Lower crustaceans. Crustaceans in nature. Lower and higher crustaceans: characteristic differences
Subclass Gill-footed
The most primitive. These small crustaceans have leaf-shaped legs and are used equally for movement and breathing. They also create a current of water that carries food particles to the mouth. Their eggs easily tolerate drying out and wait in the soil for the new rainy season. Artemia is an interesting branchiopod: 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.
Crustaceans- These are aquatic arthropods or inhabitants of wet places. Their body sizes range from a few millimeters to 1 m. They are distributed everywhere; lead a free or attached lifestyle. The class has about 20 thousand species. Only crustaceans are characterized by the presence of two pairs of antennae, two-branched limbs and gill breathing. The class Crustaceans includes 5 subclasses. Conventionally, all representatives are divided into lower (daphnia, cyclops) and higher crayfish (lobster, lobster, shrimp, river crayfish).
Representative of higher cancers - crayfish. It lives in fresh water bodies with running water, leads night image life and is a predator.
Crayfish. External and internal structure:
1 - Antennae, 2 - Claw, 3 - Walking legs, 4 - Caudal fin, 5 - Abdomen, 6 - Cephalothorax, 7 - Cephalic ganglion, 8 - Digestive tube, 9 - Green gland, 10 - Gills, 11 - Heart, 12 — Sex gland
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 there are two compound (compounded) eyes. Each eye contains up to 3 thousand small ocelli. Modified limbs (6 pairs) form the oral apparatus: the first pair are the upper jaws, the second and third are the lower jaws, the next three pairs are the maxillae. The thoracic region bears 5 pairs of jointed limbs. The first pair is the organ of attack and defense. It ends in powerful claws. The remaining 4 pairs are walking limbs. The limbs of the segmented abdomen are used in females to bear eggs and young. The abdomen ends with a caudal fin. When a crayfish swims, it scoops up water with it and moves with its tail end forward. Bundles of striated muscles are attached to the internal projections of the chitinous cover.
Cancer feeds on both living organisms and decaying animal and plant debris. The crushed food passes through the mouth into the pharynx and esophagus, then into the stomach, which has two sections. The chitinous teeth of the chewing section grind food; in the filter stomach it is filtered and enters the midgut. The channels of large digestive gland, which performs the functions of the liver and pancreas. Under the influence of its secretion, food gruel is digested. Nutrients are absorbed, and undigested residues are expelled through the hindgut and anus.
The excretory organs of cancer are a pair of green glands (modified metanephridia), which open at the base of the long antennae. Respiratory organs are gills located on the sides of the cephalothorax. They are penetrated by blood vessels in which gas exchange occurs - the blood gives off carbon dioxide and is saturated with oxygen. Circulatory system open It consists of a pentagonal heart located on the dorsal side and the vessels extending from it. Blood pigment contains copper, which is why it is blue. The nervous system of crayfish resembles the nervous system of annelids. It consists of the suprapharyngeal and subpharyngeal ganglia, united in a peripharyngeal ring, and the abdominal nerve cord. The organs of vision, touch and smell (on the antennae), and balance (at the base of the short antennae) are well developed. Cancers are dioecious. Reproduction is sexual, development is direct. Eggs are laid in winter, and small crayfish hatch from eggs in early summer. Cancer expresses concern for offspring.
The meaning of crustaceans. Crustaceans serve as food for aquatic animals and as food for humans (lobsters, crabs, shrimp, crayfish). They clear water bodies of carrion. Certain representatives of crustaceans cause fish diseases by settling on their skin or gills; some are intermediate hosts of tapeworms and roundworms.
Crustaceans, or crayfish, evolved from trilobite-like arthropods that moved to faster movement at the bottom of reservoirs and in the water column. Due to a more active lifestyle, the organization of crustaceans has become significantly more complex compared to their ancestors. This is a large and diverse class, representatives of which live in marine, fresh and brackish waters. Only a few crustaceans live on land, but only in damp places.
External building. The structure of crayfish (see Fig. 75, 80) is very diverse. Division of the body into sections different groups not similar. Often the head and thoracic regions merge together to form the cephalothorax, to which the articulated abdomen is connected. The size of the body varies widely: many forms - microscopic organisms that live mainly in the water column; bottom forms often reach large sizes. The cuticle of crustaceans, like that of all aquatic arthropods, consists of two main layers: the inner - endocuticle, and the outer - exocuticle (Fig. 78). The latter is impregnated with tannins and is therefore very durable. During molting, the endocuticle dissolves and is absorbed by the hypodermis, but the exocuticle is insoluble and is completely shed. Large crayfish are covered with strong shells. Small forms may also have armored formations, but for the most part the chitinous cuticle covering them is thin. In one order of lower crayfish (shell crustaceans), the body is enclosed in a bivalve calcareous shell. All crustaceans have two pairs of antennae, or antennae (Fig. 73, 80), the structure and functions of which are not similar in different groups of the class (see below).
Nervous system. At the row lower forms the central section of this system consists of a relatively simple brain and abdominal cords that form a ladder, not a chain (see Fig. 72); in other crustaceans, the brain becomes more complex (in varying degrees in different groups), the abdominal cords form a chain, the nodes of which, as the concentration of the body increases, can be connected until all the nodes merge into one (see Fig. 72). The behavior of the highest representatives of the class, who are, as a rule, active predators, reaching a very large value, is greatly complicated and is ensured by progressive changes in the entire nervous system. The organs of touch in the form of sensitive bristles are scattered throughout the body, but there are especially many of them on the antennae. The organs that perceive chemical irritations are quite well developed; in large crayfish they are concentrated mainly on the antennae of the first pair. Equilibrium organs (statocysts) are distributed mainly in higher crayfish and are located in the first segment of the first pair of antennae (Fig. 79).
Eyes can be simple or complex. Compound or faceted eyes (Fig. 79) consist of a large number of individual eyes, or ommatidia. Each ommatidium consists of a cornea (the transparent part of the chitinous cuticle), a crystal cone - an elongated transparent body, to which are adjacent nerve, or retinal, cells that secrete light-sensitive rods (rhabdoms) on their inner edges. Ommatidia are separated from each other by pigment cells. Rays falling obliquely on the ommatidia are absorbed by pigment cells that isolate the ommatidia from each other and do not reach the nerve cells. The latter perceive only those rays that fall perpendicular to the surface of the ommatidium. Thus, each ommatidia perceives only part of the object, yet the ommatidia perceive the entire object. The image of an object in the compound eye is made up of its individual parts and resembles mosaic paintings (or mosaics) made up of multi-colored pebbles or plates. Therefore, such vision is called mosaic. Many large crayfish have compound eyes located on special stalks.
Propulsion system. The movement of crayfish is accomplished with the help of different limbs - antennae or legs in planktonic, usually small forms (Fig. 80), special walking legs in benthic, usually large forms (see Fig. 73). In addition, the latter can swim due to the strong tuck of the abdomen under the chest. In crayfish, unlike terrestrial arthropods, two-branched limbs are widespread, which, together with the setae, have a wide surface and are convenient for use as oars. In large crayfish, for example, the crayfish, the branches of the hind pair of legs have turned into two wide plates (see Fig. 73), which, together with the last, very wide segment of the abdomen, are good for scooping up water with the abdomen.
Circulatory system. The heart, like all arthropods, located on the dorsal side, is present in most crustaceans (see Fig. 75, 80, A). The shape of the heart varies: from a long tube to a compact sac. In a number of small forms, the heart is absent and the movement of blood is caused by intestinal movements, as well as movements of the whole body. The development of the network of blood vessels mainly depends on the size of the body: in large crayfish it can be quite well developed, in small crayfish it can be completely reduced.
Respiratory system. The respiratory organs of most crustaceans are gills, which are appendages of legs that have different shapes: in small crayfish these are rounded leaves (Fig. 80, A), in large crayfish (such as crayfish) they are finely dissected (see Fig. 75), due to which their surface increases. The change of water near the gills occurs due to the movement of the legs on which they are located, as well as due to the movement of certain limbs that do not have gills. A fairly significant number of small species do not have gills and oxygen absorption occurs through the surface of the body, mainly in its thinner places.
Excretory system. The excretory system is represented mainly by a pair, rarely more, metanephridia. Reduction in the number of these organs compared to annelids, in which they are numerous, is explained mainly by the fact that in crustaceans the body cavity is continuous, not divided by partitions, like in ringlets, and it is enough for them to have a small number of excretory organs, but more complexly arranged, divided into a number of sections (Fig. 81). In higher crayfish, metanephridia reach especially great complexity; they are large (about 1 cm or more) and open at the base of the antennae of the second pair and are therefore called antennal. In other crayfish, the metanephridia are simpler in structure, they are smaller (see Fig. 80, A) and open at the base of the second pair of mandibles, or maxillae, which is why they got the name maxillary.
Digestive system. The digestive system is very diverse. Small crustaceans (see Fig. 80), living in the water column, receive food (organic pieces, bacteria, algae, microscopic animals) as a result of energetic work in some - antennae, in others - oral limbs, in others - thoracic legs, creating continuous flow of water. In the crustacean Daphnia, the rear thoracic legs beat 200-300 times per minute and ensure food enters the mouth. Large crayfish (see Fig. 73) capture prey with legs armed with claws.
Crustaceans, like all arthropods, have limbs that surround the mouth and perform a number of functions. The oral extremities of crayfish and other crayfish, for example, include (see Fig. 73) well-developed mandibles, or upper jaws, with a segmented palp and plate, inner edge which is serrated and serves for grinding food, and two pairs of lower jaws, which also serve for mechanical processing of food. In addition, three pairs of jaws, already located on the chest, help to hold food and convey it to the mouth. In the anterior part of the digestive apparatus, many species develop a large chewing stomach (see Fig. 75), the walls of which are thickened due to cuticular formations and are used for mechanical processing of food. Digestion of food occurs in the midgut, into which the ducts of the digestive gland, called the liver, flow. In fact, this gland performs the functions of the pancreas and liver glands of vertebrates, since it secretes juice that facilitates the digestion of all essential organic compounds- proteins, carbohydrates and fats: the liver of vertebrates plays a large role mainly in the digestion of fats. Therefore, it is more correct to call the digestive gland of crayfish pancreatic-hepatic. In small crustaceans these glands are moderately developed, in the form of hepatic processes (see Fig. 80, A, 10); in large crustaceans they are large organ, consisting of several lobes (see Fig. 75).
Reproduction. Reproduction is sexual. Most species are dioecious. Males, as a rule, differ greatly from females in body size, structure of limbs, etc. Parthenogenesis is widespread in some groups of lower crayfish. In cladocerans, which include many species (for example, various daphnia) that serve as food for fish, most of During the warm season, only females are found that lay unfertilized eggs, from which new crustaceans quickly develop. Males usually appear before the onset of the cold season or other unfavorable conditions. Females fertilized by males lay eggs surrounded by strong, thick shells that develop only in next year. Many crayfish carry eggs on their abdomen or in a special brood chamber (see Fig. 80, A).
Development. Development with transformation or direct. In lower crustaceans that develop with metamorphosis, larvae called nauplii(Fig. 82). These larvae have three pairs of legs and one eye. In higher crayfish living in the sea, the eggs mostly hatch into larvae called zoea (Fig. 82). Zoeas have more limbs than nauplii and two compound eyes; they are lined with spines, which increase their surface and make it easier to float in the water. Other types of larvae are also known that occupy an intermediate position between the nauplius and the zoea or between the zoea and the adult form. In many lower freshwater crustaceans and crayfish, development is direct.
The growth of crayfish is always associated with molting; For example, crayfish during the first year of its life it molts 10 times and therefore grows quickly (from 0.9 to 4.5 cm), during the second year it molts 5 times, during the third - only twice, and then females molt once a year , and males - 2 times. After 5 years they hardly grow; live 15 - 20 years.
Origin. Crustaceans originated, as noted above, from arthropods close to trilobites. In connection with adaptation to a more active and complex lifestyle, their body differentiation into sections increased, many segments merged, i.e. the concentration of the organism increased; has become more complicated nervous system; the structure of the limbs (generally the same among trilobites) in connection with the execution different functions has become diverse; the intensity of work of other organ systems has increased.
Crustaceans (Ass. F.D. MORDUCHAI-BOLTOVSKAYA)
Lower crustaceans (Entomostraca)
Lower crustaceans have a variable number of body segments and usually an unclear abdomen, which never bears limbs. In fresh and generally inland waters of the Rostov region. lower crustaceans are represented by four orders: branchiopods (Branchiopoda), cladocera (Cladocera), copepods (Copepoda) and shellfish (Ostracoda). These are in most cases small, sometimes microscopic animals that live exclusively in water.
1. Branchiopoda- these are relatively large crustaceans with a clearly dissected body with a large number of leaf-shaped swimming legs equipped with gill appendages (from 10 to 40). They inhabit very small temporary ponds and puddles, which usually dry up in the summer. In the reservoirs of the river floodplain. Don, formed during the spring flood, you can often find the most interesting representative these crustaceans are the shieldfish - Lepidurus apus. This is an extremely peculiar looking animal, up to 4-5 cm, covered on the dorsal side with a greenish armor covering the entire body, with the exception of the posterior part of the abdomen, equipped with two long tail filaments (Fig. 1). Along with Lepidurus, there is also Rpus, which is very close to it, differing from the first in the absence of a plate between the caudal filaments.
Most of the reservoirs in which these crayfish live are completely dry by mid-summer. However, next spring, scale insects appear in them again, as they lay so-called “resting” or “winter” eggs, which are not only equipped with a dense shell that allows them to withstand drying and freezing of the reservoir without harm, but even, apparently, require complete drying for further development.
In the same temporary reservoirs, other representatives of the described order are also found, devoid of armor - branchiopods. Branchiopods have an elongated body with a thin tail (abdomen) and 10-20 pairs of long legs bearing gills; the head is separate from the body and is equipped with stalked eyes and large curved antennae (“antennae”). Of the branchiopods, Branchinella spinosa was found in the reservoirs of the Don floodplain. In the salt lakes of the Mana-Chey basin, another branchiopod is common - brine shrimp (flrtemia salina v. principalis, Fig. 2). Artemia is a well-known inhabitant of salt waters, remarkable in that it cannot exist in fresh waters, but thrives in salt waters even at a salt concentration at which all other animals die. In this case, Artemia can develop in huge quantities. In some salty reservoirs of the Manych Valley, the entire mass of water, devoid of any animals, is filled with the floating remains of the leaf-shaped legs of Artemia.
In addition to shieldfishes and branchiopods, among the branchiopods there is also a group of forms equipped with a bivalve shell, similar to mollusk shells, but usually very thin and transparent. In floodplain lakes and swamp-like reservoirs you can often find these small ones (rarely more than 1a/a cm) crustaceans that swim quickly with the help of numerous (10-30 pairs) legs.
In the Rostov region. The species Leptestheria, Caenestheria, and Cyzicus were found from this group.
2. Cladocera or Cladocera- the overwhelming majority are very small animals, having an almost unarticulated body with a small number of swimming legs (no more than 6). The body is covered with a transparent, thin shell and in front bears a pair of branched antennae - antennae, which serve for movement, which occurs spasmodically. The head is usually equipped with one large eye, often of a rather complex structure. Cladocera inhabit absolutely all fresh water bodies and are one of the most widespread groups of crustaceans. The extremely wide distribution of Cladocera is largely due to the presence of “winter” or “resting” eggs, which, due to their insignificant size, can be transported over long distances by the wind along with dust. Cladocera reproduces several and sometimes many times during the year, and it is remarkable that it can for a long time move without the participation of males (parthenogenetically), but in this case only ordinary “summer” eggs are formed; with the deterioration of living conditions, males appear, fertilize females, who then lay “winter” eggs.
Cladocera represent one of the main components plankton of fresh water bodies, as well as large quantities inhabit the coastal zone and thickets. They are an important, and sometimes the main food item for various commercial and non-commercial “plank-eating” fish (herring, sprat, bleak, etc.) and juveniles of most fish that feed on bottom fauna as adults. When dried, Cladocera serves as a universal food for aquarium fish. This food is called Daphnia, although in reality Daphnia is only one of the very numerous forms of Cladocera.
In reservoirs of the Rostov region. Cladocera are represented as richly and diversely as in all water bodies of temperate and southern latitudes (at least 40 species of them were found in the Don basin). Among the planktonic forms often found in the Don River, the above-mentioned daphnia (Daphnia longispina) can be mentioned. This is a transparent crustacean 1-2 long mm, the shell of which is equipped with a long needle, and the head bears a pointed helmet (Fig. 3). Even more common than Daphnia are its close relatives - Moina and Diaphanosoma, distinguished by the absence of a helmet and a needle. Of the other Cladocera of the Don plankton, the most numerous are Bosmina longiros tris, very small (no more than 1/2 mm) a rounded crustacean with a long beak, and Chydorus sphaericus, also completely round, but without a beak. In the thickets of the coastal strip and near the bottom there live many other related cladocerans from the family Chydoridae.
In the salty reservoirs of Manychi, most Cladocera, generally adapted to fresh water, cannot exist. Only the most resistant to salinity, Moina and Diaphanosoma, remain, but they reproduce in large quantities.
Among Cladocera, Leptodora kindtii, which lives in the plankton of the Don and generally large reservoirs, stands out. It is comparatively very large - about 1 cm- a crustacean whose elongated body is almost free from the shell (covering only the “brood pouch” with eggs) (Fig. 4). Leptodora, unlike most other Cladocera, leads a predatory lifestyle and is distinguished by its extraordinary transparency. When alive, it is almost impossible to distinguish it in water, and only when killed with formaldehyde or alcohol does it turn white and become clearly visible.
Free-living copepods (Euco-pepoda) have a clearly dissected body, divided into a wide cephalothorax, equipped with 4 pairs of two-branched swimming legs and a narrow abdomen ending in a two-branched fork with setae ("furka"). The cephalothorax bears one small ocellus in front and a pair of very long antennae used for swimming.
Like Cladocera, all copepods are very small, often semi-microscopic forms, extremely widespread in all kinds of water bodies. They also form resting eggs and are part of plankton, representing an important food source for fish fry and adult planktivorous fish.
The lifestyle of copepods is similar to the lifestyle of cladocerans; It should be noted, however, that in contrast to Cladocera, which reproduce only after the water has completely warmed up and quickly disappear with cold weather, copepods are much more tolerant of low temperatures and appear in masses even early spring, and many live throughout the winter, under the ice.
The most common copepods are the Cyclops, which belong to the genus Cyclops (this genus is now divided into several others). Cyclops have an oval cephalothorax, an elongated abdomen with long tail setae, and relatively short swimming antennae. Females carry eggs in two egg sacs on the sides of the abdomen (Fig. 5). Cyclops- small crustaceans(no more than 2-3 mm in length), found in all water bodies, with the exception of heavily polluted ones, and usually leading a planktonic lifestyle. Among the numerous species of this genus (at least 20 species of Cyclops are known for the Rostov region), Cyclops strenuus, C. vernalis, and C. oithonoides are most often found in the plankton of the Don.
Along with cyclops, especially in shallow spring reservoirs, representatives of the genus Diaptomus are often found, differing slightly large sizes(up to 5 mm), longer antennae and cephalothorax and short abdomen. Many of them are red or blue in color. Among the numerous (about 15 found in the Rostov region) species of Diaptomus, interesting are D. salinus and D. (Paradiaptomus) asiatlcus, which develop in large quantities in the salty reservoirs of Manychi. Other copepods (Heterocope, Calanipeda, Eurytemora) are also found in the plankton of the Don.
Copepods belonging to the group Harpacticidae live in the coastal zone and at the bottom of reservoirs. These are extremely small crustaceans with long body and poorly developed swimming antennae, running along the bottom and, due to their small numbers and small size, usually eluding observation.
A significant part of the plankton of most water bodies is played by peculiar copepod larvae - nauplii. These are very microscopic animals with three pairs of legs and one red eye, often, especially in the spring, inhabiting water in countless numbers. All copepods in their development pass through this larval stage, which after a few weeks turns into an adult form through a series of successive molts.
Very close to copepods (but now separated into a special order of branchials - Branchiura) are also “fish or carp lice” (flrgulus). These are small (no more than 1/2 cm) crustaceans with a flat body, two compound eyes and two suckers with which they attach to the skin of fish. They suck blood from fish, but often separate from their prey and swim freely in the water for a while. One of the species of this genus, Argulus foliaceus, is often found in the Don.
4. Barnacle crustaceans (Ostracoda). Shelly crustaceans are small crustaceans that live in oval bivalve shells. The presence of a shell brings them closer together, but shell shells differ from the latter only in their smaller sizes (usually no more than 5-7 mm) and an undifferentiated body with only three pairs of legs, used not for swimming, but for running (Fig. 7). In addition, their shell, impregnated with lime, is usually very durable and preserved in fossil form, thanks to which Ostracoda have important in paleontology.
Most barnacle crustaceans live among thickets and at the bottom of various bodies of water. Although they do not have special “winter” eggs, their eggs, and often the adult crustaceans themselves, are also able to withstand drying out and freezing without harm.
In fresh water bodies they usually do not reproduce in large numbers and can easily go unnoticed by the untrained eye.
In the Rostov region. barnacle crustaceans have hardly been studied. Only a few widespread species inhabiting small floodplain lakes and puddles can be noted: Candona, one of the largest forms with a white shell; Cyclocypris, smaller, rounded; Limnicythere - with an elongated shell equipped with several large swellings.