Crustaceans are aquatic arthropods or inhabitants wet places. Their body sizes vary from a few millimeters to 1 m. They are ubiquitous; lead a free or attached lifestyle. The class includes about 20 thousand species. Only crustaceans are characterized by the presence of two pairs of antennae, biramous limbs, and gill breathing. The class Crustacea combines 5 subclasses. Conventionally, all representatives are divided into lower (daphnia, cyclops) and higher crayfish (lobster, spiny lobster, shrimp, crayfish).

Representative of higher cancers - river crayfish. It lives in fresh water running water, is nocturnal and is a predator.

Crayfish. External and internal structure:
1 - Antennae, 2 - Claw, 3 - Walking legs, 4 - Caudal fin, 5 - Abdomen, 6 - Cephalothorax, 7 - Head ganglion, 8 - Digestive tube, 9 - Green gland, 10 - Gills, 11 - Heart, 12 - gonad

The body of the cancer is covered with a dense chitinous shell. The fused segments of the head and chest form the cephalothorax. Its front part is elongated and ends with a sharp spike. Two pairs of antennae are located in front of the spine, and two complex (faceted) eyes are located on the sides on movable stalks. Each eye contains up to 3 thousand small eyes. Modified limbs (6 pairs) form the oral apparatus: the first pair is the upper jaws, the second and third are the lower jaws, the next three pairs are the jaws. The thoracic region bears 5 pairs of jointed limbs. The first pair is the organ of attack and defense. It ends with powerful pincers. The remaining 4 pairs are walking limbs. The limbs of the jointed abdomen are used in females for carrying eggs and cubs. The abdomen ends with a caudal fin. When the crayfish swims, it scoops up water with it and moves with its tail end forward. Bundles of striated muscles are attached to the internal protrusions of the chitinous cover.

Cancer feeds on both living organisms and decaying animal and plant debris. The crushed food enters through the mouth into the pharynx and esophagus, then into the stomach, which has two sections. Chitinous teeth of the chewing section grind food; in the filter stomach, it is filtered and enters the middle intestine. The ducts of a large digestive gland, which performs the functions of the liver and pancreas, also open here. Under the action of its secret, the food slurry is digested. Nutrients are absorbed, and undigested residues through the hindgut and anus are thrown out.

The excretory organs of cancer are a pair of green glands (modified metanephridia) that open at the base of long antennae. Respiratory organs - gills located on the sides of the cephalothorax. They are permeated with blood vessels in which gas exchange occurs - the blood gives off carbon dioxide and is saturated with oxygen. The circulatory system is not closed. It consists of a pentagonal heart located on the dorsal side and vessels extending from it. The blood pigment contains copper, which is why it is blue in color. The nervous system of crayfish resembles the nervous system of annelids. It consists of the supraglottic and subpharyngeal ganglia, united in the circumpharyngeal ring, and the ventral nerve cord. The organs of vision, touch and smell (on the antennae), balance (at the base of the short antennae) are well developed. Cancers are segregated. Reproduction is sexual, development is direct. Eggs are laid in winter; small crayfish hatch from eggs in early summer. Cancer expresses concern for offspring.

Significance of crustaceans. Crustaceans serve as food for aquatic animals and for humans (lobsters, crabs, shrimps, crayfish). They clean the water bodies from carrion. Some representatives of crustaceans cause diseases of fish, settling on their skin or gills, some are intermediate hosts for tapeworms and roundworms.

Description

The body of crustaceans is divided into the following sections: head, thoracic and abdominal. In some species, the head and thorax are fused together (cephalothorax). Crustaceans have an external skeleton (exoskeleton). The cuticle (outer layer) is often reinforced with calcium carbonate, which provides additional structural support (especially true for large species).

Many species of crustaceans have five pairs of appendages on the head (these include: two pairs of antennae (antennae), a pair of lower jaws (maxillas) and a pair of upper jaws (mandibles, or mandibles)). The compound eyes are located at the end of the stalks. The thorax contains several pairs of pereiopods (walking legs), and the segmented belly contains pleopods (abdominal legs). The posterior end of the crustacean body is called the telson. large species Crustaceans breathe with gills. Small species for gas exchange using the surface of the body.

reproduction

Most species of crustaceans are heterosexual and reproduce sexually, although some groups, such as barnacles, remipedians, and cephalocarids, are hermaphrodites. Life cycle Crustaceans begin with a fertilized egg that is either released directly into the water or attached to the genitals or legs of the female. After hatching from an egg, crustaceans go through several stages of development before turning into an adult.

food chain

Crustaceans occupy a key place in the sea and are one of the most common animals on Earth. They feed on organisms such as phytoplankton, in turn, crustaceans become food for larger animals such as fish, and some crustaceans such as crabs, lobsters and shrimp are a very popular food for humans.

Dimensions

Crustaceans come in a wide variety of sizes from microscopic aquatic fleas and crustaceans to the giant Japanese spider crab, which reaches a mass of about 20 kg and has legs 3-4 m long.

Nutrition

In the process of evolution, crustaceans have acquired a wide range of feeding habits. Some species are filter feeders, extracting plankton from the water. Other species, especially large ones, active predators, which capture and tear their prey with the help of powerful appendages. There are also scavengers, especially among small species, feeding on the decaying remains of other organisms.

First crustaceans

Crustaceans are well represented in the fossil record. The first representatives of crustaceans belong to the Cambrian period and are represented by fossils mined in the Burges Shale Shale Formation, located in Canada.

Classification

Crustaceans include the following 6 classes:

• Gillnopods (Branchiopoda);
• Cephalocarids (Cephalocarida);
• higher crayfish (Malacostraca);
• Maxillopods (Maxillopoda);
• Shellfish (Ostracoda);
• crested (remipedia).

Crustaceans are ancient aquatic animals with a complex dissection of the body covered with a chitinous shell, with the exception of woodlice living on land. They have up to 19 pairs of jointed legs that perform various functions: capturing and grinding food, locomotion, protection, mating, and bearing juveniles. These animals feed on worms, molluscs, lower crustaceans, fish, plants, and crayfish also eat dead prey - the corpses of fish, frogs and other animals, acting as orderlies of reservoirs, especially since they prefer very clean fresh water.

Lower crustaceans - daphnia and cyclops, representatives of zooplankton - serve as food for fish, their fry, toothless whales. Many crustaceans (crabs, shrimps, lobsters, lobsters) are commercial or specially bred animals.

2 types of crustaceans are included in the Red Book of the USSR.

general characteristics

From a medical point of view, some species of planktonic crustaceans are of interest as intermediate hosts of helminths (cyclops and diaptomus).

Until recently, the Class Crustacea was divided into two subclasses - lower and higher crayfish. In the subclass of lower crayfish, phyllopods, maxillopods and shell crayfish were combined. It is now recognized that such a union is impossible, since these groups of cancers are different in their origin.

In this section, the class Crustaceans will be considered according to the old classification.

The body of crustaceans is divided into cephalothorax and abdomen. The cephalothorax consists of segments of the head and chest, merging into a common, usually undivided body section. The abdomen is often dissected.

All crustaceans have 5 pairs of head limbs. The first 2 pairs are represented by jointed antennae; these are the so-called antennules and antennae. They carry the organs of touch, smell and balance. The next 3 pairs - oral limbs - serve to capture and grind food. These include a pair of upper jaws, or mandibles, and 2 pairs of lower jaws - maxilla. Each thoracic segment bears a pair of legs. These include: the jaws involved in holding food, and locomotor limbs (walking legs). The abdomen of higher crayfish also bears limbs - swimming legs. The lower ones don't.

Crustaceans are characterized by a two-branched structure of the limbs. They distinguish between the base, external (dorsal) and internal (ventral) branches. Such a structure of the limbs and the presence of gill outgrowths on them confirms the origin of crustaceans from polychaete annelids with biramous parapodia.

In connection with evolution in the aquatic environment, crustaceans developed organs of water respiration - gills. They often represent outgrowths on the limbs. Oxygen is delivered by the blood from the gills to the tissues. Lower cancers have colorless blood called hemolymph. Higher cancers have real blood containing pigments that bind oxygen. The blood pigment of crayfish - hemocyanin - contains copper atoms and gives the blood a blue color.

The excretory organs are one or two pairs of modified metanephridia. The first pair is localized in the anterior part of the cephalothorax; its duct opens at the base of the antennae (antennary glands). The duct of the second pair opens at the base of the maxillae (maxillary glands).

Crustaceans, with rare exceptions, have separate sexes. They usually develop with metamorphosis. A nauplius larva emerges from the egg with a non-segmented body, 3 pairs of limbs and one unpaired eye.

• Subclass Entomostraca (lower crayfish).

  Lower crayfish live both in fresh waters and in the seas. They are important in the biosphere, being an essential part of the diet of many fish and cetaceans. Highest value have copepods(Copepoda), serving as intermediate hosts of human helminths (diphyllobotriids and guinea worm). They are found everywhere in ponds, lakes and other stagnant bodies of water, inhabiting the water column.

general characteristics

The body of the crustacean is divided into segments. The complex head bears one eye, two pairs of antennae, a mouthpart, and a pair of legs-jaws. One pair of antennas is much longer than the other. This pair of antennae is highly developed, their main function being movement. They also often serve to hold the female by the male during mating. Thorax with 5 segments, pectoral legs with swimming bristles. Abdomen of 4 segments, at the end - a fork. At the base of the abdomen of the female there are 1 or 2 egg sacs in which the eggs develop. Nauplii larvae emerge from the eggs. Hatched nauplii are completely different from adult crustaceans. Development is accompanied by metamorphosis. Copepods feed on organic remains, the smallest aquatic organisms: algae, ciliates, etc. They live in water bodies all year round.

The most common genus is Diaptomus.

Diaptomuses live in the open part of water bodies. The size of the crustacean is up to 5 mm. The body is covered with a rather hard shell in connection with which it is reluctantly eaten by fish. The color depends on the nutrient base of the reservoir. Diaptomuses have 11 pairs of limbs. Antennules uniramous, antennae and peduncles of thoracic segments biramous. The antennules reach especially great lengths; they are longer than the body. Scattering them widely, the diaptomuses soar in the water, the thoracic limbs cause spasmodic movements of the crustaceans. The mouth limbs are in constant oscillatory motion and adjust particles suspended in water to the mouth opening. In diaptomus, both sexes take part in reproduction. Female diaptomus, unlike female cyclops, has only one egg sac.

Species of the genus Cyclops (Cyclops)

predominantly inhabit coastal zones of water bodies. Their antennae are shorter than those of the diaptomus, and along with the thoracic legs, they participate in jerky movement. The color of the Cyclopes depends on the type and color of the food they eat (gray, green, yellow, red, brown). Their size reaches 1-5.5 mm. Both sexes take part in reproduction. The female carries fertilized eggs in egg sacs (cyclops have two) attached at the base of the abdomen.

According to their biochemical composition, copepods are in the top ten high-protein foods. In the aquarium trade, "cyclops" is most often used for feeding grown-up juveniles and small-sized fish species.

Daphnia, or water fleas

move in leaps and bounds. The body of Daphnia, 1-2 mm long, is enclosed in a bivalve transparent chitinous shell. The head is extended into a beak-like outgrowth directed to the ventral side. There is one complex compound eye on the head and a simple eye in front of it. The first pair of antennae is small, rod-shaped. The antennae of the second pair are strongly developed, two-branched (with their help Daphnia swims). On the thoracic region there are five pairs of leaf-shaped legs, on which there are numerous feathery setae. Together they form a filtration apparatus that serves to filter out small organic residues, unicellular algae and bacteria that Daphnia feed on from the water. At the base of the thoracic pedicles are gill lobes, in which gas exchange occurs. On the dorsal side of the body is a barrel-shaped heart. There are no blood vessels. Through a transparent shell, a slightly curved tubular intestine with food, a heart, and under it a brood chamber, in which Daphnia larvae develop, are clearly visible.

• Subclass Malacostraca (higher crayfish). The structure is much more complicated than that of lower crayfish. Along with small planktonic forms, there are relatively large species.

  Higher crayfish are inhabitants of marine and fresh water bodies. Only wood lice and some crayfish (palm crayfish) live on land from this class. Some species of higher crayfish serve as an object of fishing. In the seas Far East the gigantic Pacific crab is harvested, the walking legs of which are used for food. IN Western Europe lobster and lobster are harvested. In addition, crayfish are of sanitary importance, because. free water bodies from the corpses of animals. Freshwater crayfish and crabs in the countries of the East are intermediate hosts for the lung fluke.

  A typical representative of higher crayfish is crayfish.

Crayfish lives in flowing fresh water bodies (rivers, streams), feeds mainly on plant foods, as well as dead and living animals. During the day, the crayfish hides in safe places: under stones, between the roots of coastal plants or in minks that it digs with claws in steep banks. Only at nightfall does he go out to look for food. For the winter, crayfish hide in their burrows.

The structure and reproduction of crayfish

External structure. The body of the crayfish is covered on the outside with a cuticle impregnated with calcium carbonate, which gives it strength, which is why the cuticle is called the shell. The shell protects the body of crayfish from damage and acts as an external skeleton. At a young age, during the growth period, crayfish change their shell. This process is called molting. Over time, when the crayfish reaches large sizes, it grows slowly and rarely sheds.

The color of the shell of a live crayfish depends on the color of the muddy bottom on which it lives. It can be greenish-brown, light green, dark green and even almost black. This coloration is protective and allows the cancer to become invisible. When the caught crayfish are boiled, the destruction of the part occurs. chemical substances giving color to the shell, but one of them - the red pigment astaxanthin - does not decompose at 100 ° C, which determines the red color of boiled crayfish.

The body of crayfish is divided into three sections: the head, chest and abdomen. On the dorsal side, the head and thoracic sections are covered with a single cephalothoracic solid solid chitinous shield, which carries a sharp spike in front, on its sides in recesses on movable stems there are compound eyes, a pair of short and a pair of long thin antennae. The latter are a modified first pair of limbs.

On the sides and below the oral opening of the crayfish are six pairs of limbs: upper jaws, two pairs of lower jaws and three pairs of mandibles. There are also five pairs of walking legs on the cephalothorax, and claws on the three front pairs. The first pair of walking legs is the largest, with the most well-developed claws, which are the organs of defense and attack. The mouth limbs, together with the claws, hold food, crush it and direct it into the mouth. The upper jaw is thick, serrated, powerful muscles are attached to it from the inside.

The abdomen consists of six segments. The extremities of the first and second segments in the male are modified (they participate in copulation), in the female they are reduced. On four segments there are two-branched jointed zeros; the sixth pair of limbs - wide, lamellar, are part of the caudal fin (it, together with the caudal lobe, plays an important role when swimming backwards).

Movement of crayfish. The crayfish can crawl and swim back and forth. He crawls along the bottom of the reservoir with the help of chest walking legs. Forward crayfish swims slowly, sorting through the abdominal legs. It uses its tail fin to move backwards. Straightening it and bending its abdomen, the crayfish makes a strong push and quickly swims back.

Digestive system begins with the mouth opening, then food enters the pharynx, short esophagus and stomach. The stomach is divided into two sections - chewing and filtering. On the dorsal and lateral walls of the chewing section, the cuticle forms three powerful lime-impregnated chitinous chewing plates with serrated free edges. In the sieve section, two plates with hairs act like a filter through which only highly crushed food passes. Further, the food enters the midgut, where the ducts of the large digestive gland open. Under the action of digestive enzymes secreted by the gland, food is digested and absorbed through the walls of the middle intestine and gland (it is also called the liver, but its secret breaks down not only fats, but also proteins and carbohydrates, i.e. functionally corresponds to the liver and pancreas of vertebrates). Undigested residues enter the hindgut and are excreted through the anus on the caudal lobe.

Respiratory system. Crayfish breathe with gills. Gills are feathery outgrowths of the thoracic limbs and the side walls of the body. They are located on the sides of the cephalothoracic shield inside a special gill cavity. The cephalothoracic shield protects the gills from damage and rapid drying, so the crayfish can live out of water for some time. But as soon as the gills dry out a little, the cancer dies.

Circulatory organs. The circulatory system of crayfish is not closed. Blood circulation occurs due to the work of the heart. The heart is pentagonal in shape, located on the dorsal side of the cephalothorax under the shield. Blood vessels depart from the heart, opening into the body cavity, where blood gives oxygen to tissues and organs. The blood then flows to the gills. The circulation of water in the gill cavity is provided by the movement of a special process of the second pair of lower jaws (it produces up to 200 waving movements in 1 minute). Gas exchange occurs through the thin cuticle of the gills. Oxygen-enriched blood is sent through the gill-cardiac canals to the pericardial sac, from there it enters the heart cavity through special openings. Cancer blood is colorless.

excretory organs paired, have the appearance of round green glands, which are located at the base of the head and open outwards with a hole at the base of the second pair of antennae.

Nervous system consists of a paired supraesophageal ganglion (brain), peripharyngeal connectives, and ventral nerve cord. From the brain, the nerves go to the antennae and eyes, from the first node of the ventral nerve chain, or subpharyngeal ganglion, to the mouth organs, from the following thoracic and abdominal nodes of the chain, respectively, to the thoracic and abdominal limbs and internal organs.

sense organs. Compound, or compound eyes in crayfish are located in front of the head on movable stalks. The composition of each eye includes more than 3 thousand eyes, or facets, separated from each other by thin layers of pigment. The light-sensitive part of each facet perceives only a narrow beam of rays perpendicular to its surface. The whole image is made up of many small partial images (like a mosaic image in art, so they say that arthropods have mosaic vision).

The antennae of cancer serve as organs of touch and smell. At the base of the short antennae is the organ of balance (statocyst, located in the main segment of the short antennae).

Reproduction and development. Crayfish have developed sexual dimorphism. In the male, the first and second pairs of abdominal legs are modified into a copulatory organ. In the female, the first pair of abdominal legs is rudimentary; on the remaining four pairs of abdominal legs, she bears eggs (fertilized eggs) and young crustaceans, which remain under the protection of the mother for some time, clinging to her abdominal limbs with their claws. So the female takes care of her offspring. Young crayfish grow intensively and molt several times a year. The development of crayfish is direct. Crayfish breed quite quickly, despite the fact that they have relatively few eggs: the female lays from 60 to 150-200, rarely up to 300 eggs.

Significance of crustaceans

Daphnia, cyclops and other small crustaceans consume a large number of organic remains of dead small animals, bacteria and algae, thereby purifying the water. In turn, they are an important food source for larger invertebrates and juvenile fish, as well as for some valuable planktivorous fish (eg whitefish). In pond fish farms and fish hatcheries, crustaceans are specially bred in large pools, where favorable conditions are created for their continuous reproduction. Daphnia and other crustaceans are fed to young sturgeon, stellate sturgeon and other fish.

Many crustaceans are of commercial importance. About 70% of the world's crustacean fishery is shrimp, and they are also bred in ponds created on the coastal lowlands and connected to the sea by a canal. Shrimps in ponds are fed with rice bran. There is a fishery for krill - planktonic marine crustaceans that form large aggregations and serve as food for whales, pinnipeds and fish. Food pastes, fat, fodder meal are obtained from krill. Of lesser importance is the fishing of lobsters and crabs. In our country, in the waters of the Bering, Okhotsk and Japanese seas, king crab is harvested. Commercial fishing for crayfish is carried out in fresh water, mainly in Ukraine.

• Class Crustacea (crustaceans)

lower crustaceans

Subclass Gillpods

The most primitive These small crustaceans have leaf-shaped legs and are used equally for locomotion and respiration. They also create a current of water that brings food particles to the mouth. Their eggs easily tolerate desiccation and wait in the soil for the new rainy season. Artemia is interesting among the branchiopods: it can live in salt lakes with a salt concentration of up to 300 g / l, and dies in fresh water after 2-3 days.

Subclass Maxillopods (maxillopods)

Representatives of the order of barnacles are amazing: sea acorns and sea ducks. These sea crayfish have moved to a sedentary lifestyle in houses made of calcareous plates. The larva is a typical nauplius, sinks to the bottom and is attached by antennules. The antennae and the entire anterior part of the head turn into an attachment organ (a long fleshy stalk in sea ducks, or a flat wide sole in sea acorns), antennae and compound eyes atrophy, pectoral legs stretch into long biramous "antennae" that drive food to the mouth.

The most primitive crustaceans belong to the subclass Gillnopods(Branchiopoda). Daphnia(Daphnia) are representatives of the Leaf-legged order, the branched mustache suborder. Daphnia, inhabitants of the water column, are often called water fleas, probably because of their small size and hopping mode of movement. Let's put some living D. magna in a glass jar and observe them. The body of the crustaceans is up to 6 mm long, covered with a bivalve shell flattened laterally. A large black spot stands out on a small head - an eye, and a brownish-greenish intestine clogged with food shines through in the trunk region.

Daphnia (Daphnia magna)

Daphnia never rest for a second. main role in motion, waves of long side antennas play. The legs of Daphnia are leaf-shaped, small, they do not take any part in the movement, but they regularly serve for nutrition and respiration. The legs are constantly working, making up to 500 strokes per minute. So they create a current of water that carries algae, bacteria, yeast and oxygen.

The cladocerans also include such pelagic crustaceans as a small (less than 1 mm in length) bosmina longnosed(Bosmina longirostris). It is easily recognizable by its long, curved nose - the rostrum - with a tuft of bristles in the middle. Even more petty owner brownish spherical shell - hydrorus spherical(Chydorus sphaericus) - can be found in the water column and among coastal thickets.

Also widespread copepods(Copepoda) - Cyclops and Diaptomus, which belong to the subclass Maxillopod(Maxillopoda). Their body consists of a head, segmented chest and abdomen. The main organ of movement is powerful antennae and pectoral legs bearing swimming bristles. The legs work synchronously, like oars. Hence the common name of crustaceans - "copepods".

Diaptomus (Eudiaptomus graciloides), female	Diaptomus (Eudiaptomus graciloides), male

Diaptomuses, like Daphnia, are quite peaceful animals. In a glass vessel, you can easily observe their movement. Diaptomuses(Eudiaptomus graciloides) soar smoothly, balancing with outstretched antennae, the length of which is almost equal to the length of the entire body. Having gone down, they make a sharp stroke with their pectoral legs and short abdomen and "jump" up. The current of water, carrying food, is created by crustaceans with short second antennae, making several hundred beats per minute. The elongated body of the crustacean is translucent and colorless, they need to be invisible to predators. Diaptomus females often carry a small pouch filled with eggs under their belly. Males are easily distinguished by the right antenna with a knot in the middle and the last pair of legs, which is complexly arranged, with long hooked outgrowths. These devices are used by the male to hold the female.

More common in fresh waters cyclops, named after the one-eyed hero ancient Greek myths. There is only one eye on the head of these crustaceans! Cyclops (Cyclops kolensis) have short antennae; adult females carry their eggs in two bags on the sides of the abdomen. Males hold their partners with both anterior loop-shaped antennae. Cyclopes differ in fussy, seemingly erratic movement. They "jump" often and sometimes somersault in the water. The fast and chaotic movement of the Cyclopes is aimed at achieving two main goals: firstly, not to get caught in the mouth of a fish, and secondly, to have time to grab something edible. Cyclopes are by no means vegetarians. If you get caught large seaweed, they will also eat it, but they still prefer the juveniles of their branched and copepod neighbors and other aquatic trifles, for example, ciliates and rotifers.