From the site www.esosystеma.ru

Descriptions of species and illustrations are taken from computer guide for freshwater invertebrates in Russia(Bogolyubov A.S., Kravchenko M.V., Moscow, "Ecosystem", 2018).

CLASS DOUBLE - BIVALVIA

Brief description of the class. Bivalve, or lamellar-gill (Bivalvia) - a class of marine and freshwater sedentary molluscs that in contrast from gastropods (Gastropoda) they do not have a separate head and organs associated with it (mouth, pharynx). They have a shell 2 wings- right and left (as opposed to upper and lower in brachiopods), which cover the body from the sides and are movably articulated on the dorsal side by an elastic ligament - ligament, and from the inside are connected 2 or 1 closure muscle.

Bivalve body flattened from the sides and carries 2 wide mantle blades closely adjacent to the shell valves from the inside. Leg more often keeled and can protrude from the shell; it often has a gland that secretes byssus serving to attach to the substrate. Paired gills located on the sides of the body and modified into gill plates (ctenidia), which perform not only the respiratory function, but also the role of filters for filtering food particles from the water, therefore, by the type of nutrition, bivalves are predominantly filter feeders... The central nervous system consists of 3 pairs of ganglia. There are sensitive tentacular appendages, and in some forms, complexly developed eyes. Heart usually penetrated by the hindgut and has 2 atria. Usually dioecious, rarely hermaphrodites.

To bivalve relate such famous sea molluscs as oysters, mussels, scallops, and among our freshwater ones - toothless, pearl barley, globules, peas, pearl mussels and zebra mussel. Most bivalve molluscs bury themselves in the bottom silt, thus escaping predators, some lie on the seabed or cling to rocks and other surfaces. Few species, such as scallops, are capable of short active swimming.

Shell shape and size bivalves vary greatly: among them there are both very large mollusks, for example, the giant tridacna (Tridacna gigas), which can reach 1.4 m in length and weigh up to 200 kg, and very small ones. The smallest representative of the mollusk type is the bivalve mollusk Condylonucula maya, the adults of which reach only 0.5 mm in length.

Body shape in bivalves it is also very different. For example, the heart lobes have an almost spherical body and can jump, bending and unbending the leg. At the same time, sea cuttings (Ensis), due to their specialization in the burrowing way of life, have an oblong shell and a strong leg intended for burying into the ground. Shipworms of the family Teredinidae have a highly elongated worm-like body up to 2 meters long with a reduced shell located at its front end and modified into a drilling organ, due to which the mollusk "gnaws" the branched passages in the wood. In most species of bivalves, the body is oblong, more or less flattened from the sides, bilaterally symmetrical. The head is reduced, and the mollusk, in fact, consists of a torso and a leg.

History of the study of bivalves. For the first time title Bivalvia used by Karl Linnaeus in the 10th edition of his work "The System of Nature" in 1758 to designate mollusks, the shell of which consists of two valves. In addition, this class is known by other names, for example:

• Pelecypoda (meaning "ax-footed"),
• Lamellibranchia (lamellar) and
• Acephala ("headless" because bivalves, unlike all other molluscs, have lost their heads).

In Russian science of the late 18th and early 19th centuries, bivalves (living and fossils) were called - cranial.

Bivalve shell structure. The shell valves in bivalve molluscs are most often symmetrical(for example, in the heart, toothless). However, some species may have asymmetry flaps. So, in an oyster, the valve on which the animal lies is convex, and the other valve is flat and plays the role of a lid, covering the first one. A similar phenomenon takes place in the case of a scallop lying at the bottom.

From the outer surface of the shell, a prominent and somewhat elevated part is clearly visible - top, or crown(see figure). This is the most old part of the shell, since the shell grows along the edge from the opposite side.

• The edge of the shell, on which the tip is located, is called top, or dorsal, or locking edge shells, and the opposite is lower, or abdominal.
• The wider end of the shell is in front of him, and narrower, somewhat elongated and distant from the top - hind end (edge).
• If the shell is positioned with the apex of the shell up and the front end away from you, then the valve located to the left of the closing plane of the valves is called left, and the sash on the right is right.

At the back end the shell is an elastic plate or ligament, by means of which both shell valves are connected to each other. The ligament consists of two keratin proteins - tensilium and resilium. In various groups of bivalves, the ligament can be internal and external. Its function, in addition to fastening the valves, is to participate in the opening of the shell.

The upper surface of each shell valve is concentrically drawn; some of the arches protrude more sharply among others, stretch along the entire length of the shell and have a somewhat wrinkled appearance; this is annual arcs corresponding to winter pauses in shell growth, by which, with some approximation, the age of the shell can be determined. Openings also open at the rear end of the sink siphons.

At the front end shells are located leg and byssus gland(if there is).

Sink wall consists of three layers: outer conchiolin ( periostracum ), bearing growth bands, internal calcareous ( ostracum ) and lower mother-of-pearl ( hypostracum ).

The mineral component of the shell (that is, part of ostracum and hypostracum) can be represented exclusively by calcite, like in oysters, or calcite and aragonite. Sometimes aragonite also forms a nacreous layer, as in the case of the order Pterioida. In other mollusks, the layers of aragonite and calcite alternate.

Outer layer(periostracum) consists of solid organic matter (conchiolin) and is secreted by the edge of the mantle. At the tops of the valves, the conchiolin layer is often erased. This top layer has patronizing coloration, usually brown or olive. Sometimes, under the influence of any mechanical effect, the outer stratum corneum of the shell collapses, revealing the prismatic layer lying under it, which easily dissolves in soft waters rich in free carbon dioxide, and the shells of mollusks in such waters are corroded.

The inner layer(ostracum) is called porcelain. Microscopic examination of cross-sectional sections of the porcelain layer showed that it consists of lime prisms closely lying on top of each other in a direction perpendicular to the surface of the shell.

Mother-of-pearl layer(hypostracum) consists of many thin also calcareous layers, lying on top of each other and connected by conchiolin. This structure of mother-of-pearl causes light interference, i.e. the layers refract and reflect the light falling on them, as a result of which the inner surface of the shell shines with different colors or iridescent. The mother-of-pearl tint changes depending on which side and at what angle, when looking at the shell, the light falls on it. The nacreous layer thickens with the age of the mollusk and the growth of its shell.

On the inner surface of the sink(see figure) its thickened upper edge is visible or lock plate... It is called so because it carries outgrowths or teeth, which enter the recesses of the opposite plate, forming the so-called " lock". The teeth in front of the ligament are short and massive, those behind the ligament are elongated and thin. The lock prevents the valves from displacing relative to each other. The lock can be even-toothed (taxodontic) and uneven (heterodontic).

Inner surface of pearl barley shell

The lock is most fully expressed in pearls; in the pearl mussel, some teeth are reduced, and in the toothless, they are completely absent, which is why their name comes from.

At the anterior and posterior ends of the inner surface of the shell, there are spots formed by irregularities of the nacreous layer at the attachment points closure muscles shellfish; by the force of contraction of these rather powerful muscles, the shell valves are tightly compressed with one another, and if the shell is pulled out of the water, its body retains the moisture it needs for some time. The valves open without the participation of any muscles, but only under the influence of the ligament, which plays the role of a spring. The action of the opening mechanism can be seen on dead mollusks - from the drying out of the ligament, the shell valves open by themselves.

Shell growth occurs due to the gradual deposition of the conchiolin layer by the edge of the mantle, as well as the accumulation of mineral substances in the shell. Are visible on the sink concentric lines, indicating its uneven growth in changing environmental conditions (growth lines). However, the determination of the shell age from the number of such concentric lines is not accurate enough. More precisely, the age of the mollusk can be said by the number of layers on the transverse section of the shell.

If any foreign body, for example, a grain of sand, gets between the shell valve and the adjacent mantle of a bivalve mollusk (not only a pearl mussel), then it has an irritating effect on the animal, and the skin of the mantle begins to secrete a hardening nacreous substance, which gradually envelops the foreign body in concentric layers (see figure). In the end, it is formed pearl, which is, therefore, in a mollusk a formation of a painful origin.

Pearl shell with pearl (left) and pearl formation diagram (right)

By the shape and structure of the shell three main systematic groups our freshwater bivalve molluscs. In addition to the pronounced difference in the structure of the teeth and the locking apparatus, pearl barley is distinguished by an elongated hard-walled shell with a prominent apex close to the anterior end; the edentulous shell is broadly oval, thin-walled, its apex is slightly protruding, the keel of the upper margin is high in some species. The shell of the pearl mussel is large, elongated, thick-walled, with an almost straight or even somewhat concave lower edge; the upper edge is almost parallel to the lower one.

Mantle and mantle cavity. In bivalve molluscs, the mantle has the shape of two folds of skin hanging from the back along the sides to the ventral side. From below, its folds can be free (like a toothless) or grow together, leaving only holes for the legs and siphons. Small tentacles and eyes can sometimes develop along the edge of the mantle. The outer layer of the mantle separates the shell, and the inner layer is lined with ciliated epithelium, the beating of the cilia of which ensures the flow of water in the mantle cavity. In addition to the shell, the mantle also forms a ligament, a byssus gland, and a lock.

In burrowing forms, the mantle forms siphons- two long tubes, along the lower of which (inlet siphon) water enters the mantle cavity, and along the upper (outlet siphon) it leaves. Oxygen and food particles are delivered to the mantle cavity with the flow of water.

Like all other molluscs, in bivalves, the mantle forms a mantle cavity, which includes mantle complex organs: leg, two gills, two oral lobes and osphradia. The holes of the digestive, reproductive and excretory systems also open into the mantle cavity.

In most bivalve molluscs, a line is noticeable on the inner side of the shell, running parallel to the edge of the shell and often connecting the traces of two adductor muscles (closures). They call her pallial (mantle) line, it represents the lines of attachment of the mantle to the shell valve. Attachment is carried out by a narrow row of small retractor muscles. With the help of these muscles, the mollusk can, in case of danger, hide the protruding edge of the mantle inside the shell. Siphons (normally protruding from the sink) can also be drawn in. A special pocket-like depression in the mantle cavity serves for this. On the shell valve, this depression corresponds to the pallial sinus, or the mantle sinus, or the mantle bay, or the siphon bay - an inward curvature of the pallial line.

Leg. The leg (muscular unpaired outgrowth of the abdominal wall) in bivalve molluscs is wedge-shaped, serves for burrowing into the ground and crawling. The most primitive forms (order Protobranchia) have a leg, like gastropods, with a flat crawling sole. Some bivalves that attach to the substrate have a special byssus, secreting byssus threads, with the help of which the mollusk "grows" to the bottom surface (mussels). In many bivalves, leading an immobile lifestyle, the leg is completely reduced (oysters).

Musculature. The main muscles in the body of bivalve molluscs are the anterior and posterior closing muscles (adductors), although the anterior closure in some species can be reduced or completely lost. By contracting, these strong muscles close the valves, and when they relax, the valves open. In addition, a ligament is involved in the valve opening mechanism. When the sink is closed, it is like a spring in a taut state. When the contactors relax, it returns to its original position, opening the sash.

In bivalve molluscs lying on the same valve (for example, oysters and mussels), the anterior adductor is lost, and the posterior adductor is in a central position. Mollusks of the family Limidae, which swim by flapping valves, also have a single central closure. The endpoints are composed of two types of muscle fibers: striated, designed for fast movement, and smooth, maintaining long-term muscle tension.

As mentioned above, the mantle is attached to the shell by small muscles that form arcuate trail on the shell valve - pallial line. Paired protractor (flexion) and retractor (extensor) muscles provide movement of the leg of the bicuspid. Bivalves lacking a leg do not have these muscles. Other paired muscles control the siphons and the byssus gland.

Digestive system. In connection with the passive way of feeding by filtration The digestive system of bivalves has some peculiarities. Water entering through the inlet siphon is directed to the anterior end of the body, washing the gills and 2 pairs of long triangular mouth lobes. There are sensitive cells on the gills and oral lobes ( organs of taste) and small grooves along which food particles are transported into the mouth, located near the anterior closure.

From the mouth, food enters the short esophagus and then into the saccular endodermal stomach. Since the head is reduced in bivalves, the pharynx, radula and salivary glands are absent. Several digestive glands open into the stomach, often through a couple of diverticula, such as a two-lobed liver. The liver not only secretes digestive enzymes: its cells also phagocytose food particles. Thus, bivalves have intracellular digestion.

In addition, the stomach contains crystalline stem, consisting of mucoproteins and enzymes (amylase, glycogenase, etc.). The stalk is located in a special blind saccular outgrowth and protrudes into the lumen of the stomach. The cilia located there cause the stalk to rotate, separating enzymes and stirring the contents of the stomach. Due to the constant movement of food particles in the stomach, their sorting at its posterior end: small particles are sent to the digestive glands and are absorbed there by phagocytosis, while larger particles are sent to the intestine. The midgut departs from the stomach, which then makes several bends and is directed along the dorsal side of the body to the posterior end, passes into the hindgut, which opens with the anus into the mantle cavity above the posterior closure. Excrement with a stream of water is thrown out through the outlet siphon. The hindgut usually passes through the ventricle of the heart (a specific feature of bivalves).

Nutrition and digestion in bivalves synchronized with diurnal and tidal rhythms.

The features of the digestive tract described above are characteristic of filter bivalves. In predatory bivalve stalk can be greatly reduced, but in some cases there is a gizzard lined with chitin, in which food is ground even before the beginning of digestion. In other cases, the digestive tract of carnivorous bivalves is similar to that of bivalve filter feeders.

Nervous system. Like most other molluscs, the nervous system in bivalves is of the scattered-nodal type. They have a simpler structure than that of gastropods. Due to the reduction of the head, the cerebral ganglia have merged with the pleural ganglia; this is how paired cerebropleural double nodes were formed, located on both sides of the esophagus and connected by a thin cerebral commissure above the pharynx. The formation of cerebropleural nodes by fusion of ganglia is proved by the fact that in primitive Protobranchia the pleural nodes are still separate from the cerebral ones. They innervate the mantle cavity and sense organs (except for osphradia). The leg has pedal ganglia that innervate the leg, connected by connectives to the cerebropleural nodes. Under the posterior muscle-closure there is a third pair of nodes - visceroparietal, controlling the internal organs, gills and osphradia. They are connected by even longer connectives with cerebropleural nodes. The third pair of nodes is especially well developed in swimming bivalves. Bivalves with long siphons can have special siphonal ganglia that control the siphons.

Sense organs. The sense organs of bivalve molluscs are poorly developed. There are statocysts- the organs of balance, innervated by the cerebral ganglia. In the mantle cavity at the base of the gills are osphradia- organs of chemical sense; it is possible that the osphradia of bivalves are not homologous to the osphradia of gastropods. Selected receptor cells scattered on gills, mouth lobes, along the edge of the mantle, and on siphons. The tactile function is also performed by tentacles developing along the edge of the mantle. In predatory bivalves from the order Anomalodesmata, the siphons are surrounded by tentacles sensitive to vibrations; with their help, mollusks find prey.

Many bivalves deprived of eyes; however, representatives of the groups Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea have simple eyes located along the edge of the mantle. They consist of a fossa lined with light-sensitive cells and a light-refracting lens. Scallops have inverted eyes of a rather complex structure, consisting of a lens, a two-layer retina, and a concave reflective surface. There are also known cases of the formation of eyes on siphons in the heart. All bivalves have light-sensitive cells, thanks to which the clam determines when it is completely covered by the shade.

Respiratory system. Most bivalves breathe with gills... Each of the two gills consists of a branchial axis attached to the body and two rows of branchial filaments extending from it. The aggregate of filaments of each row forms gill leaves, or half-gills... An exception is a small group of bivalve mollusks - septibranchia, whose representatives are devoid of gills, but their mantle cavity is divided by a horizontal partition pierced by rows of holes. Their food is also peculiar - they are predators. By bending the septum, they suck in small animals, such as crustaceans, along with the water.

Protobranchia, which are the most primitive bivalves, have a pair of typical ctenidia with branchial lobes.

Have filibranchia (Filibranchia) there are filiform gills. Filiform gills are characterized by the fact that their gill petals have elongated into filaments, forming first a descending and then an ascending knee. Adjacent filaments are fastened to each other with the help of rigid cilia, forming plates; in some representatives the gill filaments are free. Filiform gills are characteristic of mussels, oysters, scallops.

Have detachment Eulamellibranchia there are lamellar gills. This is a further modification of filamentous gills: septa appear in them between adjacent filaments, as well as ascending and descending sections of one filament. This is how the gill plates are formed. Each gill consists of two semi-gills: the outer one, adjacent to the mantle, and the inner one, adjacent to the leg. Thus, Eulamellibranchia has 4 gills, but each of them corresponds to only one half of the true ctenidium. Pearls and toothless have such gills.

The branchial cavity (left) and the direction of respiratory currents (right) of the bivalve mollusc

Have cloisonné (Septibranchia) the gills are reduced and transformed into a gill septum with pores. The septum fences off the upper part of the mantle cavity, forming the respiratory cavity. Its walls are permeated with blood vessels, where gas exchange takes place.

Finally, in species devoid of gills (as, for example, representatives subclass Anomalodesmata), gas exchange proceeds through the wall of the mantle cavity.

Bivalves living in the tidal zone are capable of several hours survive without water closing the flaps tightly. Some freshwater forms, when pulled out into the air, slightly open their valves, so that they exchange gas with atmospheric air.

Circulatory system. The circulatory system of bivalves, like all other molluscs, open, that is, the blood circulates not only through the vessels, but also by lacunas(gaps between organs). The heart is located on the dorsal side and consists of 1 ventricle and 2 atria. As mentioned above, the posterior intestine passes through the ventricle. This fact is explained by the fact that the heart is laid in embryogenesis as a paired one on the sides of the intestine, and then these rudiments are connected above and below the intestine (the paired origin of the heart in bivalves is confirmed by the presence of two hearts in representatives of the genus Arca). In primitive forms of the order Protobranchia, fusion occurs only above the intestine.

From the ventricle, powerful anterior and posterior aorta branching into arteries; of which blood ( hemolymph) pours out into the lacunae and gives oxygen to the tissues. Anterior artery goes forward above the intestine and supplies blood to the insides, leg and front of the mantle, and back directed back under the intestine and soon disintegrates into the posterior mantle arteries. Some bivalve molluscs have only one aorta. Further, the already become venous blood is collected in a large longitudinal gap under the heart and goes to the gill vessels. Then oxygenated arterial blood returns through the outflow vessels from the gills to the heart. Blood is also poured into the outflowing gill vessels, which, bypassing the gills, passes through the kidneys, where it is freed from metabolic products.

Bivalve blood is usually devoid of any respiratory pigment, although members of the families Arcidae and Limidae have hemoglobin dissolved directly in blood plasma. The predatory bivalve Poromya has red in its blood amoebocytes containing hemoglobin.

Excretory system. The excretory system of bivalves, like most other mollusks, is represented by paired nephridia (kidney)... Bivalve buds with glandular walls are called boyanus organs... The kidneys are long V-shaped tubes that open at one end into the pericardium of the heart, and at the other into the mantle cavity, from where the metabolic products are carried away with the flow of water.

In addition to the kidneys, the excretory function is also performed by the pericardial wall, modified into paired pericardial glands... Sometimes they separate from the rest of the pericardium in the form of two saccular formations - keber organs... The excretion products of these glands enter the pericardium, and from there they are transported outside through the kidneys.

The reproductive system. Bivalve molluscs dioecious, however, there are also cases of hermaphroditism (for example, in the species Arca noae, protandric hermaphroditism was established, in which at first individuals function as males, then as females). Some species, for example Thecaliacon camerata, have a pronounced sexual dimorphism.

The gonads and ducts (vas deferens and oviducts) are paired; the gonads lie in the front of the body, close to the intestine, going into the base of the leg, and look like two lobular uviform formations. However, in some species, the reproductive ducts are absent, and gametes leave the gonads through tissue ruptures into the mantle cavity. In primitive Protobranchia, as well as a number of other bivalves (Pecten, Ostrea, etc.), the gonads open into the kidneys.

In some species, for example, representatives of the genus Lasaea, male reproductive cells come out through a siphon, and then, with a stream of water, are drawn into the mantle cavity of females, where fertilization takes place. The offspring of such species develops in the mantle cavity of the mother and leaves her at the stage of larva - veliger or young individual. Most species fertilization external... In this case, females and males release sperm and eggs. into the water column... This process can be continuous or triggered by environmental factors such as the length of the day, the temperature of the water, and the presence of semen in the water. Some bivalves release gametes little by little, others in large parts or all at the same time. A massive release of gametes sometimes occurs when all bivalves in the area synchronously release germ cells.

Life cycle. Bivalves, like all molluscs, have spiral cleavage. It goes in about the same way as in gastropods.

Most bivalves develop with metamorphosis... Fertilized eggs usually leave planktonic larva - veliger (sailfish)... The formation of the veliger is preceded by the trochophore stage, which takes place in the egg. Trochophore formation occurs rather quickly and takes several hours or days. On the dorsal side of the trochophore, a shell is laid in the form of a solid plate, which only later bends along the median line, becoming bivalve, and the place inflection persists in the form of a ligament... The upper part of the trochophore with the corolla of the cilia becomes sail veliger - a disc covered with long cilia, which serves for swimming. The bivalve shell covers the entire body of the veliger; when swimming, the sail is exposed from the shell. The organization of the veliger is very close to the organization of an adult mollusk: it has the rudiment of a leg, mantle, ganglia, stomach, liver, and other organs, but protonephridia remains the excretory organs. In the future, the veliger settles to the bottom, is fixed with a byssus thread, loses its sail and turns into an adult mollusk.

Some freshwater molluscs (for example, pearl barley and toothless) have a special larva - glochidia with a thin-walled bivalve shell with rounded valves and hooks on the ventral margin. Most of the organs of the glochidia are still underdeveloped: there are no gills, the leg is rudimentary. In these mollusks, fertilization occurs in the mantle cavity of the female, and glochidia develop in her gills. Each mature glochidia is a small bivalve mollusk, the shell valves of which open wide and quickly close due to the contraction of a highly developed articular muscle. The lower edges of the shell are equipped with sharp teeth, and a long sticky byssus thread departs from the tiny leg of the larva.

The larva of the bivalve mollusk - glochidia and the female bitterness with ovipositor

Economic value. Since ancient times, many bivalve molluscs have been used by humans, they served and serve loot... Their shells are constantly found in the so-called "kitchen heaps" of prehistoric man who lived near the shores of seas, rivers and lakes. In the excavations of Paleolithic human sites in the Crimea, a large number of shells of oysters, mussels, scallops and other mollusks, which are still hunted, are invariably found.

Bivalve molluscs are harvested because of their delicious, very healthy and easily assimilated by the human body. meat(as, for example, oysters, mussels, scallops, cockerels-tapes and venerupis, maktr, sandy shell, heart-shaped, arches, sea cuttings and synovacula, freshwater pearl barley, lampsilin, toothless, corbicul, etc.).

In terms of calorie content, they can even surpass the meat of many fish, both marine and freshwater. The nutritional value shellfish meat is also due to the high content of vitamins A, B, C, D, etc., the high content of such rare minerals in ordinary human food as iodine, iron, zinc, copper, etc. The latter, as you know, are part of a number of enzymes, hormones, play an exceptionally large role in oxidative, carbohydrate and protein metabolism, in the regulation of hormonal activity.

In recent decades, due to the fact that the natural natural reserves of the most valuable edible shellfish (even in the seas) are depleted, and the demand for them continues to increase, in many countries they have become relocate to new areas, acclimatize, and breed artificially both in sea and fresh waters, on "farms" - specially prepared shallows and in small bays protected from predators, artificial reservoirs. Not only sea mollusks (oysters, mussels, sea cockerels, tapes), but also freshwater ones (lampsilin) ​​are successfully bred and cultivated.

In 2010, aquaculture farms grew 14.2 million tonnes of shellfish, which is 23.6% of the total mass of shellfish consumed for food. In 1950, when the Food and Agriculture Organization of the United Nations began publishing such statistics, the total mass of bivalve molluscs consumed was estimated at 1,007,419 tons. In 2000, this value was already 10 293 607, and in 2010 - 14 616 172. In particular, the consumption of mussels in 2010 amounted to 1 901 314 (a decade earlier - 1 568 417) tons, oysters - 4 592 529 (3 858 911 ) tons, scallops - 2,567,981 (1,713,453) tons. In China, the consumption of bivalve molluscs has grown 400 times from 1970 to 1997! Some countries regulate the import of bivalve and other seafood, mainly to minimize the risk of poisoning by the toxins that accumulate in these organisms.

Currently, the production of bivalve molluscs is inferior to them artificial breeding in mariculture. Thus, mussels and oysters are grown on special farms. Such farms have achieved especially great success in the USA, Japan, France, Spain, Italy. In Russia, similar farms are located on the shores of the Black, White, Barents and Japan Seas. In addition, a marine pearl mussel (Pinctada) culture is developed in Japan.

Continues also booty bivalve molluscs in natural reservoirs, where they are now harvested on large vessels with specially designed fishing gear; catching of molluscs by divers is widely used. The largest number (about 90%) of bivalve molluscs is caught in the northern hemisphere - in the Pacific and Atlantic oceans. Fishing freshwater bivalve molluscs provide only a few percent of their total world production.

Of particular importance is the bivalve mollusk fishery in countries such as Japan, the United States, Korea, China, Indonesia, the Philippine Islands and other islands in the Pacific Ocean. So, in Japan, about 90 species of bivalve molluscs are caught, of which about two dozen species are of great commercial value, and 10 species are artificially bred. In European countries, the fishing and breeding of bivalve molluscs are most developed in France and Italy.

In Russia, commercial value have mainly a large seaside scallop - Pecten (Patinopecten) yessoensis, as well as various mussels, a white shell (Spisula sachalinensis), a sandy shell - Mua (Arenomya) arenaria, cockerels (Tapes, Venerupis) and some others.

Shellfish enter the market not only fresh and dried, but especially in ice cream; the preparation of various canned shellfish has also developed greatly.

In addition to food use, humans use some types of bivalves (many freshwater pearl mussels and pearl mussels, sea pearl mussels - pinctads, pteria, etc.) as a source material for jewelry (nacre and pearl), and also as souvenirs. Pearls are used in jewelry, and mother-of-pearl from bivalve shells is used in making buttons or cheap jewelry, as well as for inlays. Among natural pearls, the pearls of the bivalve mollusks Pinctada margaritifera and Pinctada mertensi, which live in the tropical and subtropical Pacific Ocean, have the highest value. Commercial pearl production in pearl farms is based on the controlled introduction of particulate matter into oysters. The material for the embedded particles is often ground shells of other mollusks. The use of this material on an industrial scale has put some freshwater bivalve species in the southeastern United States on the brink of extinction.

Tridacna. Pearl oysters. Oysters. Scallops. Mussels

Bivalve- marine and freshwater molluscs, which are characterized by the absence of a head, the presence of a wedge-shaped burrowing leg, and the presence of a shell consisting of two valves. In attached species, the leg is reduced. Unattached species can move slowly by extending the leg and then pulling the whole body towards it.

A mantle in the form of two folds of skin hangs down the sides of the mollusc's body. In the outer epithelium of the mantle, there are glands that form the shell valves. Substances in the valve are arranged in three layers: outer organic (conchiolinic), calcareous and inner nacreous. On the dorsal side, the valves are connected by an elastic ligament (ligament) or a lock. The valves are closed with the help of the closing muscles. On the dorsal side, the mantle grows together with the body of the mollusk. In some species, the free edges of the mantle grow together, forming openings - siphons for the input and output of water from the mantle cavity. The lower siphon is called the inlet, or branchial, the upper - outlet, or cloacal.

Respiratory organs - gills - are located in the mantle cavity on both sides of the leg. The inner surface of the mantle and gills are covered with ciliated epithelium, the movement of the cilia of which creates a flow of water. Through the lower siphon, water enters the mantle cavity, and is discharged through the upper siphon.

By the way of feeding, bivalves are filter feeders: food particles trapped in the mantle cavity are glued together and sent to the mouth of the mollusk, located at the base of the leg. Food from the mouth enters the esophagus, which opens into the stomach. The midgut makes several bends at the base of the leg, then goes into the hindgut. The hindgut usually penetrates the ventricle of the heart and ends in the anus. The liver is large and surrounds the stomach on all sides. Bivalves, unlike gastropods, do not have radula and salivary glands.

rice. 1.
A - side view, B - cross section: 1 - pedal ganglion, 2 - mouth,
3 - anterior closure muscle, 4 - cerebro-pleural ganglion,
5 - stomach, 6 - liver, 7 - anterior aorta, 8 - pericardium, 9 - heart,
10 - atrium, 11 - ventricle, 12 - posterior aorta, 13 - kidney,
14 - hind gut, 15 - posterior muscle-closure, 16 - viscero-
parietal ganglion, 17 - anus, 18 - mantle,
19 - gills, 20 - sex gland, 21 - midgut, 22 - leg,
23 - ligament, 24 - shell, 25 - mantle cavity.

The nervous system of bivalve molluscs is represented by three pairs of ganglia: 1) cerebro-pleural, 2) pedal and 3) viscero-parietal ganglia. The cerebro-pleural ganglia are located near the esophagus, the pedal ganglia are located in the leg, and the viscero-parietal ganglia are located under the posterior concha-closure muscle. The sense organs are poorly developed. In the leg there are organs of balance - statocysts, at the base of the gills there are osphradia (organs of chemical sense). Tactile receptors are scattered in the integument.

The circulatory system is of an open type, it consists of the heart and blood vessels. The heart is three-chambered, has two atria and one ventricle. Blood from the ventricle goes into the anterior and posterior aortas, which break down into small arteries, then the blood pours out into the lacunae and is directed through the gill vessels to the gills. Oxidized blood through the outflowing branchial vessels enters from each side of the body into its own atrium and common ventricle.

rice. 2. Bivalve larva
shellfish - veliger.

Excretory organs - two kidneys.

Bivalves are usually dioecious animals. The testes and ovaries are paired. The reproductive ducts open into the mantle cavity. Spermatozoa are "thrown" by males through the outlet siphon into the water and then drawn through the inlet siphon into the mantle cavity of the females, where eggs are fertilized.

rice. 3. Toothless larva
- glochidia:
1 - sash, 2 - hooks,
3 - sticky (byssus).

In most species of bivalve molluscs, development occurs with metamorphosis. From fertilized eggs, a planktonic larva veliger, or sailfish, develops (Fig. 2).

rice. 4. Tridacna
(Tridacna gigas).

Tridacna gigas- the largest species of bivalve molluscs (Fig. 4). The mass of the tridacna reaches 250 kg, the body length is 1.5 m. Inhabits the coral reefs of the Indian and Pacific oceans. Unlike other bivalve molluscs, the dorsal heavy part of the tridacna shell rests on the ground. This orientation of the shell led to large changes in the arrangement of various organs; in general, we can say that the tridacna turned 180 ° inside its shell. The only muscle-closure has shifted to the abdominal edge.

The edges of the mantle are strongly expanded and grow together almost along the entire length, except for three sections, where the holes of two siphons and an opening for the exit of byssus filaments are located. In the thickened edge of the mantle, the unicellular algae zooxanthella live. Tridacna is a filter feeder, but it can also feed on these zooxanthellae.

Tridacna shells and meat have been used by the peoples of Oceania for a long time.

Pearl oysters live in the Pacific and Indian Oceans at shallow depths (Fig. 5). They are hunted for the purpose of pearl extraction. The most valuable pearls are produced by species of the genera Pinctada, Pteria.

rice. 5. Pearl oyster
(Pinctada sp.).

A pearl is formed if a foreign body (a grain of sand, a small animal, etc.) gets between the mantle and the inner surface of the mantle. The mantle begins to exude mother-of-pearl, which envelops this foreign body layer by layer, which irritates it. The pearl grows in size, gradually detaches from the inner surface of the shell, and then lies free. Often it does not connect to the sink from the very beginning. The pearl is composed of alternating layers of nacre and conchiolin. After 50-60 years after being removed from the mollusk, it becomes covered with cracks, this is due to the destruction of the conchiolin layers inside it. The maximum life of a pearl as a piece of jewelry does not exceed 150 years.

In order to have gem value, a pearl must have a certain size, shape, color, purity. Pearls that meet the "jewelry" requirements are rare in nature. At the end of the nineteenth century, a method was proposed for the artificial cultivation of pearls in sea pearl oysters. The mother-of-pearl balls turned on a lathe are tied with sections of mantle leaves and in this form are transplanted into three-year-old mollusks. The aging period of the pearl sac ("nucleolus") is from 1 to 7 years.

At present, pearl breeding technology is as follows. Some farms grow pearl mussels up to the age of three, then transfer them to pearl farms. Here pearl mussels are subjected to operations ("nucleoli" are introduced) and then placed in special sieves, which are suspended from rafts. After a few years, the sieves are raised, and pearls are extracted from the pearl oysters.

rice. 6. Oyster
(Crassostrea virginica).

The artificial breeding of marine animals is called mariculture.

Oysters(Fig. 6) have been eaten by humans since time immemorial. The oyster shell is unequal: the left valve is larger than the right one and more convex. The mollusk is attached to the substrate by the left valve. The mantle is open, does not form siphons, the water flow is through. Well-developed semicircular gills surrounding a powerful adductor (muscle-closure). In adult molluscs, the leg is missing. Oysters are dioecious. Fertilized eggs develop in the back of the female's mantle cavity. After a few days, the larvae go out into the water, swim, settle and attach to the substrate. Oysters usually form clusters, distinguishing between coastal settlements and oyster banks.

About 50 species of oysters are known, which belong to the families Ostreidae and Crassostreidae. One of the main commercial species is the edible oyster (Ostrea edulis). As a result of centuries of fishing, the number of oysters in many populations has declined sharply. Currently, along with fishing in natural habitats, oysters are grown artificially in specially organized oyster parks.

Specific conditions are required to grow oysters. First, they feed on a specific type of plankton. Secondly, they do not live at a depth below 10 meters and at a water temperature below 5 ° C. Plantations are usually set up not very far from the coast in closed bays, so as not to be swept away by a storm. The period of growing oysters is not so short and amounts to 34 years. Shellfish are kept in special containers, submerged to a certain depth and inaccessible to predators. After ripening, the oysters are placed in pools with clean sea water and special algae for a certain period of time.

rice. 7.

rice. eight.

Scallops- several dozen species of gastropods, which belong to the families Pectinidae and Propeamusiidae. The scallops have a rounded shell with a straight locking edge, which has angular protrusions in the form of ears in front and behind. The valve surface is with radial or concentric ribs. The leg is rudimentary, it looks like a dense finger-like outgrowth. The middle fold of the mantle contains numerous eyes and mantle tentacles with tactile receptors (Fig. 7). Unlike other species of bivalve molluscs, scallops can move by swimming, “flapping” their valves (Fig. 8). The flaps are closed by contraction of the powerful adductor fibers. Scallops are dioecious animals.

The adductor of scallops, sometimes their mantle, is used for food. Just like oysters, scallops are not only harvested in natural habitats, but also grown artificially (Patinopecten yessoensis). First, on the fenced area of ​​the sea, rafts are installed, to which collectors (pallets, panicles, etc.) are suspended. The mollusk larvae settle on these trays. After 1-2 years, young mollusks are removed from the collectors, placed in individual nets and grown on "farms".

rice. nine. Edible mussel
(Mytilus edulis).

Mussels- several species belonging to the Mytilidae family. They lead an attached lifestyle, in connection with which the leg is reduced, loses the ability to move and serves to excrete byssus filaments. The shell has a characteristic "mytilid" shape, very dark in color, often bluish-black. The shell of the edible mussel (Mytilus edulis) is about 7 cm long, up to 3.5 cm high, and 3.5 cm thick. The posterior adductor is much larger than the anterior one. Mussels are dioecious animals. The mussel colonies are a powerful biofilter that purifies and clarifies the water. It is estimated that mussels that settle on 1 m 2 of the bottom filter up to 280 m 3 of water per day.

Mussels are used for food. The fishery for these mollusks has been going on for a long time. In addition, mussels are currently grown artificially. In this case, approximately the same technology is used as when growing scallops.

rice. ten. Teredo
(Teredo navalis):
1 - shell,
2 - body,
3 - siphons,
4 - moves, drilled
covered with a mollusc.

Teredo(Fig. 10) belongs to the family Teredinidae. The body shape is worm-like, so these mollusks have another name - ship worms. The body is up to 15 cm long; at its anterior end there is a shell reduced to two small plates. The shell is "equipped" with a drilling machine. There are long siphons at the rear end of the body. Hermaphrodites. In wooden underwater objects, it often "drills" numerous passages, feeds on wooden "crumb". The digestion of wood is carried out by symbiotic bacteria. As a result of the activity of ship worms, the tree becomes like a sponge and is easily destroyed. Teredos pose a danger to wooden boats and buildings.

in a freshwater aquarium

Along with gastropods (snails), which are brought into the aquarium even against the will of the aquarist, interest is aroused bivalve molluscs: zebra mussels, globules, lentils, toothless, pearl barley, corbicula. They can be kept in freshwater aquariums. About the benefits / harms, conditions of detention further in the article.

Bivalve molluscs are widespread in the reservoirs of the CIS countries, and therefore arouse interest among aquarists. All bivalve molluscs have a shell of two valves and gills through which they pass water and extract oxygen and food. The food for molluscs is organic particles and planktonic microorganisms in the water. Bivalve mollusks are capable of passing through their siphons up to 40 liters or more of water per day. Therefore, the water in aquariums where bivalve molluscs live is clear as a tear, without organic suspension. It would seem such a benefit! But there is also a downside to the coin of such living water filters. After eating densely, the mollusks release their excrement into the water, which is a source of nitrogen and phosphorus, which leads to the rapid growth of algae. Another sad moment may come in connection with such a carrying capacity of bivalve molluscs. At some point, they begin to lack food in the water, especially if, in addition to the bivalves, a filter is working in the aquarium. Artificial feeding with food dissolved in water can only temporarily delay the death of mollusks from starvation, which often does occur. According to aquarists, bivalve molluscs in an aquarium, they can last from a week to a maximum of two years.

In addition to having food, mollusks desperately need oxygen, which they breathe through the gills. Since they cannot rise to the surface of the water behind it, round-the-clock good aeration is vital for them.

They do not like mollusks and the increased water temperature - 18-22 ° С for them, that's the thing.

And further. Some bivalve molluscs spend their lives motionless, attaching themselves to a stone or snag and passing water through themselves. But there are those who, albeit slowly, but move along the ground, leaving furrows behind. This often affects aquarium plants.

Now about the types and characteristics of the maintenance and reproduction of bivalve mollusks, the most common in domestic reservoirs and aquariums.

Dreissena river (Dreissena polymorpha)- a freshwater bivalve mollusc with a triangular shell.

Shell color is yellowish or greenish. The sink features a pattern of zigzag lines. An adult bivalve grows up to 4-5 cm. They lead a motionless lifestyle, attaching themselves to hard surfaces. They reproduce by planktonic larvae, which are safe for other inhabitants of the aquarium.

Balls (Sphaerium)- a genus of bivalve molluscs of the pea family.

The shell is oval or spherical, brown and olive shades. Balls grow in length from 1 to 3 cm. Hermaphrodites. Viviparous. They breed 1-2 times a year, bearing eggs in the brood chambers of their gills. Small copies of their parents are born.

Lentils (Pisidium)- small bivalve molluscs that look like globules.

The difference lies in the color of the tubes protruding from the shell. In lentils, they are white, in balls, they are red. The shell of the lentil is oval-triangular, up to 1 cm long, brown or yellowish in color. In nature, they like to settle down in places where a lot of bloodworms are found. Viviparous.

Perlovitsy (Unionidae)- large bivalve molluscs. Adults can grow up to 10 cm in length.

Toothless (Anodonta)- outwardly bivalve molluscs very similar to pearl barley.

Corbicula- bivalve molluscs, ranging in size from 2 to 6 cm, depending on the species.

The shell is oval-triangular, yellow, ribbed. They live in various types of soil: silt, sand, small pebbles. Hermaphrodites. They breed twice a year. Viviparous, in one brood there can be up to 2000 small (1 mm) corbiculi. Filters water at a rate of up to 5 l / h!

All bivalve molluscs, regardless of where they came from (from an online store or from the nearest reservoir), must be quarantined and acclimatized to aquarium water. The best method is to add aquarium water to the shellfish jar using a dropper for 6-12 hours. Then hold in this vessel for 3 to 7 days, not forgetting about aeration.

Do not tolerate bivalve molluscs any chemistry in the aquarium will die immediately. Harm bivalves and planaria. Dead molluscs can be identified by the wide-open shell valves.

Molluscs are a separate type of animal. Molluscs are subdivided into armored, shellless, gastropods, bivalves, and cephalopods.
The most characteristic features of molluscs:
their body is two-sided symmetry (the right and left sides of the body are absolutely symmetrical); the body is soft, unsegmented and without articulated limbs, for the most part with a thickened muscular organ of movement (leg) on ​​the lower (abdominal) side, usually with a thickened fold of skin (mantle) that covers a more or less significant part of the body; the body is covered with a calcareous shell (simple and, in this case, most often spirally curled, bivalve, or consisting of a number of plates), highlighted by the surface of the mantle; the mouth is always at the front end of the body, often detached in the form of a head, usually equipped with special organs for grinding food (jaw and grater); the intestines are almost always with a bulky digestive gland; the circulatory system (never completely closed) always has an arterial heart (typically consisting of 1 ventricle and 2 atria) and almost always arteries; excretory organs in the form of bags (typically two), opening with one hole into the cavity of the pericardial sac, with the other outward; the central nervous system from the supraopharyngeal mass, pharyngeal ring and associated nerve nodes or trunks; reproduction is exclusively sexual. The body of armored molluscs is oblong or worm-like, 0.5-33 cm long. The body is divided into a head, trunk and leg. The back is covered with a shell consisting of separate scutes. The digestive system consists of a pharynx with a kind of grater that grinds food, stomach and intestines. Shellfish feed on algae and small debris of organic matter. The nervous system consists of four trunks and a periopharyngeal ring. Shell molluscs have eyes and touch organs.
Shellless molluscs do not have a shell. Otherwise, the structure of their body is not much different from other mollusks. They lay up to half a million eggs at a time. Such fertility is due to the fact that shellless mollusks are the food of many fish, and if they did not lay so many eggs, they would simply die out.
Gastropods have one or two pairs of tentacles on their heads and a pair of eyes. The body of the mollusk is covered with a shell, spirally twisted to the right or left. The shell consists of horny, porcelain-like and mother-of-pearl layers. The shapes, colors and sizes of shells are very diverse and very beautiful.
The front part of the mollusk is covered with a skin fold. It covers the mantle cavity, in which the rectum with the anus, gills, chemical sense organs, heart and two kidneys are located. Gastropods move with the help of a strong leg, for which they got their name.
The shells of bivalve molluscs consist of two halves. The body of the mollusk is located completely inside the shell, but the strong muscles of the mollusk can open it. Inside, the body of the mollusk is covered with leathery folds called the mantle. On the side of the belly there is a leg, with its help the mollusk is attached to the bottom or stones.
Cephalopods are the most developed group of molluscs. They have a brain surrounded by a cartilaginous skull, well-developed sense organs and a circulatory system. Around the mouth of the cephalopods there are 8 or 10 tentacles with suction cups or horn hooks. With the help of them, the mollusk moves and gets food. On the ventral side of the cephalopods there is a skin fold - the mantle. It protects the mantle cavity in which the main internal organs are located.
The mollusks got their name due to the fact that their organs of movement are located on the head. Most cephalopods do not have a full-fledged shell, only a horny or calcareous plate. Cephalopods feed on fish, crustaceans and other molluscs. They live in warm seas.
A source:
http://web-zoopark.ru/

From

Tsiprinella is a colorful representative of the Karpov family. Inhabits the cold waters of North America. Leads a gregarious lifestyle and has a calm, peaceful character. Description of Ciprinella
Ciprinella has a body structure that is standard for most Carp. The upper part of the body, including the back, is faintly colored red. There is a large red spot on the forehead. The fins are red, slightly transparent. The dorsal fin is darker than the rest. The lower part of the body is light. The maximum size Ciprinella achieves in an aquarium is 9 cm. It should be noted that in the case of improper maintenance, Ciprinella will not even grow up to 6 cm and will have a pale color.
Breeding Ciprinella
This type of fish is not bred in aquarium conditions. Contents of Ciprinella
It is advisable to keep Ciprinella in a flock of 5-10 individuals in a spacious, well-lit species aquarium. For such a flock, an aquarium with a volume of 120 liters or more is required. It is recommended to cover the aquarium with a lid as the fish is active. It is quite common for fish to jump out of the aquarium. The water in the aquarium must meet the following criteria: Temperature 16-22. Acidity 6-8. Hardness 10-18. Cleanliness is one of the most important parameters in keeping any fish. However, it should be borne in mind that in its natural environment, Ciprinella lives in running, clean and fresh water. Accordingly, in the aquarium, you can create such conditions using a filter and a compressor.

Also, Ciprinella is quite sensitive to the level of oxygen in the water, so artificial aeration is necessary. When setting up an aquarium, it is recommended to use various plants, grottoes and driftwood. You should also leave a small empty patch of soil. Ciprinella has a calm, friendly character, so it gets along well with almost all other friendly species. It is also a good solution to keep it in the same aquarium with non-aggressive bottom catfish, since most likely these two species will not even intersect. Tsiprinella lives exclusively in the upper and middle water strata.
Feeding Ciprinella
You can feed Ciprinella with almost all types of live food (in natural conditions, it feeds on small invertebrates). She should also include dry, floating foods with spirulina in her diet.

From

The pagoda (Brotia pagodula) is a rather interesting and beautiful viviparous snail. This snail is endemic and lives only in tributaries of the river (Moei), which is located between Myamna and Thailand. The Pagoda lives exclusively in fast, flowing and clean waters with a high oxygen content. It is extremely rare in lakes. It has rather specific requirements for the content, which is why it is not very common among aquarists. When placed in a new pond, it can hide in the sink for a long time or catch on a stone or glass and is in one position for a long time. In order to distract and stir her up, it will be enough to place a scalded piece of some vegetable (for example, a vegetable marrow) next to her. Appearance

The pagoda has a grayish (or yellow) body, decorated with multiple dark (or brighter, orange) specks. The shell is unusual, it looks like a swirling, multilevel tower (conical shape) with many spines starting from the very base of the shell. The shell consists of only 5-8 curls and its maximum size is 5.5 cm, but in an aquarium this value often does not exceed 4 cm. Requirements for the aquarium and the nuances of maintenance
1. It is best to keep the Pagoda in a small flock of 5-7 individuals. Then they feel more confident and quickly get used to the new reservoir. The volume of the aquarium in the case of keeping 5 individuals should be at least 50 liters. 2. The water in the aquarium must meet the following characteristics: · Temperature 20-25. · Acidity 7-8.5. · Hardness 6-22. 3. It is recommended to create a strong current in the aquarium, to which the Pagodas are accustomed in their natural habitat, to install a powerful filter that could ensure the proper purity of the water. And also you should take care of good aeration of the water. 4. Large stones, small driftwood and tall plants should be used as decorations for the aquarium. Sand is best suited as a substrate. When choosing vegetation and decorations, special attention should be paid to their cleanliness, since the Pagoda does not tolerate even the slightest pollution. 5. The pagoda is omnivorous. However, it is still recommended to focus on plant foods in the diet. It is advisable to feed them with tablets for bottom fish with a high content of spirulina, as well as scalded spinach, green beans, broccoli, cucumbers and carrots.

From

Helena (Anentome Helena) is a freshwater snail native to the waters of Southeast Asia. Divided into Clea Anentome (Asian subspecies) and Clea Afrocanidia (African subspecies). It mainly lives in running waters, however, many cases of living in ponds and lakes have been observed. Helena is a predator that eats other snails, and also does not hesitate to scavenge. That is why quite often this species is started by aquarists to fight other snails. It is a rather tenacious and not demanding species, therefore it has gained popularity among many aquarists, especially beginners. Appearance

Helena has a light body with many dark spots (usually gray-green in color). The carapace is elongated, twisted, conical, painted black with a bright yellow stripe that winds from the beginning to the end of the shell. The maximum shell length is 15-20 mm. Keeping in the aquarium
1. Helena is a rather unpretentious snail and can live well even in nano - aquariums. 2. Water parameters are not very important for Helena. The main thing is that the water is not too cold and too soft (otherwise you will need to feed with tablets with a high calcium content). It is recommended to maintain the acidity level in the range of 6-10. It tolerates both fresh and salted water well, while it can develop and multiply normally in both maintenance options. 3. Helena has a habit of burying herself at the bottom of the reservoir. Therefore, it is best to use sand or very fine gravel as soil. It should also be noted that this habit prevents the soil from oxidizing and thereby spoiling the water in the aquarium. It is also recommended to use soft soil because Helena's young after birth go straight to the bottom, burrow and spend almost all the time there until they grow up. 4. Helena eats, as mentioned earlier, other snails. However, only by those individuals that are smaller than her. Thus, Helena is not capable of causing any harm to the adults of Ampularia, Neretina or Marise. Additionally, it can be fed with almost all types of feed. But pills for bottom fish are especially good options. In terms of cohabitation with fish, Helena shows complete indifference to them. It is not recommended to keep Helena together with cichlids that feed on snails, as Helena has a rather fragile shell.

The underwater world is full of magic and mysteries, because sometimes it is not so easy to find out what is hidden at the bottom of the reservoir. But in both salt and fresh water, many inhabitants can very often be found, and the most common of them are river shells, belonging to the class of bivalves. They are attached to the hulls of sunken ships or boats, snags, underwater piles and pipes. And a person is able to examine such peculiar growths for hours. In addition, such inhabitants play a vital role in the ecosystem.

Description, appearance

Like many zebra mussel, the river has a strong protective shell, consisting of two identical valves, forming an angle from the back. In front, the "house" of the underwater inhabitant is distinguished by its rounded shape. It reaches 5 cm in length and 3 cm in width. On the surface of the shell, dark zigzag or even stripes are clearly visible, while its main color can be yellowish, green or blue.

Remarkably, as Dreissena polymorpha, they have no locking teeth. A jumper is formed inside the valves (on their front part), on which the muscle-closure is attached. The edges of the mantle are fused, but they still have holes for short tubes-siphons and legs that help the mollusk to move. It is worth noting that the body of the shell itself is covered with cilia that can absorb water inside the mantle.

Lifestyle

Such inhabitants of fresh water bodies as zebra mussel are not active, preferring to adjoin underwater objects and not move all day. However, after sunset, the mollusks occasionally begin their "journey", having time to overcome only 10 cm during the dark. The movement is carried out with the help of a weak narrow leg with a kind of fossa located on the lower surface. Dreissena river breathes due to the gills, which consist of two parts. They are connected by threadlike petals and also serve as a filter for the process of separating water from various microparticles.

Basically, shells feed on plankton, but sometimes other elements enter the mantle cavity, which become an excellent addition to the diet. First, food enters the stomach and intestines, where digestion takes place. Then the processed mass is returned back to the mantle, from where it is washed out completely due to the water inside.

In addition, with proper nutrition, the river Dreissena grows very quickly, increasing in size every year. This process does not stop throughout the entire period of the snail's existence. Of course, among the representatives of the species, there are also centenarians, but generally the life expectancy is about 4-5 years.

How does the reproduction process take place?

With the onset of spring, when the water temperature gradually warms up, the river Dreissena absorbs male reproductive cells into the mantle cavity, where fertilization begins. After a while, she spits eggs into the water (several pieces at a time), located in sacs filled with mucus. Then external fertilization occurs, after which the larvae, called veligers, are born. They swim for several days, growing tiny shells, and grow quite intensively, quickly acquiring similarity with adults. Diving to the bottom, the larva finds a suitable place for further life and releases bead threads (special hardening mucus), which help to attach to the surface. So, young animals can be superimposed on each other in layers, which absolutely does not interfere with their usual way of life.

It should be noted that these river bivalves are dioecious, unlike other small representatives of the species.

Habitat

Despite the fact that shells are called river shells, they still prefer slightly salty water, which is why they are more often found in fresh parts of the sea. They inhabit the Black, Azov, Aral and Caspian seas very densely. The habitat extends from Europe to Western Kazakhstan. Veligers are also sometimes found in the rivers of Asia, in the Volga and Dnieper. These inhabitants of fresh water bodies are travelers, therefore they independently capture and inhabit new places, due to which they spread throughout many water bodies of the world. In addition, the snail feels comfortable at a depth of 1-2 meters, but sometimes it sinks to 10 or even 60 meters.

It is worth noting that river shells do not live in the northern regions, where it is very cold for them.

Probably, almost every aquarist seeks to diversify his little "home reservoir" in every possible way, therefore, together with fish and algae, he often acquires snails with mollusks. And this is correct, because they perform in tanks not only a decorative function, but also perfectly purify water, filtering it during the digestion process. However, when populating zebra mussel in a container, it is important to remember that in order for it to cope with the task at hand, it is necessary to adhere to certain rules:

• since the snail grows rather big, it is recommended to keep it in a container with a volume of at least 90 liters;
• an abundance of small river algae is needed;
• shellfish do not need additional feed;
• the water temperature should be at least 18-25 degrees.

It is worth noting that this representative of the species is quite peaceful, therefore it does not harm its neighbors, does not eat caviar and algae, and also does not emit harmful substances.

Role in the ecosystem

Long-term observations of the zebra mussel have allowed scientists to establish that it is an excellent filter feeder of reservoirs, since it is capable of absorbing ordinary water and releasing purified water. The liquid passed through the mantle is saturated with special substances that help the algae grow at an accelerated rate. Experts have proven that an adult shellfish purifies at least 10 liters of water every day. Small Dreissena snails (weighing 1 gram) need a lot of food to grow quickly, so they process at least 5 liters per day. So, large accumulations of mollusks quickly cleanse reservoirs.

In addition, these unpretentious lovers of fresh and brackish water are not at all averse to eating fish, crayfish, and other types of snails. Therefore, a person sometimes uses zebra mussel as a jig while fishing.

It is also often found in aquariums, as it prevents turbidity in the tank, provides additional cleaning and improves the microenvironment.