Lower crustaceans. lower crayfish

Class Crustaceans (Crustacea)

The class Crustacea (Crustacea) includes very diverse arthropods. These include, often similar friend animals like crabs and wood lice, crayfish and shrimps, hermit crabs and carp lice, lobsters and water fleas ... And since adult crustaceans are very diverse in shape, give them brief description that clearly distinguishes them from others. groups of animals, almost impossible. Therefore, evolutionary (genetic) family ties between different representatives of the class are established only by the features of their larval development. And he, in turn, usually includes a complex metamorphosis, in which only the first larval stage - the nauplius - is common to all crustaceans. But some others, and in some cases all, including the first, may be absent, and then a copy of an adult animal immediately hatches from a fertilized egg, but only a miniature one ...

Some edible and harmful species of crustaceans have been known to man since ancient times, but most representatives of this class are known only to narrow specialists. As it turned out, crustaceans are among the most numerous on our planet. Currently, scientists have described more than 25,000 of their species. At the same time, most species of crustaceans live in the seas and oceans, therefore they are figuratively sometimes called "sea insects" for their abundance and diversity. However, many species of crustaceans also live in fresh waters and on dry land. Therefore, they can practically be found in all water bodies: and under the ice in polar regions, and in hot springs with temperatures up to 50 ° C, and in deserts, and at depths of up to 6 km, and the tops of tropical trees.

Veliko and economic importance crustaceans. Wherein great importance have crabs, lobsters, crayfish and shrimp, which a person directly eats. But numerous small forms that swim en masse at the surface of water bodies as part of zooplankton and are often barely visible to the naked eye, form the main link in a number of food chains. It is these tiny crustaceans that are the link between microscopic planktonic algae with fish, whales and other large game animals. Without small crustaceans, which turn plant cells into easily digestible animal food, the existence of most representatives of the aquatic fauna would become almost impossible.

Among the crustaceans, there are many species that are harmful to humans, which in one way or another cause damage to the economy of a person or his health. Thus, the drilling forms of crustaceans, such as the wood-boring shrimp, make passages in wooden port facilities and other underwater structures. On the bottoms of ships, powerful fouling of sea acorns and sea ducks form, which interfere with navigation. Some types of crabs, crayfish and some other crustaceans are found in the tropics (and Far East Russia) as carriers of human diseases, and other crustaceans, such as wood lice and shield bugs, often damage vegetation, in particular rice crops, or farmed marine species.

Crustaceans- These are aquatic arthropods or inhabitants of 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, lobster, shrimp, crayfish).

Representative of higher cancers - river crayfish. It lives in fresh water bodies with 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. In front of the spine there are two pairs of antennae, and on the sides on movable stalks are two complex (faceted) eyes. 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 - blood gives off carbon dioxide and saturated with oxygen. Circulatory system open. It consists of a pentagonal heart located on the dorsal side and vessels extending from it. Blood pigment contains copper, so it of blue color. The nervous system of crayfish resembles nervous system 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. Crayfish are separate. 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 fish diseases by settling on their skin or gills, some are intermediate hosts tapeworms and roundworms.

Shell crustaceans belong to the lower crustaceans and make up the order of ostracods (Ostracoda). The body of crustaceans is divided into cephalothorax and abdomen. Widespread are also copepods (Copepoda) - cyclops and diaptomus, which belong to the subclass Maxillopod (Maxillopoda). Daphnia, or water fleas, are among the lower crustaceans, namely cladocerans(suborder Cladocera in the phyllopoda order).

The water donkey (Asellus aquaticus L.) is a representative of the class of crustaceans, belongs to the order of isopods (Isopoda), to the donkey family (Asellidae). Donkeys stay at the bottom of reservoirs, where they crawl between the dead parts of plants and, together with them, are carried out with a net. In these bags, clearly visible to the naked eye, the eggs develop and juveniles are formed in the form of fully formed crustaceans, generally similar to adults.

The number of eggs in one female is very different - from a few dozen to a hundred or more. A young donkey reaches maturity within two months on average. Of these, the first two pairs are called rowing tendrils, or antennae, and serve for movement. Like aquatic fleas, there is a well-developed eye on the head, which shines through a thin shell valve.

On the left - swimming of a shellfish. The arrows show the convergence and separation of the antennas. When crawling on the substrate, it plays the role of a pair of legs equipped with claws, and the second pair of antennae is also used. Some species have completely lost the ability to swim and are exclusively bottom dwellers. Ostracods feed on small organisms found in the silt, and very willingly eat the corpses of small animals.

Like water fleas, barnacles are able to reproduce parthenogenetically for some time, and such reproduction alternates with sexual reproduction. Their larvae have the same ability. The body of the water flea (in most species) is enclosed in a transparent bivalve chitinous shell, both halves of which are fastened on the dorsal side and half-opened on the ventral side.

Branched rowing antennae, or antennae, depart from the head; hence the name "branched". They should be caught with a net made of fine mesh fabric. It is recommended at the same time to drive a net along clean water, without touching the bottom and not picking up a net in the bag aquatic plants. In our country, this form is found in many lakes of the northern and middle lane Russia. The movements of water fleas can be observed even with the naked eye. The result is a series of successive jumps, which, indeed, I have: some resemblance to the movement of a flea (hence the name "water flea").

Cyclops (Cyclops coronatus). The abdomen carries six pairs of swimming legs and ends with two processes - a fork. In females, paired egg sacs can often be seen on the sides of the body. Copepods are found in a wide variety of water bodies, where they sometimes develop in huge quantities especially in spring and autumn. The most primitive crustaceans belong to the branchiopoda subclass.

lower crustaceans

Daphnia, inhabitants of the water column, are often called water fleas, probably because of their small size and hopping mode of movement. 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. An even smaller owner of a brownish spherical shell - Chydorus sphaericus - can be found both in the water column and among coastal thickets.

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. From here it happened common name crustaceans - "copepods". Diaptomuses, like Daphnia, are quite peaceful animals. The elongated body of the crustacean is translucent and colorless, they need to be invisible to predators. Among them there are large forms. More than 40 thousand species of crustaceans are known.

The cephalothorax consists of segments of the head and chest, merging into a common, usually undivided body section. The abdomen is often dissected. The first 2 pairs are represented by jointed antennae; these are the so-called antennules and antennae. Crustaceans are characterized by a two-branched structure of the limbs. Due to the evolution in aquatic environment crustaceans have developed organs of water respiration - gills. They often represent outgrowths on the limbs.

Significance of crustaceans

Crustaceans, with rare exceptions, have separate sexes. A nauplius larva emerges from the egg with a non-segmented body, 3 pairs of limbs and one unpaired eye. Lower crayfish live both in fresh waters and in the seas. They have importance in the biosphere, being an essential part of the diet of many fish and cetaceans.

Antennules uniramous, antennae and peduncles of thoracic segments biramous. 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. The color of the Cyclopes depends on the type and color of the food they eat (gray, green, yellow, red, brown).

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 their heads (these include: two pairs of antennae (antennae), a pair of lower jaws (maxillae), 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 The crustacean begins 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.

Food

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, are active predators that grab and tear their prey with 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 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).