systematic groups, for example, in the diatoms chaetoseros, peridinium-ceratium, globigerina rhizomes, numerous radiolarians, sea urchin larvae and snake stars ( Pluteus) and, especially, in various crustaceans decorated with feathery bristles.

Of the same importance is the flattening of the body in a plane perpendicular to the direction of gravity, which in the course of evolution led to the development of cake-shaped or disk-shaped forms. The most famous example of such forms is the Aurelia jellyfish, widespread in our seas, but this form is also found among plankters of other systematic groups. These include Costinodiscus, Leptodiscus, a number of radiolarians, and especially Phyllosoma foliaceae, the larva of the spiny lobster, a commercial crayfish in Western Europe.

Finally, further improvement in this direction leads to invagination of the lower surface and the development of a medusoid, parachute-like shape, so perfect that it is used in aeronautics to slow down the fall of bodies in the air. As examples of medusoid forms, in addition to various jellyfish, individual representatives of other groups can be named, such as green flagellates - medusochloris, cephalopods - cirrothauma and holothurians - pelagoturia.

Very often, the body simultaneously possesses several devices that reduce the rate of immersion. Thus, in jellyfish, in addition to the parachute-shaped form, there is a powerful development of the gelatinous intermediate plate; in some radiolarians, along with the spinous form, we find fatty inclusions; in planktonic globigerine rhizomes we have an increase in pores and numerous spines that reduces the residual weight.

All these diverse adaptations to the planktonic lifestyle were developed in the course of evolution in a wide variety of organisms, completely regardless of their evolutionary relationship. Protoplasm itself, even if you do not take into account mineral skeletal formations, is heavier than water. This circumstance gives us some right to believe that the primary way of life was benthic, and not planktic. In other words, life was initially concentrated on the bottom, and only later did organisms spread into the water column.

Planktonic crustaceans and rotifers that live in fresh waters are eaten by fish, as well as a number of relatively small invertebrate predators (cladocera Leptodora kindti, many copepods, nonbiting mosquito larvae Chaoborus and etc.). Fish and invertebrate predators that attack “peaceful” zooplankton have different hunting strategies and different most preferred prey.

In the process of hunting, fish usually rely on vision, 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 attack mainly small or medium-sized planktonic animals, since they simply cannot cope with large ones. During the hunting process, invertebrate predators orient themselves, as a rule, with the help of mechanoreceptors, and therefore many of them, unlike fish, can attack their victims in complete darkness. Obviously, invertebrate predators themselves, being the largest representatives of plankton, can easily become victims of fish. Apparently, this is why it is “not beneficial” for them to be particularly large, although this would allow them to expand the size range of their potential victims.

To protect themselves from invertebrate predators, it is more advantageous for planktonic animals to be larger in size, 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 to increase the actual size, but at the expense of some transparent outgrowths that do not make their owners particularly noticeable. Indeed, in the evolution of different groups of planktonic animals, the emergence of similar “mechanical” means of defense against invertebrate predators is observed. Yes, Cladocera Holopedium gibberum forms a spherical gelatinous shell 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. First, a similar phenomenon was discovered (Beauchamp, 1952; Gilbert, 1967) in rotifers: female prey - rotifers brachionus (Brachionus calyciflorus), grown in water that previously contained predatory rotifers of the genus Asplanchna (Asplanchna spp.), produced juveniles with especially long lateral spines of the shell (see Fig. 51). These spines greatly prevented the asplanchnids from swallowing the brachionus, since they literally stood across their throats.

Later, various body extensions induced by predators were also discovered in crustaceans. Thus, in the presence of predatory larvae Chaoborus in young individuals Daphnia pulex a “tooth-like” growth grew on the dorsal side, significantly reducing the likelihood of them being successfully eaten by these predators (Krueger, Dodson, 1981; Havel, Dodson, 1984), and in some Australian Daphnia carinata in the presence of predatory bugs Anisops calcareus(family Notonectidae) a transparent ridge was formed on the dorsal side, apparently also greatly hindering the predator in grasping and eating prey (see Fig. 51).

Such outgrowths cannot protect against most fish, and therefore it is extremely important for planktonic crustaceans, if there are fish in the reservoir, to remain invisible and (or) avoid direct encounters with them, especially in good light conditions. Since the concentration of food of planktonic crustaceans is maximum just at the surface, it is not surprising how often we find in them the existence of vertical daily migrations, expressed as rising at night into food-rich surface layers and lowering during the day into deeper layers, where there is low light, as well as the possibility reducing local density by dispersing into a larger volume prevents 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 consequently, 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. 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 stays in the upper, heated layers of the water column (epilimnion), and the second species migrates in summer and autumn, rising to the epilimnion at night and descending to great depths (to the hypolimnion) during the day. The food concentration of both species of Daphnia (mainly small planktonic algae) is quite high in the epilimnion and very low in the hypolimnion. The temperature in mid-summer in the epilimnion reaches 20°, and in the hypolimnion it barely reaches 5°. Researchers from Germany H. Stich and W. Lampert (Stich, Lampert, 1981, 1984), who studied in detail the daphnia of Lake Constance, suggested that migration D.hyalina allow it to significantly 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 and V. Lampert tested their hypothesis about the different survival strategies of these daphnia in laboratory conditions, when, in the absence of a predator, for both species they imitated the conditions of constant residence in the epilimnion (constantly maintained high temperature and a large amount of food) and the conditions of vertical migrations (changing in temperature and changing amounts of food as the day progresses). It turned out that in such artificially created conditions of the epilimnion, both species felt great and had a high birth rate. In the case of simulating conditions of vertical migrations, the survival rate and reproduction intensity of both species were significantly lower, but it is interesting that D.hyalina was characterized by much better rates of survival and reproduction than D. galeata. When simulating epilimnion conditions, some advantage (albeit insignificant) was found in D. galeata. Thus, differences in the spatiotemporal distribution of these Daphnia species corresponded to differences in their physiological characteristics.

The assumption that it is the pressure of planktivorous fish that is the factor responsible for the occurrence of vertical migrations in planktonic animals is also supported by the data obtained by the Polish hydrobiologist M. Gliwicz (Gliwicz, 1986). Having examined a number of small lakes in the Tatras, Glivich discovered that a representative of the copepod crustaceans Cyclops often found in them Cyclops abyssorum makes daily vertical migrations in those lakes where there are fish, but does not make it where there are no fish. It is interesting that the degree of severity of vertical migrations of Cyclops in a particular body of water depended on how long the permanent fish population had existed in it. In particular, weak migrations were noted in one lake, where fish were introduced only 5 years before the survey, and significantly stronger ones where fish appeared 25 years ago. But the migrations of the Cyclops were most clearly expressed in that lake, where 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. Glivich that there was no migration of Cyclops in one lake in 1962, just a few years after the fish were released there, and the presence of clear migrations there in 1985 after 25 years coexistence with fish.

Plankton - small primitive organisms drifting in the water column. The word plankton comes from the Greek planktos, meaning wandering. Plankton is divided into several groups:

• Phytoplankton. The word comes from the Greek phyton, which translates as “plant.” It consists of small algae that float near the surface of the water, where there is a lot of sunlight necessary for photosynthesis.
• Zooplankton. From zoo – animal. Consists of protozoa and multicellular animals such as crustaceans. Zooplankton feed on phytoplankton.
• Bacterioplankton. Consists of bacteria and archaea that participate in the process of remineralization, i.e. transformation of organic forms into inorganic ones.

Thus, this classification divides all plankton into three large groups: producers (phytoplankton), consumers (zooplankton) and utilizers (bacterioplankton).

Plankton are distributed throughout the world's oceans. The main condition for its formation is a sufficient amount sunlight and the presence of organic nutrients in water - nitrates and phosphates. The importance of plankton in the world's oceans can hardly be overestimated. It acts as a feeder for most fish when they are young. Currents collect plankton into so-called feeding grounds, where cetaceans and whale sharks graze. Some whales even make seasonal migrations, following the plankton fields.

Small plants on the surface of the water participate in photosynthesis and are an important element of the entire oxygen cycle system on the planet. Plankton is also the largest source of carbon on Earth. The fact is that using it as food, animals convert plankton into biological mass, which subsequently settles on the seabed, because heavier than water. This process is known in scientific circles as the "biological pump".

It is extremely important for planktonic creatures to develop structure, which would facilitate free floating in water and prevent sinking to the bottom of the reservoir. It is a matter of life or death for them; Losing the ability to maintain itself suspended in water, the planktonic organism inevitably dies.

For a planktonic organism, it is very important to have a weight that is possibly close to the weight of water, i.e., the lowest specific gravity. This is achieved primarily by the unusually high water content in the body tissues of these organisms. An excellent example is the planktonic coelenterata (Coeelenterata), which, however, with very rare exceptions (several species of freshwater jellyfish), is characteristic of marine plankton.

Various light waste products retained in the body of the body also reduce its specific gravity. In the protoplasm of planktonic rhizomes, special bubbles accumulate - vacuoles, which contain carbon dioxide released during the animal's respiration. Of course, the presence of such gas-filled vacuoles reduces the specific gravity of the animal and promotes its floating.

The cells of planktonic blue-green algae contain very small reddish inclusions called pseudovacuoles. Having lost them, these algae sink to the bottom. Thus, pseudovacuoles are also a hydrostatic device, i.e., a device used to maintain the body in water.

Sediments play a very important role in reducing the specific gravity. fat and oil. These substances are known to be lighter than water and float to the surface. It is clear that, accumulating in the body of a planktonic organism, inclusions of fat and oil reduce its specific gravity. Indeed, accumulations of fatty substances are very characteristic of planktonic algae and animals. To reduce the specific gravity, water-rich substances secreted by some planktonic organisms are also used. gelatinous membranes. An excellent example is the glassy-transparent chamber of the cladoceran crustacean Holopedium and the gelatinous cover of the bell ciliate (Tintinnidium).

Many planktonic algae, especially blue-green algae, are immersed in a lump of mucus, the presence of which relieves the body.

There are also a variety of adaptations produced by planktonic organisms for increasing resistance and increasing friction. Many of them increase the surface of the body as much as possible, which, due to friction with water particles, reduces the rate of immersion. In this case, it is especially significant that the increase in surface area often occurs due to a reduction in the volume of the organism - the latter, as it were, flattens out. Thus, planktonic diatoms for this purpose take on a disc-shaped and lamellar shape; in rotifers that are part of plankton, the shell is more or less flattened and expanded. To increase the surface, planktonic diatoms, in addition, unite into colonies consisting of many cells adjacent to each other.

Education appendages in the form of needles and thorns- a phenomenon very characteristic of a number of planktonic algae and animals. Some of them have needles and spines located in different planes and directed in all directions (freshwater, planktonic, colonial green algae Richteriella) found mainly in pond plankton.

Organs of movement of planktonic organisms are the posterior swimming antennae in cladocerans and the swimming legs in copepods, as well as the anterior pair of long antennae in the latter; The rotary apparatus of rotifers, small cilia of ciliates and some other organs are also used for movement.

In the independent movements of a planktonic organism, both horizontal and vertical, various guides. It is not enough to be able to swim, you must also be able to direct your path and, in addition, maintain body stability when swimming. For this purpose, planktonic animals develop a number of adaptations. As an example, let’s take the cladoceran crustaceans Bosmin. The front antennae of these crustaceans are very long, motionlessly fused to the end of the head like a proboscis.