The shellfish owes its name, reminiscent of a Russian-speaking person the name of a well-known cereal, to 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 valves in the clam shell from the inside. The chemical composition of the substance and appearance are really similar to pearl. This property was used by painters, mixing crushed and processed substance into paints.
Clam barley: origin
Actually, pearl barley (emphasis on the second syllable) or shells (they are also Unio in the Latin version) is just a genus of freshwater mollusks of the bivalve class, the Uniotid family (the Latin name is Unionidae). They were isolated and described by scientists at the end of the 18th century.
The ranges of river mollusks of this genus are mainly located on the Eurasian continent. In central Europeanpart there are three types of barley - thick, wedge-shaped and, of course, ordinary.
The most common types of pearls
Unio crassus - one of the most common molluscs of barley, in Russian speech is called thick barley. It develops more slowly than the other two species.
Wedge-shaped or swollen barley, also known as Unio tumidus, has a more elongated shell shape, a relatively light color and a preference for flowing water with non-stony ground. The dorsal edge of the shell is located below the umbo - it is flatter in comparison with the other two species. Stunt lines are frequent.
The painters' mollusk or common barley (Latin name Unio pictorum) grows faster than its counterparts. The shell has the shape of an ellipse, resembling an egg. Growth rings are thin, graceful. The name refers to the artists, not only because of the characteristics of mother-of-pearl, but also because of the use of shell valves by painters as a palette.
The height of barley shells usually ranges up to 3.5 centimeters, the length reaches a little more than seven, the largest - up to fifteen. However, there are abnormally large representatives with very large shells.
In these representatives of the genus of freshwater inhabitants, the walls of the valves are thick, the outer layer is smooth with noticeable, albeit thin, growth rings, indicating growth zones. Barley mollusks live on average for ten to fifteen years, but there are cases of more than twenty years of existence of representatives of the species of thick barley.
As habitats they prefer freshwater clear waters with a fast current. Due to the pollution of many rivers, the decrease in the number of fish in them, whose life is associated with the development of mollusk larvae, the number of barley has been gradually declining since the 20th century. At the moment, thick barley is threatened with extinction.
The structure of barley
All pearlworts have solid, strong walls of two valves with relatively convex or flatter shells, painted in a range of colors from yellow-gray to almost black. The valves are connected by a horn-shaped elastic ligament, in front of which the apex is located. In most barley shells, it is shifted to the anterior part of the shell and protrudes above the dorsal margin of the shell. The edge of the shell, on which there is a well-defined ligament, is considered the upper one.
There are anterior and posterior closure muscles. The castle is pronounced, consists of teeth and notches. The pearl barley mollusk has three imprints of leg muscles. This term refers to the areas of attachment of muscles to the shell, the concept refers to hard shells, and not to the soft body of the animal.
The shell of molluscs is three-layered. The outer conchiolin is usually a dirty green tone, under it is white porcelain-like, then the inner one is mother-of-pearl. The last two are formed by calcium carbonate crystals. Pearl color paletteshells can range from white to pinkish and bluish.
A clam has a body and a leg. The epidermis of pearlworts 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 the visceral sac. However, there are also fluid cavities on the torso. There is also a secondary cavity containing the heart and gonads.
The main fold, located on the border of the leg and the bag, is called the mantle. Its edges hang freely, growing together only under the outlet siphon (upper).
The clam leg is shaped like an ax or a wedge. On both sides of the body of the barley there are two half-gills, fused behind the leg. Each is a lattice plate through which water is continuously filtered. The gills are equipped with water tubes. There are two rudimentary siphon openings - inlet (gill) and outlet (cloacal). The diaphragm separates them.
The diet of Perlovitz molluscs is plankton and detritus (the smallest undecomposed organic particles). Feeding 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 mouth opening, then swallowed by pearl barley.
The digestive system is represented by three departments. At the base of the leg there is a mouth opening with two lobes on the sides, an oral cavity and a pharynx, but without grinding organs, that make up the foregut. From there, the esophagus leads to the stomach, surrounded on all sides by the liver - the digestive gland of pearl barley. Fromstomach leaves the middle intestine, curving several times. The food then enters the hindgut.
Unused or recycled substances are removed to the outside along with filtered water through the cloacal cavity. In an animal 7-8 centimeters long, the jet can be thrown out to a distance of forty centimeters.
The nervous system is represented by three pairs of nerve nodes (ganglia) - head, foot and visceral, connected by commissural nerve fibers. Nerves are stretched from the ganglia to the organs.
The pearls are equipped with skin sensitivity receptors, organs of balance and chemical sense. The latter surround the oral cavity and opening. Hearing is almost not developed - there are two auditory vesicles on the foot ganglia. No vision.
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 cavities of its body. Gills are also included in the process.
Features of movement
Barley move, slowly crawling exclusively on horizontal surfaces, at a speed of a meter and a half per hour, half immersed in sand or silt. The mollusk preliminarily burrows into it with its front part, having taken a vertical position for this. May not move for days until the onset of sunny weather. During rest, lies, buried in the ground with the whole body, except for the upper edge with the mouth opening.
The leg extends when movingforward (it depends on the moment of blood flow into it), the whole body is pulled up to it. Muscle contraction causes the leg to retract back and complete the cycle of stride, a step it takes approximately every fifty seconds.
At the end of summer, closer to the beginning of autumn, mollusks almost completely burrow into the silt for wintering. They reduce the activity of vital processes to a minimum and plunge into a state of stupor, closing the shells tightly and firmly.
Females, males and glochidia: reproduction and development of pearlworts
Shells are different sexes. There are sex glands, but there are no organs of copulation for internal fertilization. They breed in spring - starting from the end of April and throughout May.
Through the excretory siphon, the male sends spermatozoa into the reservoir, and from there they enter the female body through the inlet siphon, where they fertilize the eggs. From one female, several hundred thousand eggs can appear at a time. Embryo development takes place in the outer half-gills of the female.
Freshwater mollusk larvae have a special name - glochidia. They reach maturity and are ready to separate from the mother's body 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 then attach themselves to the gills, skin or fins of fish and parasitize them, thus developing and spreading through water bodies.
The pearlwort reaches puberty after two to three years of life.
Age of shellfishdefined in two ways. The first is according to the number of arcs of annual increment encircling the valve as a whole. Only relief stripes are taken into account. They arise due to a stop in the growth process during wintering.
You can check the result of the inspection by counting the arcuate protrusions on the inner surface of the valve near the blunt end of the shell. Age is added according to the formula: the number of these mother-of-pearl influxes plus two.
The role of pearl barley in the life of a natural reservoir
Perlowitz can be found both in flowing water bodies and in ponds and lakes. They live in shallow and deep waters. Sandy, silty or mixed soil is the most preferred for barley. They can settle on a rocky bottom with the presence of a layer of silt, but they avoid viscous. Shellfish marks on the soft bottom are visible and recognizable, resembling grooves.
An important factor in the presence of pearl barley is the good saturation of water with oxygen.
They are natural and excellent water filterers, a large specimen passes through itself about forty liters daily. Considering the abundance of mollusks in water bodies, their role in this process cannot be overestimated.
The pearls also have a binding and compacting effect on the silt, as each releases a very large amount of mucus.
Pearls in the aquarium
Despite their excellent filtration characteristics and slowness, aquarists consider mollusks to be parasites.
The reason for this is considered to be theirthe ability to produce numerous offspring living at the expense of 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 skin injuries.
