Protecting planktonic animals from being eaten by predators. The behavior of planktonic organisms What does it consist of

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

Despite the fact that this group of 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 under a microscope, scientists can infer the presence of each of these substances from the appearance of planktonic organisms. 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. In fact 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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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 a taxonomic term, but characterizes the way of life of some animals that move due to the flow of water. Zooplankton are either too small to resist the current, or large (as is the case for many jellyfish) but lack the organs 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 over 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 for zooplankton include:

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

What do zooplankton eat?

Zooplankton and food chains

Zooplankton are usually found at the second trophic level, which starts with phytoplankton. In turn, the phytoplankton are eaten by zooplankton, which are 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 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 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. 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 hovering. 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 molluscs, 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 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 silts 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 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, the "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 tint; 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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Composition of plankton. 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 have a variety of shapes and are very durable. Falling to the bottom after death, algae cover the vast areas of the ocean floor in high latitudes with the so-called diatom silt. In the fossil state, such accumulations of diatom shells give rise to a silica-rich rock - tripoli, or diatomaceous earth.

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

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

Of the unicellular animals, the most characteristic of plankton are rhizopods-globigerins with multi-chamber calcareous shells covered with long thin needles. Falling to the bottom after death, they cover vast areas of the ocean floor with lime-rich globigerin silt.

Accumulations of rayfish or radiolarians with very beautiful, the thinnest, like lace, silicon skeletons also cover large areas of the ocean floor.

Widely distributed 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 are shaped like bells, vases, pointed cylinders, tubes, etc.

Of the colorless flagellates, the most famous are undoubtedly the spherical night-lighters, or noctiluces, which have the ability to glow.

Very interesting are hydroid polyps - siphonophores, colonial coelenterates with complexly differentiated colonies, with a deep separation of functions: nourishing, protective, swimming, trapping and sexual. Very numerous and varied are jellyfish in the form of umbrellas or discs, ctenophores.

Worms are represented mainly by various larvae - trochophores and nektochaetes. Some species of worms during the breeding season lead a planktonic lifestyle, 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 all their lives (for example, copepods and cladocerans), or only during the larval period (shrimps, crabs). Copepods form the main background of the animal plankton of the seas.

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

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

Finally, the sea water column is inhabited by countless bacteria. The variety of the external form 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. For their distinction, mainly physiological characteristics are used and, to a lesser extent, external form. They play an important role in the processes of transformation of substances in the sea - from the decomposition of complex residues of plant and animal organisms to their transformation into compounds of carbon, nitrogen, sulfur and phosphorus assimilated by plants.

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

Movable plants and attached animals. The presence of plankton in the sea led to the development of an exceptionally peculiar category of animals that are not found on land at all, namely, immobile, attached, or so-called sessile. Plants on land are attached to the soil and are immobile. Herbivores must have the ability to approach food and to move about. Predators must catch their prey. In a word, all land animals must actively move.

In water, due to the presence of plankton and suspended remains of dead organisms - detritus, the animal can remain motionless, water currents will bring food to it, therefore the attached lifestyle is widespread among marine animals. Such are hydroid polyps and corals, many worms, crustaceans, or sea acorns, bryozoans, sea lilies, etc. Of the molluscs, let us give as an example the well-known oysters, densely cemented to rocks or, in general, to solid objects. All these animals either have peculiar, not found in terrestrial animals, apparatuses for straining food out of the water, or they strive to cover the space as widely as possible with numerous near-mouth tentacles, or they develop a tree-branched form.

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

Now we know that they cannot, like plants, assimilate carbon dioxide and other inorganic substances, but, like all animals, feed on ready-made organic food created by other organisms, and therefore we consider them animals, although they cannot move. Thus, due to the large specific gravity of water and the salts dissolved in it, freely 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 free-moving animals - vagil benthos: worms, crustaceans, molluscs - bivalves, gastropods and cephalopods, echinoderms, etc. Some of them feed on plankton proper, others are planktivorous animals. Thus, benthos as a whole - both mobile and attached - is directly or indirectly connected with plankton in its diet, since attached algae play a very insignificant role in the economy of the sea. Therefore, it can be expected that where there is a lot of plankton, benthos will also be abundant. However, this is not always the case. The 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 abundance of plankton, there may be little or no benthos. At considerable depths in the 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 such a 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 benthic 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, multiply at the expense of carbon dioxide, nitrates, phosphates and other inorganic compounds dissolved in water, from which they build complex organic compounds of their body in sunlight. They are organic producers. These microscopic plants are fed by crustaceans, worms and other herbivorous animals, which can only eat ready-made, created plants, organic substances and cannot use inorganic compounds from the environment. These are first-order consumers. At the expense of herbivorous predators feed - consumers of the second order. They, in turn, are eaten by larger predators - consumers of the third order, etc. Such are the relationships within this community.

In the end, 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, turn back into biogenic elements dissolved in water - the starting material for the new construction of the bodies of plant organisms with the help of solar energy, and the cycle of transformations of matter closes.

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 the elements is accomplished by solar energy captured and accumulated by plant organisms in the form of the chemical energy of complex organic substances. Animals consume only organic substances created by plants, expending the energy they have accumulated. These are, in general terms, the relationship 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, such a ratio between the plant and animal parts of plankton cannot remain permanently unchanged. On the rich food of phytoplankton, there is an increased reproduction of zooplankters and a moment may come when, for example, as a result of the depletion of the supply of biogenic compounds in water, phytoplankton production will begin to decrease. In the end, it may turn out that there will be a lot of zooplankton, and few phytoplankton, that is, the ratio will become inverse. 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, which is based on the struggle for existence.

The question of the numerical relationship between bacteria, phytoplankton, and zooplankton has not yet been studied enough. However, based on the fact that bacteria mostly live on 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 reproduction of bacteria, eating them by zooplankton is unlikely to significantly change these relationships.

In addition to purely biological internal causes, external conditions can also affect these relationships, as will be discussed below.

Adaptations to the planktonic way of life. 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 familiarize yourself with the conditions of buoyancy. The relationship between these conditions is expressed as follows:

Let's take a 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. With an increase in water temperature from 0 to + 30-40 ° C, for each degree, the viscosity decreases by about 2-3%. As salinity increases, viscosity increases, but only very slightly. The viscosity of air is 37 times less than the viscosity of water. Therefore, by virtue of this alone, a body in 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 a planktonic lifestyle should be more pronounced than in cold ones.

Form resistance - the ability of bodies to resist any influences, changes.

Residual weight is equal to the weight of the organism minus the weight of the water displaced by it. Thus, the residual weight is the smaller, the greater the weight of the displaced water, and this value is directly dependent on the specific gravity of the 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 that determine the rate of sinking (buoyancy), do not depend on the organism, then the weight of the organism itself and the resistance of the form are its features and, as such, are subject to natural selection and, therefore, in the course of evolution can be improved, adapting to changing conditions.

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

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

Reduction of heavy formations. Due to the large specific gravity of water, organisms in the aquatic environment lose almost all of their weight. Water with its pressure, as it were, supports the body. Therefore, soft, skeletonless, gelatinous forms can exist in water. Such, for example, are tender, like semi-liquid jelly, ctenophores, of which the venus belt (Cestus veneris) is especially remarkable, reaching, for all the fragility of its tissues, over a meter in length. Such are the jellyfish, especially the blue arctic, which reaches two meters in diameter. Taken out of the water, such forms are flattened and die.

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

In keeled mollusks that lead a planktonic lifestyle, we meet all stages of shell reduction: 1) the body of the mollusk can completely hide 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.

Accumulation of substances with a lower specific gravity (fats, oils) is observed in diatoms, noctules, many radiolarians, and copepods. All fatty inclusions are 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 with 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 is 2%. In one of the large planktonic copepods, anomalocera (Anomalocera) has 6% calcium and 5% fat.

Even more effective for reducing residual weight is the accumulation of gas. So, blue-green algae have special gas vacuoles. Multicellular Sargassum algae floating in the Atlantic Ocean have gas bubbles that keep them in the water. But the gas-filled hydrostatic apparatuses of the siphonophore, sailboat, aquatic flowering plant pemphigus, etc., are especially famous.

Impregnation of tissues with water and the formation of jellies are found in various unicellular and colonial algae, jellyfish, ctenophores, winged, keeled molluscs. It has been established that in the Baltic Sea, where the water is fresher and, consequently, the buoyancy conditions are worse, the body of the Aurelia jellyfish (Aurelia) contains 97.9% of water, and in the Adriatic, where the salinity is over 35% and the buoyancy conditions are better, only 95, 3%. It is possible that this is due precisely to the buoyancy conditions in these seas.

Shape resistance and plankter dimensions. It is known that the resistance provided by a medium to a moving body is associated with internal friction of the displaced water particles and is proportional to the displaced surface. Thus, the rate of immersion will be inversely proportional to the specific surface, that is, the ratio of the surface of the body to its volume. With a decrease in the size of a body, its surface decreases in proportion to the square, and volume - to the cube of linear dimensions. For a ball, the specific surface is 4r 2 π: 4 / 3 /r 3 π \u003d 3/r, that is, a ball with a radius of 1 will have a specific surface 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 an organism gives it an advantage in terms of buoyancy over a large one. From this it is clear why small forms predominate in plankton. For algae, for example, small size has the advantage of greater absorption of nutrient salts, which are found in very small amounts in the seas.

Plankters are distinguished by size.

Ultraplankton - 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 account for nannoplankton, centrifugation or sedimentation in tall vessels is used (centrifuge, or sedimentary plankton, contains bacteria of unarmored and silicon flagellates, coccolithophorides).

Microplankton are already being captured by the dense numbers of mill gas. These include armored peridines, diatoms, protozoa, small crustaceans, etc. The sizes of microplankton organisms are from 50 to 1000 microns.

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

Macroplankton is measured in centimeters. These include jellyfish, siphonophore, salp, pyrosis, keel-footed, pteropod molluscs, etc.

Megaloplankton includes very few large forms of about one meter in size, among them the already mentioned venus belt, the blue jellyfish of the Arctic and other exceptional giants. It should be noted that both macroplankton and megaloplankton consist exclusively of forms with a highly developed gelatinous body soaked in water, which obviously compensates for the large sizes that are unfavorable in terms of buoyancy.

However, to overcome the resistance of the medium, not only the relative size of the surface of the submerged body is important, but also its shape. As you know, of all geometric bodies of the same volume, the ball has the smallest surface. Despite this, spherical forms are quite widespread among planktonic organisms (some green algae, a number of flagellates, including the well-known noctiluca, radiolarian thalascol, some ctenophores, etc.).

One must think that in this case such devices as reducing the specific gravity, impregnating the body with water, and the like compensate so much for the disadvantage of a spherical shape that they completely exclude the effect of gravity. For such an organism, the water column is homogeneous. No other environment and no other habitat, except for the water column, presents such uniformity in all directions, therefore, spherical organisms are not found anywhere except the water column. It is possible that under conditions that preclude gravity, the spherical shape with its minimal surface may give 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 immersed).

With the negligible weight of plankters, a simple elongation of 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, filamentous, or ribbon-like forms of both single and colonial organisms are very often found in plankton. Examples are a number of green algae, numerous diatoms, some radiolarians, sea shooters (Sagitta), the larva of the ten-legged crustacean porcellana, and other mobile plankters. Clearly, the friction surface is further increased by numerous spines, outgrowths directed in different directions, which we also find in numerous representatives of a wide variety of systematic groups, for example, in diatoms chaetoseros, peridineum-ceratium, rhizopods of globigerin, numerous radiolarians, larvae of sea urchins and serpent 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 flattened or disk-shaped forms. The most famous example of such forms is the aurelia jellyfish, which is widespread in our seas, but this form is also found among plankters of other systematic groups. Such are costinodiscuses, leptodiscuses, a number of radiolarians, and especially leaf-shaped phyllosoma, the larva of the spiny lobster - a commercial crayfish in Western Europe.

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

Very often, the body has several adaptations at the same time that reduce the rate of immersion. So, 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 spiny form, we find fatty inclusions; in planktonic rhizopods, globigerin, we have an increase in pores that reduces the residual weight and numerous spines.

All these so diverse adaptations to the planktonic way of life have developed in the course of evolution in a wide variety of organisms, completely independently of their evolutionary relationship. The protoplasm itself, even if we do not take into account the mineral skeletal formations, is heavier than water. This circumstance gives us some right to believe that the primary mode of life was benthic, and not planktic. In other words, life was originally concentrated at the bottom, and only subsequently did organisms settle into the water column.

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