Density of water- this is a factor that determines the conditions of movement aquatic organisms and pressure at different depths. For distilled water, the density is 1 g/cm 3 at 4 °C. The density of natural waters containing dissolved salts can be greater, up to 1.35 g/cm 3 . Pressure increases with depth by an average of 1 × 10 5 Pa (1 atm) for every 10 m.

Due to the sharp pressure gradient in water bodies, aquatic organisms are generally much more eurybathic compared to land organisms. Some species, distributed at different depths, tolerate pressure from several to hundreds of atmospheres. For example, holothurians of the genus Elpidia and worms Priapulus caudatus live from the coastal zone to the ultra-abyssal zone. Even freshwater inhabitants, for example, slipper ciliates, suvoikas, swimming beetles, etc., can withstand up to 6 × 10 7 Pa (600 atm) in the experiment.

However, many inhabitants of the seas and oceans are relatively stenobatic and confined to certain depths. Stenobacy is most often characteristic of shallow- and deep-sea species. They live only in the littoral zone ringworm sandworm Arenicola, limpet molluscs (Patella). Many fish, for example from the group of anglers, cephalopods, crustaceans, pogonophores, sea ​​stars and others are found only at great depths at a pressure of at least 4 10 7 - 5 10 7 Pa (400-500 atm).

The density of water provides the ability to lean on it, which is especially important for non-skeletal forms. The density of the environment serves as a condition for floating in water, and many aquatic organisms are adapted specifically to this way of life. Suspended organisms floating in water are combined into a special ecological group of aquatic organisms - plankton (“planktos” - soaring).

Rice. 39. Increase in the relative body surface of planktonic organisms (according to S. A. Zernov, 1949):

A - rod-shaped:

1 - diatom Synedra;

2 - cyanobacterium Aphanizomenon;

3 - peridine alga Amphisolenia;

4 - Euglena acus;

5 - cephalopod Doratopsis vermicularis;

6 - copepod Setella;

7 - Porcellana larva (Decapoda)

B - dissected forms:

1 - mollusk Glaucus atlanticus;

2 - worm Tomopetris euchaeta;

3 - Palinurus crayfish larva;

4 - fish larva monkfish Lophius;

5 - copepod Calocalanus pavo

Plankton includes unicellular and colonial algae, protozoa, jellyfish, siphonophores, ctenophores, pteropods and keelfoot mollusks, various small crustaceans, larvae of bottom animals, fish eggs and fry, and many others (Fig. 39). Planktonic organisms have many similar adaptations that increase their buoyancy and prevent them from sinking to the bottom. Such adaptations include: 1) a general increase in the relative surface of the body due to reduction in size, flattening, elongation, development of numerous projections or bristles, which increases friction with water; 2) a decrease in density due to the reduction of the skeleton, the accumulation of fats, gas bubbles, etc. in the body. In diatoms, reserve substances are deposited not in the form of heavy starch, but in the form of fat drops. The night light Noctiluca is distinguished by such an abundance of gas vacuoles and fat droplets in the cell that the cytoplasm in it has the appearance of strands that merge only around the nucleus. Siphonophores, a number of jellyfish, planktonic gastropods, etc. also have air chambers.

Seaweed (phytoplankton) hover in water passively, while most planktonic animals are capable of active swimming, but in within limited limits. Planktonic organisms cannot overcome currents and are transported by them over long distances. Many types zooplankton However, they are capable of vertical migrations in the water column for tens and hundreds of meters, both due to active movement and by regulating the buoyancy of their body. A special type of plankton is an ecological group Neuston (“nein” - swim) - inhabitants of the surface film of water on the border with air environment.

The density and viscosity of water greatly influence the possibility of active swimming. Animals capable of fast swimming and overcoming the force of currents are united in an ecological group nekton (“nektos” - floating). Representatives of nekton are fish, squid, and dolphins. Rapid movement in the water column is possible only if you have a streamlined body shape and highly developed muscles. The torpedo-shaped shape is developed in all good swimmers, regardless of their systematic affiliation and method of movement in the water: reactive, due to bending of the body, with the help of limbs.

Oxygen regime. In oxygen-saturated water, its content does not exceed 10 ml per 1 liter, which is 21 times lower than in the atmosphere. Therefore, the breathing conditions of aquatic organisms are significantly complicated. Oxygen enters water mainly through the photosynthetic activity of algae and diffusion from the air. Therefore, the upper layers of the water column are, as a rule, richer in this gas than the lower ones. As the temperature and salinity of water increase, the concentration of oxygen in it decreases. In layers heavily populated by animals and bacteria, a sharp deficiency of O 2 can be created due to its increased consumption. For example, in the World Ocean, life-rich depths from 50 to 1000 m are characterized by a sharp deterioration in aeration - it is 7-10 times lower than in surface waters inhabited by phytoplankton. Conditions near the bottom of reservoirs can be close to anaerobic.

Among aquatic inhabitants there are many species that can tolerate wide fluctuations in oxygen content in water, up to its almost complete absence (euryoxybionts - “oxy” - oxygen, “biont” - inhabitant). These include, for example, the freshwater oligochaete Tubifex tubifex and the gastropod Viviparus viviparus. Among fish, carp, tench, and crucian carp can withstand very low oxygen saturation of water. However, a number of types stenoxybiont - they can exist only with sufficiently high oxygen saturation of the water (rainbow trout, brown trout, minnow, eyelash worm Planaria alpina, larvae of mayflies, stoneflies, etc.). Many species are capable of falling into an inactive state when there is a lack of oxygen - anoxybiosis - and thus experience an unfavorable period.

Respiration of aquatic organisms occurs either through the surface of the body or through specialized organs - gills, lungs, trachea. In this case, the integument can serve as an additional respiratory organ. For example, the loach fish consumes an average of 63% of oxygen through its skin. If gas exchange occurs through the integuments of the body, they are very thin. Breathing is also made easier by increasing the surface area. This is achieved during the evolution of species by the formation of various outgrowths, flattening, elongation, and a general decrease in body size. Some species, when there is a lack of oxygen, actively change the size of the respiratory surface. Tubifex tubifex worms greatly elongate their body; hydra and sea anemone - tentacles; echinoderms - ambulacral legs. Many sessile and sedentary animals renew water around them, either by creating a directed current or oscillatory movements promoting its mixing. Bivalve mollusks use cilia lining the walls of the mantle cavity for this purpose; crustaceans - the work of the abdominal or thoracic legs. Leeches, bell mosquito larvae (bloodworms), and many oligochaetes sway their bodies, sticking out of the ground.

In some species, a combination of water and air respiration occurs. These are lungfishes, siphonophores, discophants, many lung molluscs, crustaceans Gammarus lacustris, etc. Secondary aquatic animals usually retain atmospheric type respiration as more favorable energetically and therefore require contact with the air environment, for example, pinnipeds, cetaceans, water beetles, mosquito larvae, etc.

Lack of oxygen in water sometimes leads to catastrophic phenomena - I'm dying, accompanied by the death of many aquatic organisms. Winter freezes often caused by the formation of ice on the surface of bodies of water and the cessation of contact with air; summer- an increase in water temperature and a resulting decrease in oxygen solubility.

Frequent death of fish and many invertebrates in winter is characteristic, for example, of the lower part of the Ob River basin, the waters of which, flowing from the wetlands of the West Siberian Lowland, are extremely poor in dissolved oxygen. Sometimes death occurs in the seas.

In addition to a lack of oxygen, death can be caused by an increase in the concentration of toxic gases in water - methane, hydrogen sulfide, CO 2, etc., formed as a result of the decomposition of organic materials at the bottom of reservoirs.

Salt regime. Maintaining the water balance of aquatic organisms has its own specifics. If for terrestrial animals and plants it is most important to provide the body with water in conditions of its deficiency, then for hydrobionts it is no less important to maintain a certain amount of water in the body when there is an excess of it in the environment. Excessive amount of water in cells leads to a change in osmotic pressure in them and disruption of the most important vital functions.

Most aquatic life poikilosmotic: the osmotic pressure in their body depends on the salinity of the surrounding water. Therefore, the main way for aquatic organisms to maintain their salt balance is to avoid habitats with unsuitable salinity. Freshwater forms cannot exist in the seas, and marine forms cannot tolerate desalination. If the salinity of the water is subject to changes, animals move in search of a favorable environment. For example, when desalinating the surface layers of the sea after heavy rains radiolarians, sea crustaceans Calanus and others descend to a depth of 100 m. Vertebrates, higher crustaceans, insects and their larvae living in water belong to homoiosmotic species, maintaining constant osmotic pressure in the body regardless of the concentration of salts in the water.

In freshwater species, body juices are hypertonic in relation to surrounding water. They are at risk of excessive watering if the supply of water is not prevented or excess water is not removed from the body. In protozoa this is achieved by the work of excretory vacuoles, in multicellular organisms - by removing water through the excretory system. Some ciliates secrete an amount of water equal to their body volume every 2-2.5 minutes. The cell expends a lot of energy to “pump out” excess water. With increasing salinity, the work of vacuoles slows down. Thus, in Paramecium slippers, at a water salinity of 2.5%o, the vacuole pulsates at intervals of 9 s, at 5%o - 18 s, at 7.5%o - 25 s. At a salt concentration of 17.5% o, the vacuole stops working, since the difference in osmotic pressure between the cell and the external environment disappears.

If water is hypertonic in relation to the body fluids of aquatic organisms, they are at risk of dehydration as a result of osmotic losses. Protection against dehydration is achieved by increasing the concentration of salts also in the body of aquatic organisms. Dehydration is prevented by the water-impermeable integument of homoiosmotic organisms - mammals, fish, higher crayfish, aquatic insects and their larvae.

Many poikilosmotic species transition to an inactive state - suspended animation as a result of a lack of water in the body with increasing salinity. This is characteristic of species living in pools of sea water and in the littoral zone: rotifers, flagellates, ciliates, some crustaceans, the Black Sea polychaete Nereis divesicolor, etc. Salt suspended animation- a means to survive unfavorable periods in conditions of variable salinity of water.

Truly euryhaline There are not many species among aquatic inhabitants that can live in an active state in both fresh and salt water. These are mainly species inhabiting river estuaries, estuaries and other brackish water bodies.

Temperature reservoirs are more stable than on land. This is due to the physical properties of water, primarily its high specific heat capacity, due to which the receipt or release of a significant amount of heat does not cause too sudden changes in temperature. The evaporation of water from the surface of reservoirs, which consumes about 2263.8 J/g, prevents overheating of the lower layers, and the formation of ice, which releases the heat of fusion (333.48 J/g), slows down their cooling.

The amplitude of annual temperature fluctuations in the upper layers of the ocean is no more than 10-15 °C, in continental waters - 30-35 °C. Deep layers of water are characterized by constant temperature. In equatorial waters, the average annual temperature of surface layers is +(26-27) °C, in polar waters it is about 0 °C and below. In hot land-based springs, the water temperature can approach +100 °C, and in underwater geysers, at high pressure on the ocean floor, temperatures of +380 °C have been recorded.

Thus, there is a fairly significant variety of temperature conditions in reservoirs. Between the upper layers of water with seasonal temperature fluctuations expressed in them and the lower ones, where the thermal regime is constant, there is a zone of temperature jump, or thermocline. The thermocline is more pronounced in warm seas, where the temperature difference between external and deep waters is greater.

Due to the more stable temperature regime of water, stenothermy is common among aquatic organisms to a much greater extent than among the land population. Eurythermal species are found mainly in shallow continental reservoirs and in the littoral zone of seas of high and temperate latitudes, where daily and seasonal temperature fluctuations are significant.

Light mode. There is much less light in water than in air. Some of the rays incident on the surface of a reservoir are reflected into the air. The reflection is stronger the lower the position of the Sun, so the day under water is shorter than on land. For example, summer day near the island of Madeira at a depth of 30 m - 5 hours, and at a depth of 40 m only 15 minutes. The rapid decrease in the amount of light with depth is associated with its absorption by water. Rays of different wavelengths are absorbed differently: red ones disappear close to the surface, while blue-green ones penetrate much deeper. The twilight in the ocean that deepens with depth is first green, then blue, indigo and blue-violet, finally giving way to constant darkness. Accordingly, green, brown and red algae, specialized in capturing light with different wavelengths, replace each other with depth.

The color of animals changes with depth just as naturally. The inhabitants of the littoral and sublittoral zones are most brightly and variedly colored. Many deep organisms, like cave organisms, do not have pigments. In the twilight zone, red coloration is widespread, which is complementary to the blue-violet light at these depths. Rays of additional color are most completely absorbed by the body. This allows animals to hide from enemies, since their red color in blue-violet rays is visually perceived as black. Red coloring is characteristic of twilight zone animals such as sea ​​bass, red coral, various crustaceans, etc.

In some species that live near the surface of water bodies, the eyes are divided into two parts with different abilities to refract rays. One half of the eye sees in the air, the other in water. Such “four-eyedness” is characteristic of spinning beetles, the American fish Anableps tetraphthalmus, and one of the tropical species of blenny Dialommus fuscus. During low tides, this fish sits in recesses, exposing part of its head from the water (see Fig. 26).

The absorption of light is stronger, the lower the transparency of the water, which depends on the number of particles suspended in it.

Transparency is characterized by the maximum depth at which a specially lowered white disk with a diameter of about 20 cm (Secchi disk) is still visible. The clearest waters are in the Sargasso Sea: the disk is visible to a depth of 66.5 m. Pacific Ocean the Secchi disk is visible up to 59 m, in the Indian Sea - up to 50, in shallow seas - up to 5-15 m. The transparency of rivers is on average 1-1.5 m, and in the most muddy rivers, for example in the Central Asian Amu Darya and Syr Darya, only a few centimeters . The boundary of the photosynthetic zone therefore varies greatly in different bodies of water. In the most clean waters euphotic zone, or zone of photosynthesis, extends to depths not exceeding 200 m, crepuscular, or dysphotic, the zone occupies depths of up to 1000-1500 m, and deeper, in aphotic zone, sunlight doesn't penetrate at all.

The amount of light in the upper layers of reservoirs varies greatly depending on the latitude of the area and the time of year. Long polar nights severely limit the time available for photosynthesis in Arctic and Antarctic basins, and ice cover makes it difficult for light to reach all frozen bodies of water in winter.

In the dark depths of the ocean, organisms use light emitted by living things as a source of visual information. The glow of a living organism is called bioluminescence. Luminous species are found in almost all classes of aquatic animals from protozoa to fish, as well as among bacteria, lower plants and fungi. Bioluminescence appears to have occurred repeatedly in different groups on different stages evolution.

The chemistry of bioluminescence is now quite well understood. The reactions used to generate light are varied. But in all cases this is the oxidation of complex organic compounds (luciferins) using protein catalysts (luciferase). Luciferins and luciferases have different structures in different organisms. During the reaction, the excess energy of the excited luciferin molecule is released in the form of light quanta. Living organisms emit light in impulses, usually in response to stimuli coming from the external environment.

The glow may not play much ecological role in the life of a species, but to be a by-product of the vital activity of cells, as, for example, in bacteria or lower plants. It acquires ecological significance only in animals that have sufficiently developed nervous system and organs of vision. In many species, the luminescent organs acquire very complex structure with a system of reflectors and lenses that enhance radiation (Fig. 40). A number of fish and cephalopods, unable to generate light, use symbiotic bacteria that multiply in the special organs of these animals.

Rice. 40. Luminescence organs of aquatic animals (according to S. A. Zernov, 1949):

1 - a deep-sea anglerfish with a flashlight over its toothed mouth;

2 - distribution of luminous organs in fish of the family. Mystophidae;

3 - luminous organ of the fish Argyropelecus affinis:

a - pigment, b - reflector, c - luminous body, d - lens

Bioluminescence has mainly a signaling value in the life of animals. Light signals can serve for orientation in a flock, attracting individuals of the opposite sex, luring victims, for camouflage or distraction. A flash of light can act as a defense against a predator by blinding or disorienting it. For example, deep-sea cuttlefish, fleeing from an enemy, release a cloud of luminous secretion, while species living in illuminated waters use dark liquid for this purpose. In some bottom worms - polychaetes - luminous organs develop during the period of maturation of reproductive products, and females glow brighter, and the eyes are better developed in males. In predatory deep-sea fish from the order of anglerfish, the first ray of the dorsal fin is shifted to the upper jaw and turned into a flexible “rod” carrying at the end a worm-like “bait” - a gland filled with mucus with luminous bacteria. By regulating the blood flow to the gland and, therefore, the supply of oxygen to the bacterium, the fish can arbitrarily cause the “bait” to glow, imitating the movements of the worm and luring in prey.

According to modern hypotheses of the origin of life, it is generally accepted that the evolutionarily primary environment on our planet was the aquatic environment. Confirmation of the accepted statements is that the concentration of oxygen, calcium, potassium, sodium and chlorine in our blood is close to that in ocean water.

Aquatic habitat

In addition to the Sea Ocean, it includes all rivers, lakes and groundwater. The latter, in turn, are a source of food for rivers, lakes and seas. Thus, the water cycle in nature is driving force hydrosphere and important source fresh water on land.

Based on the above, the hydrosphere should be divided into:

• surface (the surface hydrosphere includes seas and oceans, lakes, rivers, swamps, glaciers, etc.);
• underground.

The main feature of the surface hydrosphere is that it does not form a continuous layer, but at the same time occupies a significant area - 70.8% of the Earth's surface.

The composition of the underground hydrosphere is represented by groundwater. The total volume of water reserves on Earth is about 1370 million km3, of which about 94% is concentrated in the ocean, 4.12% in groundwater, 1.65% - in glaciers and less than 0.02% of water is contained in lakes and rivers.

In the hydrosphere, based on the living conditions of living organisms, the following zones are distinguished:

• pelagic - water column and benthic - bottom;
• in the benthal, depending on the depth, the sublittoral is distinguished - an area of ​​smooth increase in depth up to 200 m;
• batial - bottom slope;
• abyssal - oceanic bed, up to 6 km deep;
• ultraabyssal, represented by depressions of the ocean floor;
• littoral, representing the edge of the coast, regularly flooded during high tide and drained by low tide; and sublittoral, representing the part of the coast moistened by splashes of the surf.

Based on the type of habitat and lifestyle, living organisms inhabiting the hydrosphere are divided into the following groups:

1. pelagos - are a collection of organisms living in the water column. Among the pelagos, plankton is distinguished - a group of organisms that includes plants (phytoplankton) and animals (zooplankton), which are not capable of independent movement in the water column and are moved by currents, as well as nekton - a group of living organisms capable of independent movement in the water column ( fish, shellfish, etc.).
2. benthos is a group of organisms living at the bottom and in the soil. In turn, benthos is divided into phytobenthos, represented by algae and higher plants, and zoobenthos (starfish, crustaceans, mollusks, etc.).

Ecological factors in aquatic habitats

The main environmental factors in the aquatic habitat are represented by currents and waves, operating almost non-stop. They are able to provide indirect impact on organisms, changing the ionic composition of water, its mineralization, which in turn contributes to changes in nutrient concentrations. As for the direct impact of the above factors, they contribute to the adaptation of living organisms to the flow. So, for example, fish that live in calm waters have a body that is flattened at the sides (bream), while in fast waters they have a rounded body (trout).

Being a fairly dense medium, water provides significant resistance to the movement of living organisms inhabiting it. This is why most of the inhabitants of the hydrosphere have a streamlined body shape (fish, dolphins, squid, etc.).

Note 1

It is worth noting that the human embryo in the first weeks of its development in many ways resembles a fish embryo and only at the age of one and a half to two months acquires characteristics characteristic of humans. All this indicates the critical importance of the aquatic environment in the development of life.

On our planet, living organisms, in the course of long historical development, mastered four living environments, which were distributed according to the mineral shells: hydrosphere, lithosphere, atmosphere (Fig. 1).

Rice. 1.

habitat aquatic air soil organism life

Water environment was the first in which life arose and spread. Later, in the course of historical development, organisms began to populate the ground-air environment. Land plants and animals appeared, rapidly evolving, adapting to new living conditions. The functioning of living matter on land led to the gradual transformation of the surface layer of the lithosphere into soil, in the words of V.I. Vernadsky (1978), into a kind of bio-inert body of the planet. The soil was inhabited by both aquatic and terrestrial organisms, creating a specific complex of its inhabitants.

Aquatic life environment

General characteristics. The hydrosphere as an aquatic living environment occupies about 71% of the area and 1/800 of the volume of the globe. The main amount of water, more than 94%, is concentrated in the seas and oceans (Fig. 2).

Rice. 2.

In fresh waters of rivers and lakes, the amount of water does not exceed 0.016% of the total volume of fresh water.

In the ocean with its constituent seas, two ecological areas are primarily distinguished: the water column - pelagic and the bottom - benthal. Depending on the depth, benthal is divided into subtidal zone -- area of ​​gradual decline of land to a depth of 200 m, bathyal -- steep slope area and abyssal zone -- oceanic bed with an average depth of 3-6 km. The deeper benthic regions, corresponding to the depressions of the ocean floor (6-10 km), are called ultraabyssal. The edge of the shore that is flooded during high tides is called littoral The part of the coast above the tide level, moistened by the spray of the surf, is called supralittoral.

The open waters of the World Ocean are also divided into vertical zones corresponding to the benthic zones: epipelagic, bathypelagic, abyssopelagic(Fig. 3).

Rice. 3.

The aquatic environment is home to approximately 150,000 animal species, or about 7% of the total (Fig. 4) and 10,000 plant species (8%).

It should also be noted that representatives of most groups of plants and animals remained in the aquatic environment (their “cradle”), but the number of their species is much smaller than that of terrestrial ones. Hence the conclusion - evolution on land took place much faster.

The seas and oceans of the equatorial and tropical regions, primarily the Pacific and Atlantic oceans, are distinguished by the diversity and richness of flora and fauna. To the north and south of these belts, the quality composition gradually becomes depleted. For example, in the area of ​​the East Indian archipelago there are at least 40,000 species of animals, while in the Laptev Sea there are only 400. The bulk of the organisms of the World Ocean are concentrated in a relatively small area sea ​​coasts temperate zone and among the mangroves of tropical countries.

The share of rivers, lakes and swamps, as noted earlier, is insignificant compared to seas and oceans. However, they create the supply of fresh water necessary for plants, animals and humans.

Rice. 4.

Note animals placed below the wavy line live in the sea, above it - in the land-air environment

It is known that not only the aquatic environment has a strong influence on its inhabitants, but also living matter The hydrosphere, affecting the habitat, processes it and involves it in the cycle of substances. It has been established that the water of oceans, seas, rivers and lakes decomposes and is restored in the biotic cycle over 2 million years, i.e. all of it has passed through living matter on Earth more than one thousand times.

Consequently, the modern hydrosphere is a product of the vital activity of living matter not only of modern, but also of past geological eras.

A characteristic feature of the aquatic environment is its mobility, especially in flowing, fast-flowing streams and rivers. The seas and oceans experience ebbs and flows, powerful currents, and storms. In lakes, water moves under the influence of temperature and wind.

Ecological groups of hydrobionts. Water thickness, or pelagic(pelages - sea), is inhabited by pelagic organisms that have the ability to swim or stay in certain layers (Fig. 5).

Rice.

In this regard, these organisms are divided into two groups: nekton And plankton. The third environmental group -- benthos -- form 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. These are mainly 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 insects. Many marine fish can move through the water at enormous speeds: up to 45-50 km/h for squid (Oegophside), 100-150 km/h for sailfish (Jstiopharidae) and 130 km/h for swordfish (Xiphias glabius).

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 located on the surface of the water make up special group -- Neuston. The composition of neuston also depends on the developmental stage of a number of organisms. Passing through the larval stage and growing up, they leave the surface layer that served them as a refuge and move to live on the bottom or in the underlying and deeper layers. These include the larvae of decapods, barnacles, copepods, gastropods and bivalves, echinoderms, polychaetes, fish, etc.

The same organisms, part of whose body is above the surface of the water, and the other in the water, are called plaiston. These include duckweed (Lemma), siphonophores (Siphonophora), etc.

Phytoplankton plays an important role in the life of water bodies, as it is the main producer of organic matter. Phytoplankton primarily includes diatoms (Diatomeae) and green algae (Chlorophyta), plant flagellates (Phytomastigina), peridineae (Peridineae) and coccolithophorids (Coccolitophoridae). Not only green algae, but also blue-green algae (Cyanophyta) are widespread in fresh waters.

Zooplankton and bacteria can be found at various depths. In fresh waters, mostly poorly swimming, relatively large crustaceans (Daphnia, Cyclopoidea, Ostrocoda), many rotifers (Rotatoria) and protozoa are common.

Marine zooplankton is dominated by small crustaceans (Copepoda, Amphipoda, Euphausiaceae) and protozoa (Foraminifera, Radiolaria, Tintinoidea). Large representatives include wing-footed molluscs (Pteropoda), jellyfish (Scyphozoa) and swimming ctenophora (Ctenophora), salps (Salpae), and some worms (Aleiopidae, Tomopteridae).

Planktonic organisms serve as an important food component for many aquatic animals, including such giants as baleen whales (Mystacoceti), fig. 6.

Fig 6.

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 - Zostera, Ruppia, Phyllospadix. 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 its species composition is uniform. These are mainly protozoa, sponges, ciliated and polychaete worms, leeches, mollusks, insect larvae, etc.

Freshwater phytobenthos is represented by bacteria, diatoms and green algae. Coastal plants are located from the shore inland in clearly defined belts. First belt -- semi-submerged plants (reeds, cattails, sedges and reeds); second belt -- submerged plants with floating leaves (water lilies, egg capsules, water lilies, duckweeds). IN third belt plants predominate - pondweed, elodea, etc. (Fig. 7).

Rice. 7. Plants rooting at the bottom (A): 1 - cattail; 2- rushwort; 3 -- arrowhead; 4 -- water lily; 5, 6 - pondweed; 7 - hara. Free-floating algae (B): 8, 9 -- filamentous green; 10-13 -- green; 14-17 -- diatoms; 18-20 -- blue-green

Based on their lifestyle, aquatic plants are divided into two main ones: environmental 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 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).

Temperature regime. It differs in water, firstly, by less heat influx, and secondly, by greater stability than on land. Part of the thermal energy arriving at the surface of the water is reflected, while part is spent on evaporation. The evaporation of water from the surface of reservoirs, which consumes about 2263x8 J/g, prevents overheating of the lower layers, and the formation of ice, which releases the heat of fusion (333.48 J/g), slows down their cooling.

Temperature changes in flowing waters follow its changes in the surrounding air, differing in smaller amplitude.

In lakes and ponds of temperate latitudes, the thermal regime is determined by the well-known physical phenomenon-- water has maximum density at 4°C. The water in them is clearly divided into three layers: the upper - epilimnion, whose temperature experiences sharp seasonal fluctuations; transitional layer of temperature jump, -- metalimnion, where there is a sharp temperature change; deep-sea (bottom) -- hypolimnion reaching to the very bottom, where the temperature throughout the year changes insignificant.

In summer, the warmest layers of water are located at the surface, and the coldest ones are located at the bottom. This type The layer-by-layer distribution of temperatures in a reservoir is called direct stratification In winter, as the temperature drops, reverse stratification. The surface layer of water has a temperature close to 0°C. At the bottom the temperature is about 4°C, which corresponds to its maximum density. Thus, the temperature increases with depth. This phenomenon is called temperature dichotomy. It is observed in most of our lakes in summer and winter. As a result, vertical circulation is disrupted, density stratification of water is formed, and a period of temporary stagnation begins - stagnation(Fig. 8).

With a further increase in temperature, the upper layers of water become less and less dense and no longer sink - summer stagnation sets in.

In autumn, surface waters cool again to 4°C and sink to the bottom, causing a second mixing of masses in the year with temperature equalization, i.e., the onset of autumn homothermy.

IN marine environment There is also thermal stratification determined by depth. The oceans have the following layers Surface- water is exposed to the action of wind, and by analogy with the atmosphere this layer is called troposphere or sea thermosphere. Daily fluctuations in water temperature are observed here to approximately 50 meters depth, and seasonal fluctuations are observed even deeper. The thickness of the thermosphere reaches 400 m. Intermediate -- represents constant thermocline. The temperature in it is different seas and oceans drops to 1--3°C. Extends to a depth of approximately 1500 m. Deep-sea -- characterized by a uniform temperature of about 1--3°C, with the exception of the polar regions, where the temperature is close to 0°C.

IN In general, it should be noted that the amplitude of annual temperature fluctuations in the upper layers of the ocean is no more than 10-15 °C; in continental waters it is 30--35 °C.

Rice. 8.

Deep layers of water are characterized by constant temperature. In equatorial waters, the average annual temperature of surface layers is 26-27°C, in polar waters it is about 0°C and below. The exception is thermal springs, where the temperature of the surface layer reaches 85--93°C.

In water as a living environment, on the one hand, there is a fairly significant variety of temperature conditions, and on the other hand, thermodynamic features of the aquatic environment, such as high specific heat, high thermal conductivity and expansion during freezing (in this case, ice forms only on top, and the main water column does not freeze), create favorable conditions for living organisms.

So, for wintering perennial hydrophytes in rivers and lakes great importance has a vertical temperature distribution under the ice. The densest and least cold water with a temperature of 4°C is located in the bottom layer, where the wintering buds (turions) of hornwort, bladderwort, waterwort, etc. sink (Fig. 9), as well as whole leafy plants, such as duckweed and elodea.

Rice. 9.

The opinion has been established that immersion is associated with the accumulation of starch and the weighting of plants. By spring, starch is converted into soluble sugars and fats, which makes the buds lighter and allows them to float.

Organisms in water bodies of temperate latitudes are well adapted to seasonal vertical movements of water layers, spring and autumn homothermy, and summer and winter stagnation. Since the temperature regime of water bodies is characterized by great stability, stenothermy is common among aquatic organisms to a greater extent than among terrestrial organisms.

Eurythermal species are found mainly in shallow continental reservoirs and in the littoral zone of seas of high and temperate latitudes, where daily and seasonal fluctuations are significant.

Density of water. Water differs from air in being more dense. In this respect, it is 800 times superior to the air. The density of distilled water at a temperature of 4 °C is 1 g/cm3. The density of natural waters containing dissolved salts can be greater: up to 1.35 g/cm 3 . On average, in the water column, for every 10 m of depth, pressure increases by 1 atmosphere. The high density of water is reflected in the body structure of hydrophytes. Thus, if in terrestrial plants mechanical tissues are well developed, providing the strength of trunks and stems, the arrangement of mechanical and conductive tissues along the periphery of the stem creates a “pipe” structure that is well resistant to kinks and bends, then in hydrophytes the mechanical tissues are greatly reduced, since the plants are supported by themselves. water. Mechanical elements and conductive bundles are quite often concentrated in the center of the stem or leaf petiole, which gives it the ability to bend with water movements.

Submerged hydrophytes have good buoyancy created by special devices (air sacs, swellings). Thus, the frog leaves lie on the surface of the water and under each leaf they have a floating bubble filled with air. Like a tiny life jacket, the bubble allows the leaf to float on the surface of the water. Air chambers in the stem keep the plant upright and deliver oxygen to the roots.

Buoyancy also increases with increasing body surface area. This is clearly visible in microscopic planktonic algae. Various outgrowths of the body help them “float” freely in the water column.

Organisms in the aquatic environment are distributed throughout its entire thickness. For example, in oceanic depressions, animals are found at depths of over 10,000 m and endure pressure from several to hundreds of atmospheres. Thus, freshwater inhabitants (diving beetles, slippers, suvoikas, etc.) can withstand up to 600 atmospheres in experiments. Holothurians of the genus Elpidia and worms Priapulus caudatus live from the coastal zone to the ultra-abyssal zone. At the same time, it should be noted that many inhabitants of the seas and oceans are relatively stenobatic and confined to certain depths. This applies primarily to shallow- and deep-sea species. Only the littoral zone is inhabited by the annelid worm Arenicola and mollusks - limpets (Patella). At great depths at a pressure of at least 400-500 atmospheres, fish from the group of anglers, cephalopods, crustaceans, starfish, pogonophora and others are found.

The density of water allows animal organisms to rely on it, which is especially important for non-skeletal forms. The support of the medium serves as a condition for floating in water. It is to this way of life that many aquatic organisms are adapted.

Light mode. Aquatic organisms are greatly influenced by light conditions and water transparency. The intensity of light in water is greatly weakened (Fig. 10), since part of the incident radiation is reflected from the surface of the water, while the other is absorbed by its thickness. The attenuation of light is related to the transparency of the water. In oceans, for example, with great transparency, about 1% of radiation still falls to a depth of 140 m, and in small lakes with somewhat closed water, already to a depth of 2 m, only tenths of a percent.

Rice. 10.

Depth: 1 -- on the surface; 2--0.5m; 3-- 1.5m; 4--2m

Due to the fact that the rays of different parts of the solar spectrum are absorbed differently by water, the spectral composition of light also changes with depth, and the red rays are weakened. Blue-green rays penetrate to considerable depths. The twilight in the ocean, which thickens with depth, is first green, then blue, indigo, blue-violet, later giving way to constant darkness. Accordingly, living organisms replace each other with depth.

Thus, plants living on the surface of the water do not experience a lack of light, while submerged and especially deep-sea plants are classified as “shadow flora”. They have to adapt not only to the lack of light, but also to changes in its composition by producing additional pigments. This can be seen in the known pattern of coloration in algae living at different depths. In shallow water zones, where plants still have access to red rays, which are absorbed to the greatest extent by chlorophyll, green algae tend to predominate. In deeper zones there are brown algae, which, in addition to chlorophyll, contain brown pigments phycaffeine, fucoxanthin, etc. Red algae containing the pigment phycoerythrin live even deeper. The ability to capture solar rays of different wavelengths is clearly visible here. This phenomenon is called chromatic adaptation.

Deep-sea species have a number of physical traits characteristic of shade plants. Among them, it is worth noting the low point of compensation for photosynthesis (30-100 lux), the “shadow nature” of the light curve of photosynthesis with a low saturation plateau; algae, for example, have large chromatophores. Whereas for the surface and floating forms these curves are of a “lighter” type.

To use weak light in the process of photosynthesis, an increased area of ​​assimilating organs is required. Thus, arrowhead (Sagittaria sagittifolia) forms leaves of different shapes when developing on land and in water.

The hereditary program encodes the possibility of development in both directions. The “trigger mechanism” for the development of “water” forms of leaves is shading, and not the direct action of water.

Often the leaves of aquatic plants, immersed in water, are strongly dissected into narrow thread-like lobes, as, for example, in hornwort, uruti, bladderwort, or have a thin translucent plate - underwater leaves of egg capsules, water lilies, leaves of submerged pondweeds.

These features are also characteristic of algae, such as filamentous algae, dissected thalli of Characeae, and thin transparent thalli of many deep-sea species. This makes it possible for hydrophytes to increase the ratio of body area to volume, and therefore to develop a larger surface area at a relatively low cost of organic mass.

In plants partially submerged in water, the heterophilia, i.e., the difference in the structure of above-water and underwater leaves of the same plant: This is clearly visible in the aquatic buttercup (Fig. 11). The above-water ones have features common to the leaves of above-ground plants (dorsoventral structure, well-developed integumentary tissues and stomatal apparatus) , underwater - very thin or dissected leaf blades. Heterophily was also noted in water lilies and egg capsules, arrowheads and other species.

Rice. eleven.

Leaves: 1 - above-water; 2 -- underwater

An illustrative example is the caddisfly (Simn latifolium), on the stem of which you can see several forms of leaves, reflecting all the transitions from typically terrestrial to typically aquatic.

The depth of the aquatic environment also affects animals, their color, species composition etc. For example, in a lake ecosystem, the main life is concentrated in the layer of water, into which the amount of light sufficient for photosynthesis penetrates. The lower boundary of this layer is called the compensation level. Above this depth, plants release more oxygen than they consume, and the excess oxygen can be used by other organisms. Below this depth, photosynthesis cannot provide respiration; therefore, only oxygen is available to organisms, which comes with water from the more surface layers of the lake.

Brightly and variously colored animals live in light, surface layers of water, while deep-sea species are usually devoid of pigments. In the twilight zone of the ocean, animals live that are colored with a reddish tint, which helps them hide from enemies, since the red color in blue-violet rays is perceived as black. Red coloring is characteristic of twilight zone animals such as sea bass, red coral, various crustaceans, etc.

The absorption of light in water is stronger, the lower its transparency, which is due to the presence of mineral particles (clay, silt) in it. Water transparency also decreases with the rapid growth of aquatic vegetation in the summer or with the mass reproduction of small organisms suspended in the surface layers. Transparency is characterized by extreme depth, where a specially lowered Secchi disk (a white disk with a diameter of 20 cm) is still visible. In the Sargasso Sea (the clearest waters), the Secchi disk is visible to a depth of 66.5 m, in the Pacific Ocean - up to 59, in the Indian Ocean - up to 50, in shallow seas - up to 5-15 m. The transparency of rivers does not exceed 1 - 1.5 m, and in the Central Asian rivers Amu Darya and Syr Darya - several centimeters. Hence, the boundaries of photosynthesis zones vary greatly in different bodies of water. In the cleanest waters, the photosynthetic zone, or euphotic zone, reaches a depth of no more than 200 m, the twilight (disphotic) zone extends to 1000-1500 m, and deeper into the aphotic zone, sunlight does not penetrate at all.

Daylight hours in water are much shorter (especially in deep layers) than on land. The amount of light in the upper layers of reservoirs varies with the latitude of the area and the time of year. Thus, long polar nights greatly limit the time suitable for photosynthesis in the Arctic and Antarctic basins, and ice cover makes it difficult for light to access all frozen water bodies in winter.

Salt regime. The salinity of water or salt regime plays an important role in the life of aquatic organisms. The chemical composition of waters is formed under the influence of natural historical and geological conditions, as well as anthropogenic impact. Content chemical compounds(salts) in water determines its salinity and is expressed in grams per liter or in per mile(°/od). According to the general mineralization, waters can be divided into fresh with a salt content of up to 1 g/l, brackish (1-25 g/l), sea salinity (26-50 g/l) and brines (more than 50 g/l). The most important solutes in water are carbonates, sulfates and chlorides (Table 1).

Table 1

Composition of basic salts in various reservoirs (according to R. Dazho, 1975)

Among fresh waters, there are many that are almost pure, but there are also many that contain up to 0.5 g of dissolved substances per liter. Cations according to their content in fresh water are arranged as follows: calcium - 64%, magnesium - 17%, sodium - 16%, potassium - 3%. These are average values, and in each specific case fluctuations, sometimes significant, are possible.

An important element in fresh water is calcium content. Calcium may act as a limiting factor. There are “soft” waters, low in calcium (less than 9 mg per 1 liter), and “hard” waters, which contain large amounts of calcium (more than 25 mg per 1 liter).

In sea water, the average content of dissolved salts is 35 g/l, in marginal seas it is much lower. 13 metalloids and at least 40 metals have been found in seawater. In terms of importance, table salt ranks first, then barium chloride, magnesium sulfate and potassium chloride.

Most aquatic life poikilosmotic. Osmotic pressure in their body depends on salinity environment. Freshwater animals and plants live in environments where the concentration of dissolved substances is lower than in body fluids and tissues. Due to the difference in osmotic pressure outside and inside the body, water constantly penetrates into the body, as a result of which fresh water aquatic organisms are forced to intensively remove it. They have well-expressed osmoregulation processes. In protozoa this is achieved by the work of excretory vacuoles, in multicellular organisms by removing water through the excretory system. Some ciliates secrete an amount of water equal to their body volume every 2-2.5 minutes.

With increasing salinity, the work of vacuoles slows down, and at a salt concentration of 17.5% it stops working, since the difference in osmotic pressure between the cells and the external environment disappears.

The concentration of salts in the body fluids and tissues of many marine organisms is isotonic with the concentration of dissolved salts in the surrounding water. In this regard, their osmoregulatory functions are less developed than in freshwater animals. Osmoregulation is one of the reasons that many sea ​​plants and the animals failed to populate fresh water bodies and turned out to be typical marine inhabitants: coelenterata (Coelenterata), echinoderms (Echinodermata), sponges (Spongia), tunicates (Tunicata), pogonophora (Pogonophora). On the other hand, insects practically do not live in the seas and oceans, while freshwater basins are abundantly populated by them. Typically marine and typically freshwater organisms do not tolerate significant changes in salinity and are stenohaline. Euryhaline There are not so many organisms, in particular animals, of freshwater and marine origin. They are found, often in large quantities, in brackish waters. These are such as bream (Abramis brama), freshwater pike perch (Stizostedion lucioperca), pike (Ezox lucios), and from the sea - the mullet family (Mugilidae).

The habitation of plants in an aquatic environment, in addition to the features listed above, leaves an imprint on other aspects of life, especially on the water regime of plants literally surrounded by water. Such plants do not have transpiration, and therefore there is no “upper engine” that maintains the flow of water in the plant. And at the same time, the current delivering to the tissues nutrients, exists (though much weaker than in land plants), with a clearly defined daily periodicity: more during the day, absent at night. An active role in its maintenance belongs to root pressure (in attached species) and the activity of special cells that secrete water - water stomata or hydathodes.

In fresh waters, plants fixed on the bottom of the reservoir are common. Often their photosynthetic surface is located above the water. These include reeds (Scirpus), water lilies (Nymphaea), egg capsules (Nyphar), cattails (Typha), arrowhead (Sagittaria). In others, the photosynthetic organs are submerged in water. These are pondweed (Potamogeton), urut (Myriophyllum), elodea (Elodea). Selected species higher plants fresh waters are devoid of roots and freely float or overgrow underwater objects, algae, which are attached to the ground.

Gas mode. The main gases in the aquatic environment are oxygen and carbon dioxide. The rest, such as hydrogen sulfide or methane, are of secondary importance.

Oxygen for the aquatic environment it is the most important environmental factor. It enters water from the air and is released by plants during photosynthesis. The diffusion coefficient of oxygen in water is approximately 320 thousand times lower than in air, and its total content in the upper layers of water is 6-8 ml/l, or 21 times lower than in the atmosphere. The oxygen content in water is inversely proportional to temperature. As the temperature and salinity of water increase, the concentration of oxygen in it decreases. In layers heavily populated by animals and bacteria, oxygen deficiency may occur due to increased oxygen consumption. Thus, in the World Ocean, life-rich depths from 50 to 1000 m are characterized by a sharp deterioration in aeration. It is 7-10 times lower than in surface waters inhabited by phytoplankton. Conditions near the bottom of reservoirs can be close to anaerobic.

With stagnation in small bodies of water, the water is also sharply depleted of oxygen. Its deficiency can also occur in winter under ice. At a concentration below 0.3-3.5 ml/l, the life of aerobes in water is impossible. The oxygen content under reservoir conditions turns out to be a limiting factor (Table 2).

table 2

Oxygen requirements for different species of freshwater fish

Among aquatic inhabitants there are a significant number of species that can tolerate wide fluctuations in oxygen content in water, close to its absence. These are the so-called euryoxybionts. These include freshwater oligochaetes (Tubifex tubifex), gastropods (Viviparus viviparus). Carp, tench, and crucian carp can withstand very low oxygen saturation of fish from fish. However, many species are stenoxybiont, that is, they can exist only with a sufficiently high saturation of water with oxygen, for example, rainbow trout, brown trout, minnow, etc. Many species of living organisms are capable of falling into an inactive state, the so-called anoxybiosis, and thus experience an unfavorable period.

Respiration of aquatic organisms occurs both through the surface of the body and through specialized organs - gills, lungs, trachea. Often the integument of the body can serve as an additional respiratory organ. In some species, a combination of aquatic and air respiration occurs, for example, lungfishes, siphonophores, discophants, many pulmonary mollusks, crustaceans Yammarus lacustris, etc. Secondary aquatic animals usually retain the atmospheric type of respiration as energetically more favorable, and therefore require contact with the air environment. These include pinnipeds, cetaceans, water beetles, mosquito larvae, etc.

Carbon dioxide. In the aquatic environment, living organisms, in addition to a lack of light and oxygen, may lack available CO 2, for example, plants for photosynthesis. Carbon dioxide enters water as a result of the dissolution of CO 2 contained in the air, the respiration of aquatic organisms, the decomposition of organic residues and release from carbonates. The carbon dioxide content in water ranges from 0.2-0.5 ml/l, or 700 times more than in the atmosphere. CO 2 dissolves in water 35 times better than oxygen. Sea water is the main reservoir of carbon dioxide, as it contains from 40 to 50 cm 3 of gas per liter in free or bound form, which is 150 times higher than its concentration in the atmosphere.

Carbon dioxide contained in water takes part in the formation of calcareous skeletal formations of invertebrate animals and ensures photosynthesis of aquatic plants. With intensive photosynthesis of plants, there is an increased consumption of carbon dioxide (0.2-0.3 ml/l per hour), which leads to its deficiency. Hydrophytes respond to an increase in CO 2 content in water by increasing photosynthesis.

An additional source of CO for photosynthesis of aquatic plants is also carbon dioxide, which is released during the decomposition of bicarbonate salts and their transformation into carbon dioxide:

Ca(HCO 3) 2 CaCO 3 + CO, + H 2 O

The poorly soluble carbonates that form in this case settle on the surface of the leaves in the form of a limescale or crust, which is clearly visible when many aquatic plants dry out.

Hydrogen ion concentration(pH) often affects the distribution of aquatic organisms. Freshwater pools with a pH of 3.7-4.7 are considered acidic, 6.95-7.3 neutral, and with a pH of more than 7.8 alkaline. In fresh water bodies, pH experiences significant fluctuations, often during the day. Sea water is more alkaline and its pH changes less than fresh water. pH decreases with depth.

From plants with a pH less than 7.5, grasshopper (Jsoetes) and hogweed (Sparganium) grow. In an alkaline environment (pH 7.7-8.8), many species of pondweed and elodea are common; at pH 8.4-9, Typha angustifolia reaches strong development. The acidic waters of peat bogs promote the development of sphagnum mosses.

Most freshwater fish can withstand a pH of 5 to 9. If the pH is less than 5, there is a massive death of fish, and above 10, all fish and other animals die.

In lakes with an acidic environment, larvae of dipterans of the genus Chaoborus are often found, and in the acidic waters of swamps, shell rhizomes (Testaceae) are common, elasmobranch mollusks of the genus Unio are absent, and other mollusks are rarely found.

Ecological plasticity of organisms in the aquatic environment. Water is a more stable environment, and abiotic factors undergo relatively minor fluctuations, and hence aquatic organisms have less ecological plasticity compared to terrestrial ones. Freshwater plants and animals are more plastic than marine ones, since freshwater as a living environment is more variable. The breadth of ecological plasticity of aquatic organisms is assessed not only as a whole to a complex of factors (eury- and stenobionticity), but also individually.

Thus, it has been established that coastal plants and animals, in contrast to the inhabitants of open zones, are mainly eurythermic and euryhaline organisms, due to the fact that the temperature conditions and salt regime near the shore are quite variable - warming by the sun and relatively intense cooling, desalination by the influx of water from streams and rivers, in particular during the rainy season, etc. An example is the lotus, which is a typical stenothermic species and grows only in shallow, well-warmed reservoirs. Inhabitants of the surface layers, compared to deep-sea forms, for the above reasons, turn out to be more eurythermic and euryhaline.

Ecological plasticity is an important regulator of the dispersal of organisms. It has been proven that aquatic organisms with high ecological plasticity are widespread, for example, Elodea. The opposite example is the brine shrimp (Artemia solina), which lives in small reservoirs with very salty water and is a typical stenohaline representative with narrow ecological plasticity. In relation to other factors, it has significant plasticity and is found quite often in salt water bodies.

Ecological plasticity depends on the age and developmental phase of the organism. For example, the marine gastropod Littorina, as an adult, goes without water for a long time every day during low tides, but its larvae lead a planktonic lifestyle and cannot tolerate drying out.

Features of plant adaptation to the aquatic environment. Water paradise| Sthenias have significant differences from terrestrial plant organisms. Thus, the ability of aquatic plants to absorb moisture and mineral salts directly from the surrounding environment is reflected in their morphological and physiological organization. Characteristic of aquatic plants is the poor development of conductive tissue and the root system. The root system serves mainly for attachment to the underwater substratum and does not perform the functions of mineral nutrition and water supply, as in terrestrial plants. Aquatic plants feed on the entire surface of their body.

The significant density of water makes it possible for plants to inhabit its entire thickness. Lower plants that inhabit various layers and lead a floating lifestyle have special appendages for this purpose that increase their buoyancy and allow them to remain suspended. Higher hydrophytes have poorly developed mechanical tissue. How yni As noted above, in their leaves, stems, and roots there are air-bearing intercellular cavities that increase the lightness and buoyancy of organs suspended in water and floating on the surface, which also contributes to the washing away of the internal cells by water with salts and gases dissolved in it. Hydrophytes are distinguished| They have a large leaf surface with a small total volume of the plant, which provides them with intense gas exchange with a lack of oxygen and other gases dissolved in water.

A number of aquatic organisms have developed diversity of leaves, or heterophilia. Thus, in Salvinia, submerged leaves provide mineral nutrition, while floating leaves provide organic nutrition.

An important feature of plant adaptation to living in water | This environment is also due to the fact that leaves immersed in water are usually very thin. Often the chlorophyll in them is located in the epidermal cells, which helps to increase the intensity of photosynthesis in low light. Such anatomical and morphological features are most clearly expressed in water mosses (Riccia, Fontinalis), Vallisneria spiralis, and pondweeds (Potamageton).

The protection against leaching or leaching of mineral salts from the cells of aquatic plants is the secretion of mucus by special cells and the formation of endoderm from thicker-walled cells in the form of a ring.

The relatively low temperature of the aquatic environment causes the death of vegetative parts of plants immersed in water after the formation of winter buds and the replacement of thin, tender summer leaves with tougher and shorter winter leaves. Low temperature water negatively affects the generative organs of aquatic plants, and its high density makes it difficult to transfer pollen. In this regard, aquatic plants reproduce intensively by vegetative means. Most floating and submerged plants carry flowering stems into the air and reproduce sexually. Pollen is carried by wind and surface currents. The fruits and seeds that are produced are also distributed by surface currents. This phenomenon is called hydrochoria. Hydrochorous plants include not only aquatic plants, but also many coastal plants. Their fruits are highly buoyant, remain in water for a long time and do not lose their germination. For example, water transports the fruits and seeds of arrowhead (Sagittaria sagittofolia), commonweed (Butomus umbellatus), and chastukha (Alisma plantago-aguatica). The fruits of many sedges (Carex) are enclosed in peculiar air sacs and are carried by water currents. In the same way, the humai weed (Sorgnum halepense) spread along the Vakht River along the canals.

Features of animal adaptation to the aquatic environment. In animals living in an aquatic environment, compared to plants, adaptive features are more diverse, these include such as anatomical-morphological, behavioral and etc.

Animals that live in the water column primarily have adaptations that increase their buoyancy and allow them to resist the movement of water and currents. These organisms develop adaptations that prevent them from rising into the water column or reduce their buoyancy, which allows them to stay at the bottom, including fast-flowing waters.

In small forms living in the water column, a reduction in skeletal formations is noted. Thus, in protozoa (Radiolaria, Rhizopoda), the shells are porous, and the flint spines of the skeleton are hollow inside. The specific density of ctenophora and jellyfish (Scyphozoa) decreases due to the presence of water in the tissues. The accumulation of fat droplets in the body (nightlights - Noctiluca, radiolarians - Radiolaria) helps to increase buoyancy. Large accumulations of fat are observed in some crustaceans (Cladocera, Copepoda), fish and cetaceans. The specific density of the body is reduced and thereby increased buoyancy by gas-filled swim bladders, which many fish have. Siphonophores (Physalia, Velella) have powerful air cavities.

Animals that passively swim in the water column are characterized not only by a decrease in mass, but also by an increase in the specific surface area of ​​the body. This is due to the fact that the greater the viscosity of the medium and the higher the specific surface area of ​​the body of the organism, the slower it sinks into water. In animals, the body is flattened, spines, outgrowths, and appendages are formed on it, for example, in flagellates (Leptodiscus, Craspeditella), radiolarians (Aulacantha, Chalengeridae), etc.

A large group of animals that live in fresh water use the surface tension of water (surface film) when moving. Water strider bugs (Gyronidae, Veliidae), whirling beetles (Gerridae), etc. run freely across the surface of the water. Arthropods touching the water with the ends of their appendages covered with water-repellent hairs cause deformation of its surface with the formation of a concave meniscus. When the lifting force (F) directed upward is greater than the mass of the animal, the latter will be held on the water due to surface tension.

Thus, life on the surface of water is possible for relatively small animals, since mass increases in proportion to the cube of size, and surface tension increases as a linear value.

Active swimming in animals is carried out with the help of cilia, flagella, bending of the body, and in a reactive manner due to the energy of the ejected stream of water. The greatest perfection of the reactive method of locomotion is achieved by cephalopods. Thus, some squids develop speeds of up to 40-50 km/h when throwing out water (Fig. 12).

Rice. 12.

Large animals often have specialized limbs (fins, flippers), their body is streamlined and covered with mucus.

Only in the aquatic environment are motionless animals leading an attached lifestyle found. These are such as hydroids (Hydroidea) and coral polyps(Anthozoo), sea lilies (Crinoidea), bivalves (Br/aMa), etc. They are characterized by a peculiar body shape, slight buoyancy (the density of the body is greater than the density of water) and special devices for attachment to the substrate.

Aquatic animals are mostly poikilothermic. In homoothermic mammals (cetaceans, pinnipeds), for example, a significant layer of subcutaneous fat is formed, which performs a thermal insulation function.

Deep-sea animals are distinguished by specific organizational features: the disappearance or weak development of the calcareous skeleton, an increase in body size, often a reduction in the organs of vision, increased development of tactile receptors, etc.

Osmotic pressure and ionic state solutions in the body of animals is provided by complex mechanisms of water-salt metabolism. The most common way to maintain constant osmotic pressure is to regularly remove water entering the body using pulsating vacuoles and excretory organs. So, freshwater fish excess water is removed hard work excretory system, and salts are absorbed through the gill filaments. Marine fish are forced to replenish their water reserves and therefore drink sea water, and excess salts supplied with water are removed from the body through the gill filaments (Fig. 13).

Rice. 13.

The abbreviations hypo-, iso- and hyper- indicate the tonicity of the internal environment in relation to the external one (from N. Green et al., 1993)

A number of hydrobionts have a special feeding pattern - this is the filtering or sedimentation of particles of organic origin suspended in water, numerous small organisms. This method of feeding does not require large expenditures of energy in search of prey and is typical for elasmobranch mollusks, sessile echinoderms, ascidians, planktonic crustaceans, etc. Filter-feeding animals play an important role in biological treatment reservoirs.

Freshwater daphnia, cyclops, as well as the most abundant crustacean in the ocean, Calanus finmarchicus, filter up to 1.5 liters of water per individual per day. Mussels living on an area of ​​1 m 2 can drive 150-280 m 3 of water per day through the mantle cavity, precipitating suspended particles.

Due to the rapid attenuation of light rays in water, life in constant twilight or darkness greatly limits the visual orientation capabilities of aquatic organisms. Sound travels faster in water than in air, and aquatic organisms have a better-developed visual orientation to sound. Some species even detect infrasounds. Sound signaling serves most of all for intraspecific relationships: orientation in a flock, attracting individuals of the opposite sex, etc. Cetaceans, for example, look for food and orient themselves using echolocation - the perception of reflected sound waves. The principle of the dolphin locator is to emit sound waves that travel in front of the swimming animal. When encountering an obstacle, such as a fish, the sound waves are reflected and returned to the dolphin, which hears the resulting echo and thus detects the object causing the sound reflection.

About 300 species of fish are known that are capable of generating electricity and using it for orientation and signaling. A number of fish (electric stingray, electric eel, etc.) use electric fields for defense and attack.

Characteristic of aquatic organisms ancient way orientation - perception of the chemistry of the environment. The chemoreceptors of many hydrobionts (salmon, eels, etc.) are extremely sensitive. In migrations of thousands of kilometers, they find spawning and feeding grounds with amazing accuracy.

Changing conditions in the aquatic environment also causes certain behavioral reactions of organisms. Changes in illumination, temperature, salinity, gas regime and other factors are associated with vertical (descending into the depths, rising to the surface) and horizontal (spawning, wintering and feeding) migrations of animals. In the seas and oceans, millions of tons of aquatic organisms take part in vertical migrations, and during horizontal migrations, aquatic animals can travel hundreds and thousands of kilometers.

There are many temporary, shallow bodies of water on Earth that appear after river floods, heavy rains, snow melting, etc. General Features inhabitants of drying up reservoirs is the ability to give birth to numerous offspring in a short time and endure long periods without water, passing into a state of reduced vital activity - hypobiosis.

Aquatic habitat

HABITAT AND THEIR CHARACTERISTICS

In the process of historical development, living organisms have mastered four habitats. The first is water. Life originated and developed in water for many millions of years. The second - ground-air - plants and animals arose on land and in the atmosphere and rapidly adapted to new conditions. Gradually transforming the upper layer of land - the lithosphere, they created a third habitat - soil, and themselves became the fourth habitat.

Aquatic habitat

Water covers 71% of the earth's area. The bulk of water is concentrated in the seas and oceans - 94-98%, polar ice contains about 1.2% of water and a very small proportion - less than 0.5%, in fresh waters of rivers, lakes and swamps.

About 150,000 species of animals and 10,000 plants live in the aquatic environment, which is respectively only 7 and 8% of the total number of species on Earth.

In the seas-oceans, as in the mountains, vertical zoning is expressed. The pelagic - the entire water column - and the benthic - the bottom - differ especially greatly in ecology. The water column, the pelagic zone, is vertically divided into several zones: epipeligal, bathypeligal, abyssopeligal and ultraabyssopeligal(Fig. 2).

Depending on the steepness of the descent and the depth at the bottom, several zones are also distinguished, which correspond to the indicated pelagic zones:

Littoral - the edge of the coast that is flooded during high tides.

Supralittoral - the part of the coast above the upper tidal line where surf splashes reach.

Sublittoral - a gradual decrease in land up to 200m.

Bathial - a steep depression of land (continental slope),

Abyssal - a gradual decrease in the bottom of the ocean floor; the depth of both zones together reaches 3-6 km.

Ultra-abyssal - deep-sea depressions from 6 to 10 km.

Ecological groups of hydrobionts. The greatest diversity of life is found in warm seas and oceans (40,000 species of animals) in the equator and tropics, to the north and south the flora and fauna of the seas is hundreds of times depleted. As for the distribution of organisms directly in the sea, the bulk of them are concentrated in the surface layers (epipelagic) and in the sublittoral zone. Depending on the method of movement and stay in certain layers, Marine life are divided into three ecological groups: nekton, plankton and benthos.

Nekton (nektos - floating) - actively moving large animals that can overcome long distances and strong currents: fish, squid, pinnipeds, whales. In fresh water bodies, nekton includes amphibians and many insects.

Plankton (planktos - wandering, soaring) - a collection of plants (phytoplankton: diatoms, green and blue-green (fresh water bodies only) algae, plant flagellates, peridineans, etc.) and small animal organisms (zooplankton: small crustaceans, of the larger ones - pteropods mollusks, jellyfish, ctenophores, some worms) living at different depths, but not capable of active movement and resistance to currents. Plankton also includes animal larvae, forming a special group - Neuston . This is a passively floating “temporary” population of the uppermost layer of water, represented by various animals (decapods, barnacles and copepods, echinoderms, polychaetes, fish, mollusks, etc.) in the larval stage. The larvae, growing up, move into the lower layers of the pelagel. Above the neuston is located plaiston - these are organisms in which the upper part of the body grows above water, and the lower part in water (duckweed - Lemma, siphonophores, etc.). Plankton plays an important role in the trophic relationships of the biosphere, because is food for many aquatic inhabitants, including the main food for baleen whales (Myatcoceti).

Benthos (benthos – depth) – bottom hydrobionts. It is represented mainly by attached or slowly moving animals (zoobenthos: foraminephores, fish, sponges, coelenterates, worms, mollusks, ascidians, etc.), more numerous in shallow water. In shallow water, benthos also includes plants (phytobenthos: diatoms, green, brown, red algae, bacteria). At depths where there is no light, phytobenthos is absent. Rocky areas of the bottom are richest in phytobenthos.

In lakes, zoobenthos is less abundant and diverse than in the sea. It is formed by protozoa (ciliates, daphnia), leeches, mollusks, insect larvae, etc. The phytobenthos of lakes is formed by free-floating diatoms, green and blue-green algae; brown and red algae are absent.

The high density of the aquatic environment determines the special composition and nature of changes in life-supporting factors. Some of them are the same as on land - heat, light, others are specific: water pressure (increases with depth by 1 atm for every 10 m), oxygen content, salt composition, acidity. Due to the high density of the environment, the values ​​of heat and light change much faster with an altitude gradient than on land.

Thermal mode. The aquatic environment is characterized by less heat gain, because a significant part of it is reflected, and an equally significant part is spent on evaporation. Consistent with the dynamics of land temperatures, water temperatures exhibit smaller fluctuations in daily and seasonal temperatures. Moreover, reservoirs significantly equalize the temperature in the atmosphere of coastal areas. In the absence of an ice shell, the seas have a warming effect on the adjacent land areas in the cold season, and a cooling and moistening effect in the summer.

The range of water temperatures in the World Ocean is 38° (from -2 to +36°C), in fresh water bodies – 26° (from -0.9 to +25°C). With depth, the water temperature drops sharply. Up to 50 m there are daily temperature fluctuations, up to 400 – seasonal, deeper it becomes constant, dropping to +1-3°C. Since the temperature regime in reservoirs is relatively stable, their inhabitants tend to stenothermicity.

Due to to varying degrees heating of the upper and lower layers throughout the year, ebbs and flows, currents, and storms constantly mix the water layers. The role of water mixing for aquatic inhabitants is extremely important, because at the same time, the distribution of oxygen and nutrients within reservoirs is equalized, ensuring metabolic processes between organisms and the environment.

In stagnant reservoirs (lakes) of temperate latitudes, vertical mixing takes place in spring and autumn, and during these seasons the temperature throughout the reservoir becomes uniform, i.e. comes homothermy. In summer and winter as a result of a sharp increase in heating or cooling upper layers mixing of water stops. This phenomenon is called temperature dichotomy, and the period of temporary stagnation is stagnation(summer or winter). In summer, lighter warm layers remain on the surface, located above heavy cold ones (Fig. 3). In winter, on the contrary, there is warmer water in the bottom layer, since directly under the ice the temperature of surface waters is less than +4°C and they, due to physical and chemical properties waters become lighter than water with a temperature above +4°C.

During periods of stagnation, three layers are clearly distinguished: the upper (epilimnion) with the sharpest seasonal fluctuations in water temperature, the middle (metalimnion or thermocline), in which there is a sharp jump in temperature, and bottom ( hypolimnion), in which the temperature varies little throughout the year. During periods of stagnation, oxygen deficiency occurs in the water column - in the bottom part in summer, and in the upper part in winter, as a result of which fish kills often occur in winter.

Light mode. The intensity of light in water is greatly weakened due to its reflection by the surface and absorption by the water itself. This greatly affects the development of photosynthetic plants.

The absorption of light is stronger, the lower the transparency of the water, which depends on the number of particles suspended in it (mineral suspensions, plankton). It decreases with the rapid development of small organisms in summer, and in temperate and northern latitudes even in winter, after the establishment of ice cover and covering it with snow on top.

Transparency is characterized by the maximum depth at which a specially lowered white disk with a diameter of about 20 cm (Secchi disk) is still visible. The clearest waters are in the Sargasso Sea: the disk is visible to a depth of 66.5 m. In the Pacific Ocean, the Secchi disk is visible up to 59 m, in the Indian Ocean - up to 50, in shallow seas - up to 5-15 m. The transparency of rivers is on average 1-1.5 m, and in the muddiest rivers only a few centimeters.

In the oceans, where the water is very transparent, 1% of light radiation penetrates to a depth of 140 m, and in small lakes at a depth of 2 m only tenths of a percent penetrates. Rays different parts spectrum are absorbed differently in water; red rays are absorbed first. With depth it becomes darker, and the color of the water first becomes green, then blue, indigo and finally blue-violet, turning into complete darkness. Hydrobionts also change color accordingly, adapting not only to the composition of light, but also to its lack - chromatic adaptation. In light zones, in shallow waters, green algae (Chlorophyta) predominate, the chlorophyll of which absorbs red rays, with depth they are replaced by brown (Phaephyta) and then red (Rhodophyta). At great depths, phytobenthos is absent.

Plants adapted to the lack of light by developing large chromatophores, as well as increasing the area of ​​assimilating organs (leaf surface index). For deep-sea algae, strongly dissected leaves are typical, the leaf blades are thin and translucent. Semi-submerged and floating plants are characterized by heterophylly - the leaves above the water are the same as those of land plants, they have a solid blade, the stomatal apparatus is developed, and in the water the leaves are very thin, consisting of narrow thread-like lobes.

Animals, like plants, naturally change their color with depth. In the upper layers they are brightly colored in different colors, in the twilight zone (sea bass, corals, crustaceans) they are painted in colors with a red tint - it is more convenient to hide from enemies. Deep-sea species lack pigments. In the dark depths of the ocean, organisms use light emitted by living beings as a source of visual information. bioluminescence.

High density(1 g/cm3, which is 800 times the density of air) and water viscosity ( 55 times higher than that of air) led to the development of special adaptations of aquatic organisms :

1) Plants have very poorly developed or completely absent mechanical tissues - they are supported by water itself. Most are characterized by buoyancy due to air-carrying intercellular cavities. Characterized by active vegetative reproduction, the development of hydrochory - the removal of flower stalks above the water and the distribution of pollen, seeds and spores by surface currents.

2) In animals living in the water column and actively swimming, the body has a streamlined shape and is lubricated with mucus, which reduces friction when moving. Developed devices to increase buoyancy: accumulations of fat in tissues, swim bladders in fish, air cavities in siphonophores. In passively swimming animals, the specific surface area of ​​the body increases due to outgrowths, spines, and appendages; the body is flattened, and skeletal organs are reduced. Different ways locomotion: bending of the body, with the help of flagella, cilia, reactive mode of locomotion (cephalopods).

In benthic animals, the skeleton disappears or is poorly developed, body size increases, vision reduction is common, and tactile organs develop.

Currents. A characteristic feature of the aquatic environment is mobility. It is caused by ebbs and flows, sea currents, storms, at different levels elevation marks of river beds. Adaptations of hydrobionts:

1) In flowing reservoirs, plants are firmly attached to stationary underwater objects. The bottom surface is primarily a substrate for them. These are green and diatom algae, water mosses. Mosses even form a dense cover on fast river riffles. In the tidal zone of the seas, many animals have devices for attaching to the bottom (gastropods, barnacles), or hide in crevices.

2) Fish in running waters have a round body in diameter, while fish that live near the bottom, like bottom-dwelling invertebrate animals, have a flat body. Many have attachment organs to underwater objects on the ventral side.

Salinity of water.

Natural reservoirs are characterized by a certain chemical composition. Carbonates, sulfates, and chlorides predominate. In fresh water bodies, the salt concentration is no more than 0.5 ‰ (with about 80% being carbonates), in the seas - from 12 to 35 ‰ (mainly chlorides and sulfates). When the salinity is more than 40 ppm, the water body is called hypersaline or oversaline.

1) In fresh water (hypotonic environment), osmoregulation processes are well expressed. Hydrobionts are forced to constantly remove water penetrating into them; they are homoyosmotic (ciliates “pump” through themselves an amount of water equal to its weight every 2-3 minutes). In salt water (isotonic environment), the concentration of salts in the bodies and tissues of hydrobionts is the same (isotonic) with the concentration of salts dissolved in water - they are poikiloosmotic. Therefore, the inhabitants of salt water bodies do not have developed osmoregulatory functions, and they were unable to populate fresh water bodies.

2) Aquatic plants are able to absorb water and nutrients from water - “broth”, with their entire surface, therefore their leaves are strongly dissected and conductive tissues and roots are poorly developed. The roots serve mainly for attachment to the underwater substrate. Most freshwater plants have roots.

Typically marine and typically freshwater species, stenohaline, do not tolerate significant changes in water salinity. There are few euryhaline species. They are common in brackish waters (freshwater pike perch, pike, bream, mullet, coastal salmon).

The inhabitants of the aquatic environment received a common name in ecology hydrobionts. They inhabit the World Ocean, continental reservoirs and groundwater. In any body of water, zones with different conditions can be distinguished.

In the ocean and its seas, there are primarily two ecological areas: the water column - pelagic and the bottom - benthal. The inhabitants of the abyssal and ultra-abyssal depths exist in darkness, at constant temperature and enormous pressure. The entire population of the ocean floor was named benthos.

Basic properties of the aquatic environment.

Density of water is a factor that determines the conditions for the movement of aquatic organisms and pressure at different depths. For distilled water, the density is 1 g/cm 3 at 4 °C. The density of natural waters containing dissolved salts can be greater, up to 1.35 g/cm 3 . Pressure increases with depth by an average of 1 × 10 5 Pa (1 atm) for every 10 m. The density of water makes it possible to rely on it, which is especially important for non-skeletal forms. The density of the environment serves as a condition for floating in water, and many aquatic organisms are adapted specifically to this way of life. Suspended organisms floating in water are combined into a special ecological group of aquatic organisms - plankton(“planktos” – soaring). The plankton is dominated by unicellular and colonial algae, protozoa, jellyfish, siphonophores, ctenophores, pteropods and keelfoot mollusks, various small crustaceans, larvae of bottom animals, fish eggs and fry, and many others. Seaweed (phytoplankton) hover in water passively, while most planktonic animals are capable of active swimming, but within limited limits.. A special type of plankton is an ecological group Neuston(“nein” - swim) - inhabitants of the surface film of water at the border with the air. The density and viscosity of water greatly influence the possibility of active swimming. Animals capable of fast swimming and overcoming the force of currents are united in an ecological group nekton(“nektos” – floating).

Oxygen regime. In oxygen-saturated water, its content does not exceed 10 ml per 1 liter, which is 21 times lower than in the atmosphere. Therefore, the breathing conditions of aquatic organisms are significantly complicated. Oxygen enters water mainly through the photosynthetic activity of algae and diffusion from the air. Therefore, the upper layers of the water column are, as a rule, richer in this gas than the lower ones. As the temperature and salinity of water increase, the concentration of oxygen in it decreases. In layers heavily populated by animals and bacteria, a sharp deficiency of O 2 can be created due to its increased consumption. Conditions near the bottom of reservoirs can be close to anaerobic.

Among aquatic inhabitants there are many species that can tolerate wide fluctuations in oxygen content in water, up to its almost complete absence (euryoxybionts – “oxy” – oxygen, “biont” – inhabitant). These include, for example, gastropods. Among fish, carp, tench, and crucian carp can withstand very low oxygen saturation of water. However, a number of types stenoxybiont– they can exist only with sufficiently high oxygen saturation of the water (rainbow trout, brown trout, minnow).

Salt regime. Maintaining the water balance of aquatic organisms has its own specifics. If for terrestrial animals and plants it is most important to provide the body with water in conditions of its deficiency, then for hydrobionts it is no less important to maintain a certain amount of water in the body when there is an excess of it in the environment. Excessive amount of water in cells leads to a change in osmotic pressure in them and disruption of the most important vital functions. Most aquatic life poikilosmotic: the osmotic pressure in their body depends on the salinity of the surrounding water. Therefore, the main way for aquatic organisms to maintain their salt balance is to avoid habitats with unsuitable salinity. Freshwater forms cannot exist in the seas, and marine forms cannot tolerate desalination. Vertebrates, higher crustaceans, insects and their larvae living in water belong to homoiosmotic species, maintaining constant osmotic pressure in the body regardless of the concentration of salts in the water.

Light mode. There is much less light in water than in air. Some of the rays incident on the surface of a reservoir are reflected into the air. The reflection is stronger the lower the position of the Sun, so the day under water is shorter than on land. In the dark depths of the ocean, organisms use light emitted by living things as a source of visual information. The glow of a living organism is called bioluminescence. The reactions used to generate light are varied. But in all cases this is the oxidation of complex organic compounds (luciferins) using protein catalysts (luciferase).

Methods of orientation of animals in the aquatic environment. Living in constant twilight or darkness greatly limits your options visual orientation hydrobionts. Due to the rapid attenuation of light rays in water, even those with well-developed visual organs can only use them to navigate at close range.

Sound travels faster in water than in air. Sound orientation is generally better developed in aquatic organisms than visual orientation. A number of species detect even very low frequency vibrations (infrasounds) , arising when the rhythm of waves changes, and descends from the surface layers to deeper ones in advance of the storm (for example, jellyfish). Many inhabitants of water bodies - mammals, fish, mollusks, crustaceans - make sounds themselves. A number of hydrobionts find food and navigate using echolocation– perception of reflected sound waves (cetaceans). Many perceive reflected electrical impulses , producing discharges of different frequencies while swimming. A number of fish also use electric fields for defense and attack (electric stingray, electric eel, etc.).

For orientation in depth it is used perception of hydrostatic pressure. It is carried out using statocysts, gas chambers and other organs.

Filtration as a type of nutrition. Many hydrobionts have a special feeding pattern - this is the filtering or sedimentation of particles of organic origin suspended in water and numerous small organisms.

Body shape. Most hydrobionts have a streamlined body shape.