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Density of water- This is a factor that determines the conditions of 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 higher, up to 1.35 g / cm 3. The pressure increases with depth by about 1 · 10 5 Pa (1 atm) on average for every 10 m.

Due to the sharp pressure gradient in water bodies, aquatic organisms are generally much more eurybatic than terrestrial organisms. Some species, common at different depths, tolerate pressures from several to hundreds of atmospheres. For example, sea cucumbers of the genus Elpidia, Priapulus caudatus worms live from the coastal zone to the ultraabyssal. Even freshwater inhabitants, for example, ciliates, slippers, suvoys, 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 stenobathic and confined to certain depths. Stenobath is most often characteristic of shallow and deep-water species. Only the littoral zone is inhabited by the annelid sandworm Arenicola and the sea saucers (Patella). Many fish, for example from the angler group, cephalopods, crustaceans, pogonophores, starfish, etc. 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 the water provides the ability to rely on it, which is especially important for skeletal forms. The density of the environment serves as a condition for soaring in water, and many aquatic organisms are adapted to this particular way of life. Suspended, floating in water organisms are combined into a special ecological group of aquatic organisms - plankton ("Planktos" - soaring).

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

A - rod-shaped:

1 - diatom Synedra;

2 - cyanobacterium Aphanizomenon;

3 - Peridineal alga Amphisolenia;

4 - Euglena acus;

5 - the cephalopod Doratopsis vermicularis;

6 - copepod Setella;

7 - Porcellana larva (Decapoda)

B - dismembered forms:

1 - mollusk Glaucus atlanticus;

2 - worm Tomopetris euchaeta;

3 - Palinurus crayfish larva;

4 - larva of the fish of the monkfish Lophius;

5 - copepod Calocalanus pavo

Plankton includes unicellular and colonial algae, protozoa, jellyfish, siphonophores, comb jellies, pterygopods and keeled molluscs, various small crustaceans, larvae of benthic 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 devices include: 1) a general increase in the relative surface of the body due to a decrease in size, flattening, elongation, the development of numerous outgrowths or bristles, which increases friction against water; 2) a decrease in density due to the reduction of the skeleton, the accumulation of fats, gas bubbles in the body, etc. In diatoms, reserve substances are deposited not in the form of heavy starch, but in the form of fatty drops. Nightlight Noctiluca is distinguished by such an abundance of gas vacuoles and droplets of fat in the cell that the cytoplasm in it looks like strands that fuse 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 within limited limits. Planktonic organisms cannot overcome currents and are carried over long distances by them. Many kinds zooplankton are capable, however, of vertical migrations in the water column for tens and hundreds of meters, both due to active movement and due to the regulation of the buoyancy of their body. A special type of plankton is the ecological group neuston ("Nein" - to swim) - the inhabitants of the surface film of water at the border with the air environment.

The density and viscosity of water greatly affects the ability to swim actively. Animals capable of fast swimming and overcoming the force of currents are united into an ecological group nekton ("Nektos" - floating). Representatives of nekton are fish, squid, dolphins. Rapid movement in the water column is possible only if there is a streamlined body shape and highly developed muscles. The torpedo 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 mode. In oxygenated water, its content does not exceed 10 ml per 1 liter, which is 21 times lower than in the atmosphere. Therefore, the conditions for the respiration of aquatic organisms are significantly complicated. Oxygen enters the water mainly through the photosynthetic activity of algae and diffusion from the air. Therefore, the upper layers of the water column, as a rule, are richer in this gas than the lower ones. With increasing temperature and salinity of water, the concentration of oxygen in it decreases. In layers heavily populated with animals and bacteria, a sharp O 2 deficiency can be created due to its increased consumption. For example, in the World Ocean, depths from 50 to 1000 m rich in life are characterized by a sharp deterioration in aeration - it is 7-10 times lower than in surface waters inhabited by phytoplankton. Near the bottom of water bodies, conditions can be close to anaerobic.

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

Respiration of aquatic organisms is carried out 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, a loach fish consumes up to 63% of oxygen through the skin on average. If gas exchange occurs through the integuments of the body, then they are very thin. Breathing is also made easier by the increased surface area. This is achieved during the evolution of species by the formation of various outgrowths, flattening, lengthening, and a general decrease in body size. Some species, with a lack of oxygen, actively change the size of the respiratory surface. The Tubifex tubifex worms stretch the body strongly; hydras and anemones - tentacles; echinoderms - ambulacral legs. Many sedentary and sedentary animals renew the water around them, either by creating a directed current of it, or by oscillating movements contributing to its mixing. For this purpose, bivalves are served by cilia lining the walls of the mantle cavity; crustaceans - the work of the abdominal or thoracic legs. Leeches, larvae of bell mosquitoes (bloodworms), many oligochaetes wiggle their bodies, sticking out of the ground.

In some species, a combination of water and air respiration is found. These are lung-breathing fish, siphonophores discophants, many pulmonary mollusks, crustaceans Gammarus lacustris, etc. Secondary animals usually retain the atmospheric type of respiration as energetically more beneficial and therefore need contacts with the air environment, for example, pinnipeds, cetaceans, water beetles, mosquito larvae and others.

Lack of oxygen in water sometimes leads to catastrophic phenomena - zamora, accompanied by the death of many aquatic organisms. Winter zamory often caused by the formation of ice on the surface of reservoirs and the termination of contact with air; summer- an increase in the temperature of water and a decrease, as a result, of the solubility of oxygen.

Frequent death of fish and many invertebrates in winter is typical, for example, for the lower part of the Ob river basin, the waters of which, flowing down from the swampy areas of the West Siberian lowland, are extremely poor in dissolved oxygen. Sometimes deaths occur in the seas.

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

Salt mode. 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 aquatic organisms it is equally important to maintain a certain amount of water in the body with its excess in the environment. An excessive amount of water in cells leads to a change in osmotic pressure 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, for aquatic organisms, the main way to maintain their salt balance is to avoid habitats with inappropriate salinity. Freshwater forms cannot exist in the seas, sea forms cannot tolerate desalination. If the salinity of the water is subject to changes, animals move in search of a favorable environment. For example, during desalination of the surface layers of the sea after heavy rains, radiolarians, sea crustaceans Calanus and others descend to a depth of 100 m. Vertebrates, higher crayfish, insects and their larvae living in water belong to homeosmotic species, maintaining a 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 the surrounding water. They are at risk of excessive watering if the excess water is not prevented from entering or 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 release an amount of water equal to the volume of the body every 2-2.5 minutes. The cell spends a lot of energy to "pump out" excess water. With an increase in salinity, the work of vacuoles slows down. So, in Paramecium shoes with a water salinity of 2.5% o, the vacuole pulsates with an interval 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 environment disappears.

If the 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 in the body of aquatic organisms. Dehydration is prevented by water-impermeable covers of homoiosmotic organisms - mammals, fish, higher crayfish, aquatic insects and their larvae.

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

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

Temperature regime water bodies are more stable than on land. This is due to the physical properties of water, primarily the high specific heat capacity, due to which the receipt or release of a significant amount of heat does not cause too sharp changes in temperature. Evaporation of water from the surface of reservoirs, in which about 2263.8 J / g is spent, prevents overheating of the lower layers, and the formation of ice, in which the heat of fusion (333.48 J / g) is released, 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 water bodies - 30-35 ° C. Deep layers of water are characterized by constant temperature. In equatorial waters, the average annual temperature of the surface layers is + (26-27) ° C, in polar waters - about 0 ° C and below. In hot terrestrial springs, the water temperature can approach +100 ° C, and in underwater geysers at high pressure at the bottom of the ocean, a temperature of +380 ° C is recorded.

Thus, there is a fairly significant variety of temperature conditions in water bodies. 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 the external and deep waters is stronger.

Due to the more stable temperature regime of water among aquatic organisms, stenotherm is widespread to a much greater extent than among the land population. Eurythermal species are found mainly in shallow continental water bodies and in the littoral of the 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. Part of the rays falling on the surface of the reservoir is reflected into the air. The lower the position of the Sun, the stronger the reflection, so the day is shorter under water than on land. For example, a summer day near Madeira Island 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 with different wavelengths are not absorbed in the same way: reds disappear already close to the surface, while blue-greens penetrate much deeper. Twilight deepening with depth in the ocean is first green, then blue, blue 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 in the same way. The inhabitants of the littoral and sublittoral zones are most vividly and variedly colored. Many deep-seated organisms, like cave organisms, do not have pigments. In the twilight zone, a red coloration is widespread, which is complementary to the blue-violet light at these depths. The rays complementary in color are most fully 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 coloration is typical for such animals of the twilight zone as sea bass, red coral, various crustaceans, etc.

In some species living near the surface of water bodies, the eyes are divided into two parts with different ability to refract rays. One half of the eye sees in the air, the other in the water. Such "four-eyed" is typical for beetles-twists, American fish Anableps tetraphthalmus, one of the tropical species of sea dogs Dialommus fuscus. At low tide, this fish sits in recesses, exposing part of its head out of the water (see Fig. 26).

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

Transparency is characterized by the extreme depth at which a specially lowered white disc with a diameter of about 20 cm (Secchi disc) is still visible. The most transparent waters are in the Sargasso Sea: the disc is visible to a depth of 66.5 m. In the Pacific Ocean, the Secchi disc 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 1-1 on average , 5 m, and in the most muddy rivers, for example, in the Central Asian Amu Darya and Syrdarya, only a few centimeters. The boundary of the photosynthetic zone therefore varies greatly in different water bodies. In the purest waters euphotic zone, or zone of photosynthesis, extends to depths of no more than 200 m, twilight, or dysphotic, the zone occupies depths of up to 1000-1500 m, and deeper, in aphotic zone, sunlight does not penetrate at all.

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

In the dark depths of the ocean, organisms use the 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, apparently, appeared many times in different groups at different stages of evolution.

The chemistry of bioluminescence is now fairly 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 pulses, usually in response to stimuli from the external environment.

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

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

1 - deep-sea angler with a flashlight over the toothed mouth;

2 - distribution of luminous organs in fish of this. Mystophidae;

3 - the luminous organ of the fish Argyropelecus affinis:

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

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