Benthos - what is it? Marine benthos (Encyclopedia) Benthos organisms.

Water thickness, or pelagic (pelages - sea) is inhabited by pelagic organisms that have the ability to swim or stay in certain layers.

In this regard, these organisms are divided into two groups: nekton And plankton. The third environmental group - benthos- formed by the inhabitants of the bottom.

Nekton (nektos - floating) is a collection of pelagic actively moving animals that do not have a direct connection with the bottom. Nekton is represented mainly by large animals that are able to overcome long distances and strong water currents. They have a streamlined body shape and well-developed organs of movement. Typical nektonic organisms include fish, squid, whales, and pinnipeds. In addition to fish, nekton in fresh waters includes amphibians and actively moving aquatic insects. Many marine fish can move through the water at enormous speeds: up to 45-50 km/h for squid, 100-150 km/h for sailfish and 130 km/h for swordfish.

Plankton (planktos - wandering, soaring) is a set of pelagic organisms that do not have the ability for rapid active movements. As a rule, these are small animals - zooplankton and plants - phytoplankton , who cannot resist the currents. Plankton also includes the larvae of many animals “floating” in the water column. Planktonic organisms are located both on the surface of the water, at depth, and in the bottom layer.

Organisms that inhabit the surface film of water at the border with the air constitute a special group - Neuston .

Benthos (benthos - depth) is a set of organisms that live at the bottom (on the ground and in the ground) of reservoirs. It is divided into zoobenthos And phytobenthos . Mostly represented by attached or slowly moving or burrowing animals. In shallow water, it consists of organisms that synthesize organic matter (producers), consume it (consumers) and destroy it (decomposers). At depths where there is no light, phytobenthos (producers) is absent. The marine zoobenthos is dominated by foraminiphores, sponges, coelenterates, worms, brachiopods, mollusks, ascidians, fish, etc. Benthic forms are more numerous in shallow waters. Their total biomass here can reach tens of kilograms per 1 m2.

The phytobenthos of the seas mainly includes algae (diatoms, green, brown, red) and bacteria. Along the coasts there are flowering plants - zoster, ruppia, phyllospodix. Rocky and stony areas of the bottom are richest in phytobenthos.

In lakes, as in seas, there are plankton, nekton And benthos.

However, in lakes and other fresh water bodies there is less zoobenthos than in seas and oceans, and their species composition is uniform. These are mainly protozoa, sponges, ciliated and oligochaete worms, leeches, mollusks, insect larvae, etc.

Based on their lifestyle, aquatic plants are divided into two main ecological groups:

- hydrophytes - plants that are immersed in water only with the lower part and usually take root in the ground.

- hydatophytes - plants that are completely submerged in water, and sometimes float on the surface or have floating leaves.

In the life of aquatic organisms, an important role is played by the vertical movement of water, density, temperature, light, salt, gas (oxygen and carbon dioxide content) regimes, and the concentration of hydrogen ions (pH).

BENTHOS (from the Greek benthos - depth), a set of organisms living on the ground and in the soil of marine and continental reservoirs. Benthos is divided into plant (phytobenthos) and animal (zoobenthos). In zoobenthos, there are animals living in the soil - infauna (mainly many polychaete worms and bivalves, echiurids, sipunculids, some echinoderms, etc.), moving along the surface of the soil - onfauna (polychaete worms and mollusks, most echinoderms, various crustaceans) , attached to the substrate - epifauna (sponges, hydroids, sea anemones and various corals, bryozoans, sea acorns, some bivalves, etc.), as well as floating near the bottom and only periodically sinking to the bottom - nektobenthos (shrimps, mysids, some holothurians, bottom fish, etc.). By size, benthos organisms are divided into macrobenthos - from 5-10 mm and larger (the vast majority of bottom animals), meiobenthos - from 0.5 to 5-10 mm (population of the very top layer of soil) and microbenthos - less than 0.5 mm ( bacteria and other single-celled organisms). The basis of shallow-water phytobenthos in the seas are macrophytes (algae and sea grasses); Aggregations of benthic diatoms may also play a significant role. In the depths, in addition to animals, only bacteria and lower fungi live. The biomass of benthos in the seas decreases with depth: in the littoral and upper sublittoral zones - up to 5-10 kg/m2 and more, deeper in the sublittoral zone - hundreds and tens of g/m2, in the bathyal zone - grams, in the abyssal zone - usually not more than 1 g/m2, and in the central regions of the oceans, which are poor in life - 0.01 g/m2 or less. The share of shallow waters located near the continents (up to 200 m), occupying less than 8% of the ocean floor area, accounts for about 60% of the biomass of all oceanic benthos, and the share of the abyssal waters (deeper than 3000 m), occupying 3/4 of the bottom area, accounts for only less than 10 %. The total biomass of benthos in the ocean is estimated at 10-12 billion tons. In some areas of the eastern Pacific Ocean, at a depth of 2.5-3 km, so-called oases of life were discovered (in 1979) near outlets of hot underground waters (hydrotherms). In these areas, benthos biomass reaches several kg/m2; their fauna includes many previously unknown species of animals: giant bivalves and representatives of pogonophora. In fresh water bodies, benthos is qualitatively and quantitatively poorer than in sea water bodies. Animals it includes protozoa, sponges, roundworms, oligochaete worms, leeches, mollusks, crustaceans and the larvae of many aquatic insects. Phytobenthos is represented mainly by algae (especially blue-green and characeae) and various flowering plants (pondweed, water lilies, cattails, reeds and many others). Benthos serves as food for many fish, and in the seas also for some pinnipeds. Many species of shallow-water marine benthos are subject to fishing and aquaculture.

Benthos- a set of organisms living at the bottom of water bodies. In terms of the number of species, diversity of structure and lifestyle, benthos is undoubtedly the leader among aquatic organisms. Thus, among marine animals, about 200 thousand species belong to the benthos, and only about 2 thousand belong to the benthos (Zenkevich, 1951). According to taxonomic origin, benthos is divided into zoobenthos(animal organisms) and phytobenthos(higher plants and algae); there is also bacterial benthos.

The most diverse group of benthic organisms are invertebrates. They are divided into size groups: macro-, meio- and microbenthos, and for each of these groups their own research methods are effective.

Macrobenthos- multicellular benthic invertebrates over 2 mm long. These are mollusks, higher crustaceans, insects, annelids, echinoderms and many other groups of relatively large animals. Organisms of macrobenthos are always visible to the naked eye; they can be selected from a sample without a microscope, using tweezers. To catch them, benthic nets and dredges with a mesh diameter of 0.5 or 1 mm, or large-area bottom grabs (in marine research - up to 1 sq.m.) are used. Macrobenthos is the most studied group of benthic organisms, but it also accounts for most of the species diversity of marine fauna.

Meiobenthos- multicellular submicroscopic benthic invertebrates (up to 2 mm long). These include most lower crustaceans (especially Harpacticoida, Cyclopoida and Ostracoda), nematodes, rotifers and early-stage larvae of larger animals. Meiobenthos organisms reach their greatest density in the thickness of soft soils - silt and sand; they stay in the crevices between individual grains of sand or make tunnels for themselves in the silt. Meiobenthos is almost invisible to the naked eye, and to collect it, the soil is washed through a very fine net (mesh 100-200 microns), after which the search for organisms in the sample is carried out under a binocular microscope at 30-50 times magnification. Identification of most meiobenthos groups is difficult and requires detailed microscopy. Meiobenthos has been studied relatively poorly; Most of the species in this group have most likely not yet been described.

Microbenthos- single-celled bottom animals (usually classified as Protozoa), almost always microscopic in size. The most diverse among them are ciliates. Methods for extracting microbenthos from soil are quite complex, and its diversity reaches dozens of species in just one square centimeter of soil. On the contrary, the microbenthos faunas of different regions of the world differ rather little, and its global diversity is relatively small.

Benthic organisms live on all possible bottom substrates, and for each habitat a whole set of structural and lifestyle variants (the so-called life forms) is formed. Thus, on rocky soils the following options are most common: photosynthetic fouling of the surface of stones (various algae), attached and sedentary filter feeders (sponges, bryozoans, hydroids, some bivalves, larvae of midges and caddis flies), sedentary fouling scrapers (many gastropods, sea urchins , caddisflies), crevice detritivores-gatherers (various worms and insect larvae), mobile predators (found among most groups). In general, on dense substrates, bottom epifauna predominates - organisms that inhabit the surface of the soil, but do not go deep into it. On the contrary, on loose soils (silty and sandy) infauna is most abundant - burrowing animals that use the soil thickness as a refuge from predators and, sometimes, a source of food.

Phytobenthos is represented in water bodies by two size groups. This microphytobenthos(single-celled bottom algae) and macrophytes- large aquatic flowering plants and algae that take root on the bottom. The latter, due to their significant size (macrophytes often pierce the entire thickness of a reservoir and reach the surface, and can also protrude above the water), are often considered as a special ecological group. Macrophytes create habitat for their own (phytal) community of animals and algae, and also serve as the main substrate for periphyton.

Periphyton- a set of organisms that grow on dense substrates rising above the surface of the bottom of reservoirs (including macrophytes). The most typical periphyton are attached organisms (algae, sponges, bryozoans, hydroid polyps). Some authors understand periphyton more broadly, including also mobile organisms that live on dense substrates (for example, on stones and snags) and in thickets of macrophytes. Thus, the boundary between benthos and periphyton can be interpreted in different ways.

Benthos is the general name for organisms that live on the bottom. This includes thousands of plants and animals in a wide variety of sizes and shapes. They all belong to different groups and have different lifestyles and diets. Apart from their habitat, there is little that unites them. Classifications of benthos, photos of its representatives and their descriptions can be found below.

Aquatic life

Inhabitants of marine and fresh water bodies are usually divided into three large groups: plankton, nekton and benthos. They include both plant and animal species. The main characteristic for their classification is what layers of water they live in and what kind of life they lead.

Benthos is a community of organisms living on the bottom or in its upper layers. Among them there are microscopic bacteria, tiny crustaceans and worms, as well as giant mollusks, huge sponges and bottom fish. Animals that belong to it are called zoobenthos, and plants, respectively, phytobenthos.

Nekton is a group of actively swimming organisms that move independently, can resist currents, and often move throughout the entire water column. It includes not only cetaceans, fish, mobile mollusks, but also animals that lead a semi-aquatic lifestyle, such as penguins, turtles, sea snakes, seals and walruses.

Plankton includes a variety of organisms, the main difference of which is their inactivity. These are mainly gray-green algae, small crustaceans, floating fish eggs, animal larvae, ciliates, radiolaria, and pteropods. The size of some representatives can reach several meters, but most of them are still small. Plankton differs from nekton and benthos in its inability to overcome the force of currents. He simply drifts in the water, succumbing to its excitement.

Features of benthos

The benthos includes a huge number of living creatures, most of which inhabit temperate and subtropical geographic zones. They mainly live in shallow water - in small rivers, lakes or close to the shore. For example, in the area of ​​continental shelves and coral reefs, more than 200 thousand species of marine benthos are found.

Not everyone can adapt to the cold, darkness and pressure of great depths, therefore, with increasing depth, the number of bottom inhabitants decreases significantly. The most desolate places are oceanic depressions, pits and faults, but life can be found there, especially near thermal springs.

Marine and freshwater benthos provide food for many animals. Fish alone eat about 2 billion tons of benthic mass annually. In addition, its representatives are important commercial targets. They are caught for sale, and some are raised on special farms. Particularly popular are lobsters, crabs, oysters, mussels, sea urchins and stars, scarlet algae, kelp, ahnfeltsia and others.

Types and classifications

Representatives of the benthos are very heterogeneous and can differ from each other in almost all aspects. Some lead a sedentary, or attached, lifestyle, while others are nomadic, covering long distances. Some of them bury themselves in the ground or drill holes in underwater rocks, and some simply lie on the bottom.

The lower layers of reservoirs are rich in all kinds of sediments from organic and inorganic substances - the remains of tissues, skeletons, foreign secretions and sedimentary rocks. These “delicacies” are often included in the diet of local inhabitants, although there are also species among them that survive by hunting. According to the method of feeding, benthic organisms are:

• predatory;
• filters;
• ground eaters;
• scrapers;
• suspended matter eaters.

Species that live on the surface of the bottom are called epibenthos, those that burrow deeper are called endobenthos. They are all divided by size:

• On macrobenthos - they can be distinguished by the naked eye and reach more than 1 mm in length.
• Meiobenthos are organisms difficult to distinguish with the naked eye, the size of which reaches from 0.1 mm to 1 mm.
• Microbenthos - animals and plants smaller than 0.1 mm in size, for example bacteria, protozoa, nematodes.

Attached

Unlike nekton, benthos does not always move freely in water. Some of its representatives are attached to some substrate and remain motionless throughout all or part of their life.

A significant part of sessile benthos is plants. They are attached to stones and soil using developed rhizomes or special organs - rhizoids. Sessile animals include corals, sponges, bryozoans, bivalves, crinoids, some worms and crayfish. They are held in place by suckers, hooks, stems, tendrils, or rhizome-like organs. Some settle inside calcareous shells that are attached to the substrate.

A sedentary lifestyle has a number of disadvantages: sessile organisms cannot run away from the enemy, go in search of food and actively hunt, or change their habitat to a more favorable one. In this regard, they acquired many useful devices. They feed by filtering the water with plankton swimming by. Some species have acquired stinging tentacles and poisonous body parts, which allow them to catch unwary animals and protect themselves from enemies.

Reproducing by budding, they expand their colonies and spread throughout the surrounding areas. Many of them live in colonies and can form large aggregations. For example, corals sometimes extend for hundreds of kilometers, and the limestone deposits in which they live form entire islands. Barnacles and other species have a mobile stage of development. At a young age, they can move actively, traveling throughout the entire body of water.

Loose lying

Lying benthos are organisms that are not attached to the substrate in any way, but are freely located on the bottom surface. As a rule, its representatives have a wide, flat body and a camouflage color matching the color of silt and underwater rocks, which makes them invisible to prying eyes.

For protection and hunting, they can have a variety of growths on the skin and shell, and emit a variety of repellent and poisonous secretions. Many of them do not have developed sense organs and limbs. For example, in scallops, the eyes are able to recognize only changes in lighting, and animals move using jet thrust, which is created by the sharp closing of the shell valves.

Burrowing

Burrowing benthos is also called inifauna. Its representatives live in a layer of soil or bottom sediment, into which they are completely immersed. There are no plants among the burrowing benthos. It includes various gastropods and bivalves, brachiopods, round and annelids, sea cucumbers, sea urchins, insect larvae and other invertebrates.

Some animals burrow only a little into the ground, while others dig numerous passages and tunnels in it, which can significantly exceed their own size. Irregular sea urchins prefer to burrow in soft soil and sand. Sometimes for shelter they choose not bottom rocks, but algae, in the thickets of which they hide. Bivalve mollusks burrow into stones, soil, sand, and silt with the help of a muscular leg. They often live in the intertidal zone near coasts and, when the water recedes, can remain out of the water for long periods.

Drilling

Unlike burrowing benthos, boring organisms do not live in soft soil, but prefer hard rocks. They inhabit rocks, limestone, slate and granite rocks, wood and even the shells of other animals. Freshwater insect larvae live in clay or bore holes in plants, attaching themselves to their leaves and stems.

They make moves thanks to hard teeth and ridges on their body. Amphipods use powerful oral appendages for this, literally gnawing their way through, and some mollusks secrete a secretion that dissolves lime.

Boring organisms feed on plankton, various particles floating in the water and the sedimentary layer of reservoirs. A number of species use substances in which they make moves. Eating them, they go deeper and deeper into the rock, forming real labyrinths.

Crawling

Crawling organisms belong to mobile benthos and are classified as epifauna. They move freely along the bottom of reservoirs in search of food, but cannot swim in the water column. Sea urchins walk using spines or ambulatory legs. Polychaete worms move using parapodia - dermal-muscular outgrowths with which they cling to the substrate and pull the body forward. Starfish move with ambulatory legs, which are additionally equipped with suction cups and allow them to cling to various surfaces.

Among the crawling creatures there are filter feeders, soil and detritus eaters, but there are also many predators. Almost all starfish obtain their food by hunting. They eat sea urchins and worms, and with their strong muscular arms they can open the shells of bivalve mollusks. Many crawlers are characterized by bilateral symmetry of the body, different from each other in the abdominal and dorsal regions.

Free floating

Together with the group of crawling organisms, this type of organisms is usually classified as mobile benthos. They have well-developed limbs and can move freely within bodies of water. In addition to invertebrate crustaceans and worms, this includes various bottom-dwelling fish. Some of them spend their entire lives close to the bottom, others can rise to the surface from time to time.

Typical representatives of these animals are flounders. They have a flat body, flattened on top with wide sides. The eyes, unlike most fish, are not on the sides, but on the back. Flounders live in shallow water and are rarely found at depths greater than 50 meters. In search of food, they swim near the seabed, and in case of danger they bury themselves in silt or sand.

5.1 Plankton Plankton (from the Greek planktos - soaring, wandering) is a set of pelagic organisms that do not have the ability for fast and active movements, but are passively transported by the current. Planktonic organisms cannot resist currents. Plankton includes mainly microscopic algae - phytoplankton, small animals – zooplankton and bacterioplankton. The plankton periodically includes the larvae of many animals floating in the water column. Phytoplankton inhabit layers of water with sufficient illumination, zooplankton and bacterioplankton inhabit the entire water column (Figure 5.1). Phytoplankton is of great importance in the life of water bodies, since it is the main producer of organic matter. Planktonic organisms are located either on the surface of the water, or in its thickness, or even in the bottom layer. To maintain a suspended state in water, plankton organisms have developed various adaptations: reduction of the skeleton (in some mollusks); soaking in water (scyphoid jellyfish); fatty inclusions (in cladocera and copepods); gas inclusions (in blue-green algae - gas vacuoles); increasing shape resistance - elongated along the axis (ribbon-shaped colonies of diatoms) or by forming outgrowths (ceratium from algae). Depending on body size, plankton organisms are divided into groups:

ultraplankton – sizes do not exceed several micrometers (bacteria);

nannoplankton (dwarf, small plankton) – body sizes are micrometers and dozens of them;

microplankton – tenths and hundredths of a millimeter;

mesoplankton - body length measured in millimeters (large representatives of phytoplankton, the main part of the zooplankton of the seas;

macroplankton - body length is centimeters;

megaloplankton - body sizes reach tens of centimeters.

Planktonic organisms are an important food component of many aquatic animals (including such giants as baleen whales, especially considering that they, and especially phytoplankton, are characterized by seasonal outbreaks of mass reproduction. ^ Figure 5.1 – Profile of the ocean and its inhabitants Prominent Ukrainian hydrobiologist Yu.P. Zaitsev identified a thin, several centimeters, layer in the planktonic film of life, calling it “the incubator of the sea.” The name “neustal” has been assigned to this specific pelagic biotope in the scientific literature. This author also discovered such an interesting phenomenon as “anti-rain of corpses”, caused by the floating of dead organisms for some time. Before they sink, they, being in the surface layer, manage to enrich it with organic matter, which during storms is churned into the well-known foam. Therefore, the base of the trophic pyramid in the surface film is made up of saprotrophic microorganisms: they process organic matter in excess. 5.2 Nekton Nekton (from the Greek nektos - floating) is a collection of pelagic actively moving animals that do not have a direct connection with the bottom, capable of resisting the force of the current and independently moving over considerable distances. These are mainly large animals that can overcome long distances and strong water currents. They are characterized by a streamlined body shape and well-developed organs of movement. Typical nektonic organisms are fish, squid, pinnipeds, and whales. In fresh waters, in addition to fish, nekton includes amphibians and actively moving insects. Many marine fish can move through the water at great speed. Some squid swim very fast, up to 45–50 km/h, sailfish reach speeds of up to 100–110 km/h, and swordfish reach speeds of up to 130 km/h. ^ 5.3 Plaiston and neuston Organisms that float passively on the surface of the water or lead a semi-submerged lifestyle are called plaiston (from Greek pleusis - swimming). They often use a surface tension film as a support or form air cavities and other floats. Typical pleistonic animals are siphonophores, some mollusks, etc. Plant organisms classified as pleistonic include sargassum algae and duckweed. Neuston is a unique type of plankton. Neuston (from the Greek neustos - floating) - a community of organisms living near the surface film of water. Organisms that live on top of the surface film - epineuston, below – hyponeuston. Neuston also includes the inhabitants of the upper five-centimeter layer of water. Neuston consists of some protozoa, unicellular algae, small pulmonate mollusks, water striders, whirligigs, mosquito larvae, etc. ^ 5.4 Benthos and periphyton Benthos (benthos - depth) - a set of organisms living at the bottom (on the ground and in the ground) of reservoirs. Benthos is divided into phytobenthos And zoobenthos. Mainly represented by attached or slowly moving animals, as well as burrowing animals. Only in shallow water does it consist of organisms that synthesize organic matter (producers), consume (consumers) and destroy (decomposers) it. At great depths, where light does not penetrate, phytobenthos (producers) is absent. Benthic organisms vary:

by way of life – mobile (wandering), slightly mobile (lying down) and immobile (attached);

by feeding method – photosynthetic, herbivorous, carnivorous, detritivorous;

by size - macro-, meso-, microbenthos.

The phytobenthos of the seas mainly includes algae. Flowering plants are also found along the coasts. The richest phytobenthos is in rocky and stony areas of the bottom. Along the coasts, kelp and fucus sometimes form a biomass of up to 30 kg per 1 m2. On soft soils, where plants cannot firmly attach, phytobenthos develops mainly in places protected from waves. Freshwater phytobenthos is represented by diatoms and green algae. Coastal plants are abundant, located inland from the shore in clearly defined belts. In the first zone, semi-submerged plants grow (reeds, reeds, cattails and sedges). The second zone is occupied by submerged plants with floating leaves (water lilies, water lilies, duckweeds). The third zone is dominated by submerged plants - pondweed, elodea, etc. All aquatic plants according to their lifestyle can be divided into two main ecological groups: hydrophytes– plants that are immersed in water only with their lower part and usually root in the ground, and hydatophytes- plants that are completely submerged in water, but sometimes float on the surface or have floating leaves. Benthic forms are most numerous in shallow waters. With depth, the number of benthos drops sharply. There is less zoobenthos in fresh water bodies than in seas and oceans. These are mainly protozoans, some sponges, ciliated and oligochaete worms, leeches, bryozoans, mollusks and insect larvae. Benthic organisms are divided into groups:

attached forms (sponges, corals, crinoids, representatives of mollusks, crustaceans);

lying organisms (scallops from bivalves, polychaetes from annelids, flat sea urchins from echinoderms);

burrowing organisms (peskozhil);

boring organisms (shipworm from bivalve mollusks);

freely moving organisms (crayfish, crabs, echinoderms, insect larvae, adult insects - water bugs, swimming beetles and water lovers).

A unique group of aquatic organisms is periphyton (from Greek peri - about and phyton - plant). This is a set of organisms that settle on underwater objects or plants and form so-called fouling on natural or artificial hard surfaces - stones, rocks, underwater parts of ships, piles, hydraulic structures (algae, barnacles, mollusks, bryozoans, sponges, etc.) . Periphyton organisms are fouling organisms. The morphological plasticity of aquatic organisms varies. They have less ecological plasticity than terrestrial ones, since water is a more stable environment and its abiotic factors undergo relatively minor fluctuations. Marine plants and animals are the least plastic. They are very sensitive to changes in water salinity and temperature. Freshwater animals and plants, as a rule, are much more plastic than marine ones, since freshwater as a living environment is more variable. The most flexible are the brackish-water inhabitants. They are adapted to both high concentrations of dissolved salts and significant desalination. However, there are a relatively small number of their species, since environmental factors undergo significant changes in brackish waters. Ecological plasticity serves as an important regulator of the dispersal of organisms. As a rule, aquatic organisms with high ecological plasticity are quite widespread.

Ecological plasticity of organisms (ecological valence) is the degree of adaptability of a species to changes in environmental factors. It is expressed by the range of values ​​of environmental factors within which a given species maintains normal life activity. The wider the range, the greater the environmental plasticity. There are more stenobionts in the aquatic environment, since it is relatively stable in its properties and the amplitudes of fluctuations of individual factors are small

Stenobionts are opposed to ubiquists. They, unlike stenobionts, can exist in a wide variety of conditions.

Adaptive features of aquatic plants. Aquatic plants, unlike terrestrial ones, absorb moisture and mineral salts directly from the surrounding water, so their organization has its own characteristics. Their conductive tissues and root systems are poorly developed. Since the roots serve primarily for attachment to the submerged substrate, they lack root hairs. The powerful development of the root system in some of them - water lilies, egg capsules - ensures vegetative reproduction and storage of certain substances.

The main structural feature of hydrophytes is the presence of large intercellular spaces and cavities that create a special air tissue that ensures the buoyancy of the organs. Underwater hydrophytes differ from above-water ones in the absence of functioning stomata, thin dissected leaves, and poor development of mechanical tissues. Intense gas exchange with a lack of dissolved oxygen in the water is ensured either by very long and thin stems and leaves, the covers of which are easily permeable to oxygen, or by strong leaf segmentation.

A number of plants have developed heterophyly (various leaves). In water lilies and egg capsules, floating leaves are very different from submerged ones - their upper surface is dense and leathery, with a large number of stomata, which promotes better gas exchange with air; there are no stomata on the underside.

Due to the low water temperature, which negatively affects the reproductive organs, and the high density of the environment, which impedes the transfer of pollen, plants immersed in water reproduce vegetatively. However, many of them carry flowering stems into the air and reproduce sexually. Their pollen, fruits and seeds are carried by wind and surface currents. Coastal plants also use surface currents. Their fruits are highly buoyant and can remain viable for a long time while in water.

Adaptive features of aquatic animals. The adaptations of animals to the aquatic environment are more diverse than those of plants. Animals that live in the water column are characterized, first of all, by adaptations that increase their buoyancy and allow them to withstand the movement of water and currents. In small forms, a reduction of skeletal formations is observed. They have porous shells or hollow skeletal spines. Specific density of the body is reduced by the presence of water, air or fat in the tissues.

Animals passively swimming in the water column are also characterized by an increase in the specific surface area of ​​the body. This is achieved by flattening the body, forming all kinds of spines and outgrowths.

Active swimming is carried out with the help of cilia, flagella, and also bending of the body. Jet swimming has become widespread due to the energy of the ejected stream of water. Thus, some squids reach speeds of 40–50 km/h. Larger animals have specialized limbs - fins, flippers. The body of such animals is covered with mucus and has a streamlined shape.

Freshwater animals use the surface film of water when moving. Whirling beetles and water strider bugs run freely across it. Their covers are not wetted by water, and their limbs have a special structure.

Bottom organisms, on the contrary, develop adaptations that reduce buoyancy and allow them to stay on the bottom even in fast-flowing waters. The heavy shells of the tridacna mollusk are well known, which lie freely at shallow depths and are held on reefs due to their mass.

Only in the aquatic environment are there animals that lead a sedentary lifestyle. The most famous of those attached to the ground are sponges, hydroid and coral polyps, sea lilies, bivalves, etc.

For aquatic animals, environmental pressure matters. Among the eurybates - living at both high and low pressure - holothurians are distinguished, living at depths from 100 to 9000 m. Among the stenobats - crinoids, pogonophora, living at depths from 3000 to 10,000 m. Deep-sea animals are characterized by poor development or absence calcareous skeleton, reduction of the organs of vision, increased development of tactile receptors, lack of body pigmentation or, conversely, dark coloring.

Compared to terrestrial communities, the biocenoses of aquatic organisms have some significant differences. The main one is the microscopic size of the vast majority of producers. Another characteristic feature is the presence of close interorganismal connections through the environment, in particular, the existence of various biochemical influences that play a crucial role in the organization of communities.