With its name, reminiscent of the Russian-speaking person the name of the well-known cereals, the mollusk pearl barley owes a slightly less prosaic concept. Its origin is associated with the peculiarity of the inner surface of the shell and the English word perl - pearl. Mother of pearl covers the flaps in the shell of the mollusk from the inside. The chemical composition and appearance are really pearl-like. This property was used by painters, mixing crushed and processed substance into paints.
Clam pearl barley: origin
Actually, perlovichami (emphasis on the second syllable) or shells (they are also Unio in the Latin version) is called just the genus of freshwater mollusks of the bivalve class, the Uniotid family (Latin name - Unionidae). They were isolated and described by scientists at the end of the XVIII century.
The ranges of river mollusks of this genus are mainly located on the Eurasian continent. In the central European part, there are three types of barley - thick, wedge-shaped and, of course, ordinary.
The most common types of barley
Unio crassus is one of the species of frequently found mollusks of peritles, which in Russian is called thick pearl barley. It develops more slowly than the other two species.
The barley is wedge-shaped or swollen, it is also Unio tumidus, it has a more elongated shell shape, a relatively lighter color and a preference for running water with non-rocky soil. The dorsal margin of the concha is located below the crown - it is flatter in comparison with the other two species. Lines of stunting are often located.
The mollusk of painters or the pearl barley (Latin name Unio pictorum) grows faster than brothers. The shell has the shape of an ellipse, resembles an egg. Growth rings are thin, graceful. The name refers to the artists not only because of the peculiarities of mother of pearl, but also due to the use of shells by the painters as a palette.
The height indicators of the pearl barley shells usually range up to 3.5 centimeters, the length reaches just over seven, the largest - up to fifteen. However, abnormally large representatives are found with very large shells.
In these representatives of the genus of freshwater inhabitants, the leaf walls are thick, the outer layer is smooth with noticeable, albeit thin, annual rings indicating growth zones. On average, mollusk shellfish live for ten to fifteen years, but there are cases of more than twenty years of existence of representatives of the species of thick pearl barley.
As habitats, they prefer fresh-water clear water bodies with a fast current. Due to the pollution of many rivers, the decrease in the number of fish in them, the life of which is associated with the development of mollusk larvae, the number of pearl barley is gradually decreasing since the 20th century. At the moment, the thick barley is threatened with complete disappearance.
The structure of the barbs
All the perlovka have solid, durable walls of two flaps with relatively convex or more flat shells, painted in a gamut of colors from yellow-gray to almost black. The hinges are connected by a horn-shaped elastic ligament, in front of which the apex is located. In most of the barley, it is shifted to the front of the shell and protrudes above the dorsal edge of the shell. The edge of the shell, on which there is a well-defined ligament, is considered the top.
There are front and rear muscle closures. The castle is pronounced, consists of teeth and recesses. Clam pearl barley has three prints of leg muscles. This term refers to the areas of muscle attachment to the shell, the concept refers to hard flaps, and not to the soft body of an animal.
The mollusk shell is three-layered. The external conchiole is usually a dirty green tone, below it is white porcelain, then the inside is mother-of-pearl. The last two are formed by crystals of calcium carbonate. The color palette of the mother-of-pearl layer of the shell can vary from white to pinkish and bluish.
The mollusk has a body and a leg. The epidermis of the peritoneum is glossy, smooth or superficially uneven. The body has folds. On the dorsal part there is an outgrowth in which most of the internal organs are placed. It is called an internal bag. However, there are also cavities with fluid on the body. There is a secondary cavity containing the heart and gonads.
The main fold located at the border of the leg and bag is called the mantle. Its edges hang freely, growing together only under the outlet siphon (upper).
The leg of the mollusk has the shape of an ax or wedge. On both sides of the body of the pearl barley are two half-gills fused behind the leg. Each is a trellis plate through which water is continuously filtered. The gills are equipped with water tubes. There are two primordial siphon openings - inlet (gill) and excretory (cloacal). They are separated by a diaphragm.
The diet of mollusks of perlovitsa is plankton and detritus (the smallest undecomposed organic particles). Nutrition occurs due to the filtration of water by particles remaining on the gills. They are covered with mucus and cilia of the ciliated epithelium move to the oral opening, then are swallowed by the barley.
The digestive system is represented by three departments. At the base of the leg there is a mouth opening with two blades on the sides, an oral cavity and a pharynx, but without grinding organs that make up the anterior intestine. From there, the esophagus leads to the stomach, surrounded on all sides by the liver - the digestive gland of the barley. From the stomach, the middle intestine bends several times. Then the food enters the hind gut.
Unused or recycled substances are discharged out along with filtered water through the clown cavity. In an animal with a length of 7-8 centimeters, a jet can be thrown over a distance of forty centimeters.
The nervous system is represented by three pairs of nerve nodes (ganglia) - the head, foot and viscera, connected by commissural nerve fibers. Nerves are stretched from the ganglia to the organs.
Perlitsa are equipped with receptors of skin sensitivity, organs of balance and chemical sensation. The latter surround the oral cavity and opening. Hearing is almost not developed - two auditory vesicles are located on the foot ganglia. Vision is absent.
The circulatory system is represented by a three-chambered heart (two atria and one ventricle) and vessels - arterial and venous. Part of the open circulatory system of the mollusk passes through the cavity of its body. Gills are also included in the process.
Travel Features
Barley moves, slowly crawling exclusively on horizontal surfaces, at a speed of one and a half and an hour per hour, half immersed in sand or silt. The mollusk previously digs into it with the front part, taking a vertical position for this. May not move for days before sunny weather. During rest, it lies, buried in the ground with the whole body, except for the upper edge with a mouth opening.
The leg moves forward when moving (it depends on the moment of blood flow into it), the whole body is pulled to it. The contraction of the muscles allows the leg to retract and complete the stroke cycle - a step that it takes approximately every fifty seconds.
At the end of summer, closer to the beginning of autumn, mollusks almost entirely burrow in silt for wintering. They reduce the activity of life processes to a minimum and plunge into a state of numbness, tightly and firmly closing the shell flaps.
Females, males, and glochidia: reproduction and development of barley
Mollusks are heterogeneous. There are gonads, but there are no copulation organs for internal fertilization. They breed in the spring - beginning in late April and throughout May.
The male sends spermatozoa to the pond through the outlet siphon, and from there they enter the female’s body through the inlet siphon, where the eggs are fertilized. From one female, several hundred thousand eggs can appear at a time. Embryo development occurs in the external gills of the female.
The larvae of freshwater mollusks have a special name - glochidia. They reach maturity and are ready for separation from the mother's body in 20-40 days after fertilization - from the end of May to August. The first days (usually no more than three), the larvae swim in the water, and after that they attach to the gills, skin or fins of the fish and parasitize on them, developing and spreading in this way through the water bodies.
The pearl barley reaches puberty after two to three years of life.
The age of mollusks is determined in two ways. The first one is based on the number of arches of annual growth surrounding the sash entirely. Only relief bands are taken into account. They arise due to a halt in the growth process during wintering.
You can check the result of the inspection by counting the arc-like protrusions on the inner surface of the sash near the blunt end of the shell. Age is summarized by the formula: the number of these pearly influx plus two.
The role of perlovits in the life of a natural reservoir
Perlovits can be found both in flowing reservoirs, and in ponds, lakes. They live in shallow water and in depth. The most preferable for pearl barley is sandy, silty or mixed soil. They can settle on a rocky bottom with the presence of a layer of silt, but viscous is avoided. Traces of mollusks on a soft bottom are noticeable and recognizable, resembling grooves.
An important factor in the presence of pearl barley is a good oxygen saturation of the water.
They are natural and excellent water filters; a large individual passes about forty liters daily through itself. Given the number of mollusks in water bodies, their role in this process is difficult to overestimate.
Peritoneum also has a binding and sealing effect on sludge, since each secretes a very large amount of mucus.
Perlits in the aquarium
Despite the excellent filtration characteristics and slowness, aquarists classify mollusks as parasites.
The reason for this is their ability to produce numerous offspring that live off many species of fish. They do not cause much harm, but they can cause itching and the desire of the fish to rub against objects, which often leads to injuries of the skin.