The species composition of the hydrosphere of planet Earth is about 250 thousand species of representatives of all kingdoms of organisms . This is much less than the species diversity of sushi. But hydrobionts (living in the aquatic environment) are represented by all types of organisms, which makes up 90% of all animals, and 85% of them are exclusively aquatic inhabitants.
Biota structure
Hydrobionts are organisms adapted to life in the aquatic environment. Moreover, their entire life cycle can take place in water (echinoderms, crustaceans, mollusks, fish), or only part of their life (amphibians, many insects) passes in the aquatic environment. They inhabit fresh and salt waters, occupy all layers of the aquatic environment. The following types of aquatic organisms are distinguished:
- Neuston (from the Greek " floating " ) - all organisms that live on the border of water and air and occupy the surface layer of a reservoir of several millimeters.
- Pleiston (from the Greek " swim " ) is a hydrobiont, leading a semi-submerged way of life or living on the surface of the water.
- Rheophiles (from the Greek. " Current and love " ) - animals that are adapted to live in flowing waters.
- Necton (from the Greek. " Floating " ) - these are hydrobionts that can withstand the current and are actively swimming.
- Plankton (from the Greek " wandering " ) - a collection of small organisms drifting in the water column and unable to withstand the current.
- Benthos (from the Greek " depth " ) - organisms that live on soils and in the soil itself, forming the bottom of reservoirs .
Ecological niches
The ecological habitats of hydrobionts are considered pelagic (water column), benthal (bottom of the reservoir ), neustal (surface layer). Examples of hydrobionts of the pelagic zone are zooplankton and zoonecton, as well as rheophiles . Benthal organisms are epibenthos (live on the surface of the soil), endobenthos (live in the soil itself) and periphyton (organisms that attach to objects and bodies of other organisms). The group of hydrobionts of neustal is neuston and pleiston.
Life specifics
Most hydrobionts are characterized by the presence of a rather weak vision, but a good orientation to sound. This is due to the habitat, because light rays quickly decay in the water. Therefore, those organisms that have developed organs of vision, see well only at close range. Sound waves propagate in water much better than in air. Some hydrobionts are capable of picking up sound vibrations of even a very low frequency. For example, jellyfish pick up low-frequency changes in the rhythm of the waves and when they approach a storm they sink to a depth. Many aquatic animals themselves make various sounds to ensure intraspecific communication, attract a partner or orientation in a group. Various crustaceans rub against each other, fish use a swimming bladder, teeth, and rays of pectoral fins to make sounds.
Orientation elements of aquatic organisms
For orientation in the aquatic environment, the search for food and partners, many aquatic organisms are capable of perceiving reflected sound vibrations (echolocation). Many organisms are able to produce and perceive electronic impulses. Ichthyologists know about 300 species of fish that can generate electricity, navigate and signal with it. And, for example, electric ramps and eels use electricity as a means of defense or attack. In addition, all hydrobionts have a pronounced perception of hydrostatic pressure.
Hydrobionts-filter
Only among representatives of the aquatic environment do organisms with a specific method of nutrition — filtration — occur. This is the nature of nutrition associated with straining or sedimentation of particles or small organisms in water. All water filtrators play an important role in water purification. For example, a colony of mussels on an area of 1 square meter sends up to 300 cubic meters of water through itself per day. And according to environmentalists, all the water in the oceans is passed through the filtration apparatus of hydrobiont- filtering equipment for one day.
Hydrobionts and light mode
As you know, solar ultraviolet is an important component of life. There is not much light in the aquatic environment: part of it is reflected from the surface, part is absorbed by water. The rays of light are absorbed differently. Deep twilight, first green , then blue, blue and blue-violet, and they end in complete blackness. Algae - green , brown and red - change with light. And animals are most brightly colored in the area from 50 to 200 meters. The deeper the habitat, the more red in the color of aquatic organisms (red corals and sea bass). The absorption of light depends on the transparency of the waters, which also affects the life of hydrobiotic organisms and the boundaries of the photosynthesis zone. In the most transparent sea - Sargassovo - the border of photosynthesis is at a depth of 200 meters. But at a depth of more than 1,500 meters, the light does not reach at all . And then there are many hydrobionts that are capable of bioluminescence - glow as a way of orientation and food production.
Aquatic organisms and salinity
In relation to the concentration of salt in water, living aquatic organisms are divided into freshwater and marine. Water is considered fresh when the concentration of inorganic dissolved substances in it is 0.5 grams per liter. The average salt content in seawater is 35 grams per liter. But a more serious indicator is the ability of organisms to tolerate fluctuations in water salinity . All inhabitants of the aquatic environment with respect to salinity fluctuations are divided into euryhaline and stenohaline . Euryhaline organisms can tolerate fairly large ranges of vibration. For example, the edible mussels Mutilus edulis or the crab Carcinus maenas survive at fluctuations in salt concentrations from 50 to 1600 millimoles per milligram of water. Most hydrobionts do not have mechanisms to maintain constant concentrations of osmotically active substances in the internal environment and belong to stenohaline organisms.
Optimum living temperatures
Depending on what kind of living environment hydrobionts inhabit, they are divided into cryophiles and thermophiles. The former prefer cold waters. On our planet, more than 80% of the biosphere is cold areas, where the average temperature is + 5 ° C. These are the depths of the seas and oceans, the Arctic and Antarctic zones. The resistance to hydrobionts is given by the mechanisms of the enzymatic system, which can support metabolism in body cells at a temperature of 0 ° C. Thermophiles can not only exist at high ambient temperatures, but also tolerate boundary values. For example, in the vents of black smokers of ocean ridges, where the temperature reaches + 400 ° C, chemosynthetic bacteria were found .
Among the hydrobionts there are few warm-blooded animals
All homoothermal (warm-blooded) hydrobionts are secondary-water organisms. Whales, seals, dolphins returned to the aquatic environment during their evolution after the acquisition of such an expensive aromorphosis as the ability to maintain a constant body temperature. Expensive, because they spend almost 90% of endogenous heat on maintaining a stable internal temperature. And this requires acceleration of metabolic processes, the main of which is oxidation. In an aqueous medium, the oxygen concentration is within 1%, and its diffusion is a thousand times less than in an air environment. This is what makes the existence of warm-blooded organisms in the aquatic environment disadvantageous from an energy point of view. And so most hydrobionts are poikilothermic (cold-blooded) animals.