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see marine biologymarine biology,
study of ocean plants and animals and their ecological relationships. Marine organisms may be classified (according to their mode of life) as nektonic, planktonic, or benthic. Nektonic animals are those that swim and migrate freely, e.g.
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an aggregation of organisms living on or in the bottom soil of marine and continental bodies of water. In contrast to benthos, organisms living in open water and not bound to the bottom are called pelagic organisms (neuston, pleuston, plankton, and nekton). Benthos is divided into animal (zoobenthos) and plant (phytobenthos) categories. Based on the mode of living on the bottom, the following types of animals are distinguished: living on or in the soil, mobile, slightly mobile, and immobile (partially embedded in the soil or adhering to it). The forms of zoobenthos are subdivided according to their feeding habits into predatory (carnivorous), herbivorous, detritus-eating (feeding on organic particles), and so on. It is difficult to classify many bottom-dwelling animals as either pelagic or benthic; they are called planktobenthos and nektobenthos. Benthic organisms are divided according to size into large (macrobenthos), medium (mesobenthos), and small (microbenthos). The term “meio-benthos” (small benthos without bacteria) is also used.

Marine zoobenthos consists mainly of foraminifers, sponges, coelenterates, nemerteans, polychaete worms, si-punculids, bryozoans, brachiopods, mollusks, crustaceans, echinoderms, ascidians, and fish. Most of the benthos is concentrated in shallow waters. On the littoral and the top layer of the sublittoral, the mass of organisms per sq m may be as great as several dozen kilograms (mainly mollusks). At depths as great as 100–150 m, the biomass weighs hundreds of grams. At 500 to 1,000 m it is also calculated in grams, further down in fractions of a gram, and at great depths (abyssal region) in milligrams. The distribution of benthos is characterized by vertical zonality, with mollusks and crustaceans predominating in the top layers; mollusks, polychaetes, and echinoderms in the middle layers; and polychaetes, crustaceans, and echinoderms in the deeper layers.

Among plant organisms, the bulk of the benthos in the seas is made up of bacteria and algae (diatomic, green, brown, and red). Some flowering plants are common offshore: eelgrass, phyllospadix, widgeon grass, and others. The phytobenthos is most abundant and diverse on the rocky and stony portions of the bottom, which serve as a firm substrate for the attachment of algae. Along the Murmansk, White Sea, and Far East shores, kelps and fucus algae (from the brown algae) often produce a mass of 15–30 kg per sq m of bottom on the littoral and top layer of the sublittoral. There are beds of red Phyllophora algae in the northwestern Black Sea at depths of 20–60 m in which the mass averages 1.7 kg per sq m of bottom and amounts overall to millions of tons. On soft soils, phytobenthos develops only in shallow places that are more or less protected from the waves. It consists chiefly of flowering plants (eelgrass and others), whose root system enables them to take a firm hold in sandy and silty soils.

The vertical distribution of the algae varies with the composition of the solar spectrum reaching different depths because of the unequal absorption of rays with different wavelengths. Green algae are usually concentrated in the top layer, brown algae below, and mostly red algae still further down.

There is a much smaller quantity of zoobenthos in bodies of fresh water than in salt water, and its composition is less diverse. It includes protozoans, sponges, worms of the classes Turbellaria and Oligochaeta, leeches, bryozoans, mollusks, and insect larvae. It sometimes consists mainly of chironomid and oligochaete larvae that produce a mass weighing several dozen grams per sq m and serve as a very valuable source of food for fish. Freshwater phytobenthos consists of bacteria, diatoms, green (chara and filamentous) algae, and numerous littoral plants found in distinct zones away from the shore. The first zone contains semisubmerged plants (cane, scirpus, cattail, sedge, and others), the second consists of submerged plants with leaves floating on the water (water lilies, brandy bottles, and others), and the third includes submerged plants of which only the flowers usually rise above the water (mostly pondweed, water thyme, and so on).

A substantial part of the marine benthos—especially mollusks (oysters, mussels, and so on) and crustaceans (crabs, shrimp, and lobsters)—is consumed as food or used as industrial raw material. Of the 12 million centners of marine invertebrates caught every year, 62 percent are mollusks and 30 percent are crustaceans. Many mollusks are food for fish and, in addition, yield mother-of-pearl and pearls. Other benthic animals of commercial significance are household sponges and coral.

Some benthic animals are harmful, especially marine borers (bivalve mollusks, shipworms). Many marine organisms settle in large numbers on the bottoms of ships, thereby slowing them. Among the marine phytobenthos used as food and industrial raw material are blade kelp, sea lettuce, red laver, Ahnfeltia, Phyllophora, and eelgrass. Some plants— for example, cane and reeds—are used in industry and agriculture. An increase in freshwater phytobenthos is sometimes harmful because it leads to overgrowth of the basin. Mowing is a means of controlling the growth.


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Bottom-dwelling forms of marine life. Also known as bottom fauna.
The floor or deepest part of a sea or ocean.
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