Freshwater bivalve shellfish perch: description, habitat, breeding


Its name, reminiscent of the Russian-speaking person name of the well-known cereals, shellfish perm owes a little 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 clamshell inside the clam shell. The chemical composition of the substance and appearance are really similar to pearl. This property was used by painters, mixing the crushed and processed substance into paints.


Clam perch: origin

Actually, perlits (accent on the second syllable) or shells (they are also Unio in the Latin version) are called just the kind of bivalve class freshwater mollusks of the Uniotide family (the Latin name is Unionidae). They were isolated and described by scientists at the end of the XVIII century.

Areas of river mollusks of this genus are mainly located on the Eurasian continent. In the central European part there are three types of perlovits - thick, wedge-shaped and, of course, ordinary.


The most common types of pearl barley

Unio crassus is one of the species of commonly found mollusc of perlovitsa, in Russian speech called thicker perch. It develops more slowly than the other two species.


The pearl is wedge-shaped or swollen, it is also Unio tumidus, it is distinguished by a more elongated shell shape, a relatively light color and a preference for running water with non-quaternary soil. The dorsal edge of the shell is located below the crown; it is flatter compared to the other two species. Growth lines are often located.


The mollusk of painters or ordinary pearl (Latin name Unio pictorum) grows faster than its brethren. 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 shell valves by painters as a palette.

The height of the shells of perlovits usually range 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.

These representatives of the genus of freshwater inhabitants have sash walls that are thick, the outer layer is smooth with noticeable, albeit thin, annual rings denoting 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 thick barley species.

As habitats, they prefer freshwater uncontaminated reservoirs with a rapid flow. Due to the pollution of many rivers, the decrease in the number of fish in them, whose life is associated with the development of the larvae of mollusks, the number of perlovits starting from the twentieth century is gradually decreasing. At the moment, thick perlovitsy threatens complete extinction.


Perlovitsa structure

All perlovitsy have firm strong walls of two shutters of rather convex or more flat sinks painted in color scale from yellow-gray to almost black. Sash connects the corneal elastic ligament, in front of which the top is located. In most pearls, 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 the well-defined ligament is located, is considered the upper.

There are anterior and posterior closing muscles. The pronounced lock consists of teeth and grooves. The pearl barley has three foot muscle imprints. This term refers to the area of ​​attachment of muscles to the shell, the concept refers to a solid casement, and not to the soft body of the animal.

The shell of mollusks is three-layered. The outer konkholinovy ​​- usually a dirty green tone, under it white porcelain, then the inner - mother-of-pearl. The latter two are formed by calcium carbonate crystals. The color palette of the nacre shell layer can vary from white to pinkish and bluish.

The mollusk has a body and a leg. The epidermis perlovits glossy, smooth or superficially uneven. The body has folds. On the dorsal part is an outgrowth in which most of the internal organs are located. It is called the inner sac. However, there are also cavities with fluid on the body. There is also a secondary cavity containing the heart and the sex glands.

The main fold, located on the border of the foot and the bag, is called the mantle. Its edges hang loosely, growing together only under the expelled siphon (top).

The leg of mollusks has the form of an ax or wedge. On both sides of the perlovi calf there are two half-gabs, 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 germinal holes-siphon - introductory (gill) and output (cloacal). They are separated by the diaphragm.

The diet of perlovitsa mollusks is plankton and detritus (the smallest non-decomposed organic particles). Nutrition occurs by filtering water with particles remaining on the gills. They are covered with mucus and cilia of the ciliated epithelium are moved to the oral opening, then swallowed with a perlovitsa.

The digestive system is represented by three sections. At the base of the leg there is an oral opening with two lobes 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 perlovitsy. The middle intestine, which bends several times, departs from the stomach. Then the food gets into the back intestine.

Unused or recycled substances are discharged to the outside with filtered water through the cloacal cavity. In an animal with a length of 7-8 centimeters, the jet can be thrown up to a distance of forty centimeters.

The nervous system is represented by three pairs of ganglia (ganglia) - the head, foot, and the internal, connected commissural nerve fibers. Nerves stretched from the ganglia to the organs.

Perlitsy are equipped with receptors for skin sensitivity, organs of balance and chemical sense. The latter surround the oral cavity and the hole. The 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-chamber heart (two atria and one ventricle) and arterial and venous vessels. A part of the unclosed system of the mollusk's blood circulation passes through the cavities of its body. The gills are also included in the process.


Features of movement

Perlovits are moving, slowly crawling exclusively on horizontal surfaces, with a speed of one and a half per hour, half submerged in sand or silt. The mollusk is previously buried in its front part, assuming a vertical position for this. May not move days from place to the onset of sunny weather. During rest, it lies buried in the ground with the whole body, except for the upper edge with the oral opening.

When moving, the leg moves forward (it depends on the moment of blood inflow into it), the whole body pulls up to it. Muscle contraction ensures that the leg retracts and completes the cycle of a stroke - a step that it performs approximately every fifty seconds.

At the end of summer, closer to the beginning of autumn, mollusks almost completely burrow into silt for wintering. They reduce the activity of life processes to a minimum and plunge into a state of stupor, tightly and tightly closing the shell flaps.


Females, males and glochidia: breeding and development of perlice

Mollusks are heterogeneous. There are sex glands, but there are no organs of intercourse for internal fertilization. They breed in the spring - from the end of April and throughout May.

Through the excretory siphon, the male sends spermatozoa into the reservoir, and from there they get through the entrance siphon into the female's body, where the eggs are fertilized. From one female several hundreds of thousands of eggs can appear at a time. The development of embryos occurs in the outer half-gaps of the female individual.

The larvae of freshwater mollusks have a special name - glochidia. They reach maturity and are ready for separation from the maternal organism 20–40 days after fertilization - from the end of May to August. The first days (usually not more than three) of the larvae swim in the water, and then they attach to the gills, skin or fins of fish and parasitize on them, developing and spreading in this way through reservoirs.

Sexual maturity of the pearl reaches after two or three years of life.

The age of mollusks is determined in two ways. The first is in the number of enclosures that enclose the entire arch of the annual increment. Only embossed stripes are counted. They arise as a result of stopping the growth process during wintering.

You can check the result of the inspection, considering the arc-like protrusions on the inner surface of the sash near the blunt end of the shell. The age is made up according to the formula: the number of these nacreous nodules plus two.


The role of pearl in the life of a natural reservoir

Perlovits can be met both in flowing reservoirs, and in ponds, lakes. They live in shallow water and in depth. Sandstone, muddy or mixed soil is the most preferable for perlice. They may settle on a rocky bottom with the presence of a layer of silt, but they are viscous. Traces of mollusks on a soft bottom are noticeable and recognizable, resemble grooves.

An important factor in the presence of perlovits is good water saturation with oxygen.

They are natural and excellent filtering water, a large individual passes through about forty liters daily. Given the number of mollusks in reservoirs, their role in this process is difficult to overestimate.

Pearls also have a binder and a sealing effect on the sludge, as each produces a very large amount of mucus.


Pearls in aquarium

Despite excellent filtration characteristics and slowness, aquarists include mollusks to parasites.

The reason for this is their ability to produce numerous offspring living due to many species of fish. They do not cause any harm, but they can cause itching and the desire of the fish to rub against objects, which often leads to injuries of the skin.

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