Crustaceans

External structure

Body Sizes

The size and body shape of crustaceans vary widely. The smallest crustaceans are parasites and belong to the group of tantulocarides; their body length is 0.15-0.3 mm ^{[8] [9]} . The smallest arthropod - the parasitic crustacean Stygotantulus stocki , whose body length is less than 0.1 mm ^[10], belongs to the same group. Kamchatka crab ( Paralithodes camtschatica ) reaches a weight of 10 kg, a giant Tasmanian crab ( Pseudocarcinus gigas ) up to 14 kg ^[11] , and the Japanese crab spider ( Macrocheira kaempferi ) up to 20 kg and 3.8 m in leg span. Sedentary forms with calcareous shell, as well as parasitic cancers, have a strongly modified appearance ^[12] .

Segmentation and limbs

Initially, the body of crustaceans includes 3 departments: head, thoracic and abdominal. In some primitive species, the thoracic and abdominal sections are segmented almost homonomously (that is, they consist of almost identical segments) ^[12] . The number of body segments varies greatly: from 5-8 to 50. Currently, it is believed that during the evolution of crustaceans, like other arthropods, there was a decrease in the number of segments. In higher cancers, the number of segments is constant: acron, four segments of the head, eight thoracic segments and six abdominal ^[6] .

Limbs

Body segments carry a pair of bifurcated limbs. In a typical case, the crustacean extremity consists of the basal part - the protopodite , bearing two branches: the outer - exopodite and the inner - endopodite . The protopodite includes two segments: the coxopodite , usually bearing the gill appendage, and the basipodite , to which the exopodite and endopodite are attached. The exopodite is often reduced, and the limb takes on a single-branch structure. Primarily, the limbs of crustaceans performed several functions: motor, respiratory, and auxiliary when feeding, but the majority have morphofunctional differentiation of the limbs ^[12] .

Head

The head consists of a head lobe - an acron and four segments. The head carries the acronis appendages — the first antennas ( antennals ) and the limbs of the next four segments: the second antennas , mandibles , or mandibles ^[14] (upper jaws) and two pairs of maxillas (lower jaws) ^[12] . Sometimes the first pair of lower jaws is called maxilla , and maxilla is called the second ^[15] . Antennals are usually single-branching and homologous to the palms of polychaete worms ^[16] . The exopodite of the second antenna is called scafoceritis . The antennae perform the function of the sensory organs, sometimes movements, other head appendages participate in the capture and grinding of food ^[14] . Mandibles play a major role in chopping food. In the larva - nauplius - the mandible is a typical bifurcated limb with a chewing process. Adults rarely have a similar form of the mandible, usually both branches are reduced, and the protopodi with the chewing process forms the upper jaw, to which the muscles are attached. Maxillae usually have the form of tender leaf-shaped legs with chewing processes on the protopodite and, to some extent, reduced branches ^[17] .

The head can be either fused ( syncephalon ^[15] ), or subdivided into two articulated departments: the protocephalon , which is formed by the fusion of the acron and the first head segment and carries the first two pairs of antennas, and the gnatocephalon , formed by the fusion of the last three head segments and bearing the mandibles and maxilla. The latter option occurs in the units: gills, mysids, euphausiae, decapods, and rotopods ^[14] . The front opening of the mouth is covered with an unpaired cuticle fold - the upper lip ^[16] . Often in higher cancers (like, for example, crayfish), the gnatocephalon fuses with the thoracic region, forming the jaw-chest ( gnatothorax ), covered with dorsal carapace - carapace . The body of higher cancers is divided into the following departments: head - protocephalon (acron and one segment), jaw - gnatothorax (three head and eight thoracic segments) and abdomen (6 segments and telson). In other cases, there is a fusion of the entire head, not subdivided into protocephalon and gnatotcephalon, with one or more thoracic segments. This forms the cephalothorax , followed by the chest and abdomen ^[18] . In some crustaceans (for example, cladocera), the head is elongated in the downward beak - rostrum ^[15] .

Thoracic department

The thoracic region, like the abdominal, can have a different number of segments. Some cancers, for example, gills, have multifunctional abdominal limbs, while others have a separation of functions. For example, in crayfish, the first three pairs of pectoral legs are bifurcated maxillae , which serve to hold and strain food, the next three are single-branched walking and at the same time grasping, with a claw on the end, but all pectoral legs at the base carry gills ^[19] .

Abdominal

The abdominal region consists of several segments and telson; as a rule, he is devoid of limbs. Only in higher crayfish ^[15] on the abdomen are bifurcated limbs that perform various functions: in shrimp - swimming, in roton-footed crayfish - respiratory, in male crayfish, the first two pairs are modified into copulative organs, and in females the first pair is reduced, the remaining abdominal legs are intended for carrying juveniles. In most decapods, the last pair of abdominal legs has a lamellar form ( uropods ) and together with telson forms a five-lobed fin ^[19] .

Crustaceans, deprived of abdominal extremities, usually have a fork (furka) at the end of the body, formed by articulated appendages of telson. At the same time, both the breast and the abdominal legs are found only in the Nebalia crustacean. In crabs, the abdominal region is reduced ^[20] .

In some parasitic crustaceans, the limbs of the body significantly decrease or even completely disappear ( Sacculina , females of Dendrogaster ) ^[17] .

Veils

Like other arthropods, crustaceans have a strong chitinous exoskeleton ( cuticle ). The cuticle consists of several layers, its peripheral layers are impregnated with lime, and the inner ones consist mainly of soft and elastic chitin. In small lower forms, the skeleton is soft and transparent ^[21] . In addition, the composition of the chitinous cuticle includes a variety of pigments that give the animal a protective color. Pigments are also found in the hypodermis. Some crustaceans are able to change color due to changes in the distribution of pigment grains in the cells (if the pigment is concentrated in the center of the cell, then the color disappears, if the pigment is distributed evenly in the cell, the color will appear in the integument). This process is regulated by neuro-humoral factors ^[20] .

The function of the external skeleton is not limited to protecting the animal; various muscles are also attached to the cuticle. Often, for their attachment, on the underside of the cuticle there are special processes in the form of ridges and crossbars ^[22] .

The mobility of such parts of the body of the crustacean is ensured by special soft membranes located between the fused parts of the body, segments or segments of limbs and appendages. The densified portions of the segments on the dorsal side are called tergites , and on the ventral side they are called sternites . The carapax already mentioned above is a special fold of covers. It can take the form of a shield, a bivalve shell or a half cylinder ^[15] . Carapax can cover various departments: the head, chest (crayfish, shield) or the entire body (daphnia, shell crayfish), in higher crayfish its side parts cover the gills ^[20] .

Internal structure

Musculature

The crustacean muscles are represented by striated muscle tissue, as in all arthropods. They do not have a single skin-muscle bag, and the muscles are represented by separate more or less large bundles. Typically, one end of a muscle attaches to the wall of one segment of the body or segment of a limb, the other to the wall of another segment. Shell crustaceans having a bivalve shell have a special closure muscle that runs across the body and connects two shell leaves ^[22] .

Digestive system

The crustacean digestive system is well developed; it looks like a straight or slightly bent tube ^[23] . As with all arthropods, it consists of an ectodermal anterior, endodermal middle, and ectodermal hindgut ^[20] .

The anterior intestine is represented by the esophagus and stomach and is lined with chitinous cuticle. The stomach can be divided into chewing (cardiac) , in which food is crushed using chewing plates - jagged, lime-saturated cuticle thickenings on the walls of the stomach, and pyloric , in which food is filtered using thin cuticular outgrowths that form something like a filter, departments (for example, in crayfish) ^{[20] [23]} .

The ducts of paired hepatic appendages, which are lateral protrusions of the wall, flow into the middle intestine. In case of abundant development, these appendages are called the liver. Crustacean liver not only secretes digestive enzymes, but also absorbs digested food. Its enzymes act on fats, proteins and carbohydrates. Thus, functionally, the liver of crustaceans corresponds to the liver and pancreas of vertebrates. Both abdominal and intracellular digestion are carried out in the liver. There is an inverse relationship between the sizes of the middle intestine and the liver ^[24] . In copepods, the middle intestine has the appearance of a simple tube and is devoid of hepatic protrusions. In its infancy, the liver is present in some branched, in amphipods and isopods, the liver has the appearance of two pairs of long tubular bags ^[25] .

The hind rectum is lined with chitinous cuticle. The anus opens on the ventral side of the telson (anal lobe) ^[23] . During molting, in crustaceans, in addition to the outer chitinous cover, the lining of the front and rear sections is also discarded. Until the new integument hardens, the cancer does not feed ^[26] .

In some parasitic crustaceans (for example, Sacculina ), the intestines are completely atrophied ^[25] .

Respiratory system

Most crustaceans breathe with skin gills, which are cirrus or lamellar outgrowths - epipodites extending from the protopodites of the legs. As a rule, they are located on the chest extremities, only in the rotopod and isopods the abdominal legs are completely turned into gills. In decapod crustaceans, gills also form on the body wall in gill cavities under the carapace, gradually transitioning from protopodites to the body wall. In this case, the gills of decapods are located in three longitudinal rows: in the first row, the gills retain their primary location on the body protopodites, in the second they sit at the junction of the protopodites with the body, in the third - they have completely switched to the side wall of the body. In the gills, the body cavity continues, into which the hemolymph enters. Gas exchange occurs through a very tender cuticle of the gills ^[25] .

The flow of water in the gills is as follows. Water enters the gill cavities from one end of the body through the gap between the carapace and the body, and is pushed from the other, and the direction of the water flow can change. The conduction of water is also facilitated by the movement of special processes of the second pair of maxillas, making up to 200 swinging movements in 1 min ^[25] .

Many small crustaceans with thin carapace have no gills, and breathing goes through the entire surface of the body. In land crustaceans, there are special devices for breathing atmospheric oxygen, for example, pseudotracheas (deep bulges) on the abdominal legs of woodlice. The limb cavity is filled with hemolymph washing washing and carrying out gas exchange ^[27] . Land crabs breathe oxygen dissolved in water, covering a thin film of the membrane of the gill cavity and protected from evaporation by carapace. However, for the respiration of terrestrial crustaceans, high humidity is still required ^[26] .

Circulatory system

Like all arthropods, crustaceans have a mixed body cavity (myxocel) and an open circulatory system (that is, hemolymph flows through the vessels and sinuses of the mycelium). The heart is located above the intestines, on the dorsal side of the body ^[15] and is located near the respiratory organs (if the gills are only on the pectoral legs, the heart in the thoracic region, etc.). In the most primitive crustaceans, the heart is metameric, multi-chamber, and is represented by a long tube running along the entire body (some gill-footed) and having a pair of rests (holes) in each segment (chamber). In other crustaceans, the heart is shortened: in water fleas, the heart is shortened to the extent of a barrel-shaped sac with one pair of ostia, in decapods the heart is a small sac with three pairs of ostia. Among the higher cancers there are representatives with both a long and a shortened heart ^[27] .

The heart of crustaceans is located in the pericardial sinus of the myxocele. From there, hemolymph through the ostia enters the heart. When the chambers of the heart contract, the rest of the valves close, the valves of the heart chambers open, and the hemolymph is expelled into the arteries: front and back ^[27] . From there, the hemolymph is poured into the spaces between the organs, where it gives off oxygen and is saturated with carbon dioxide. It performs the gas exchange function due to the presence of respiratory pigments — hemocyanin (in higher cancers) or hemoglobin (in copepods, shell, barnacles and gill-footed cancers), which bind oxygen ^[28] . Partially hemolymph washes the kidneys, where it is freed from metabolic products. Then it is collected in a system of venous vessels, delivered to the gill system of capillaries, gives off carbon dioxide and is saturated with oxygen. Then the efferent gill vessels deliver it to the pericardial sinus ^[29] .

The degree of development of the circulatory system is associated with the development of the respiratory system. In small crustaceans carrying out gas exchange through the wall of the body, only the heart remains from the circulatory system or it completely disappears.

Excretory system

The excretory system of crustaceans is represented by kidneys, which are altered co-products. Each kidney consists of a sac of coelomic origin and a convoluted excretory tubule, which can expand, forming a bladder. Depending on the place where the excretory pores open, two types of kidneys are distinguished: antennal (first pair; excretory pores open at the base of the second antennas) and maxillary (second pair; at the base of the second pair of maxillas). Higher cancers in adulthood have only antennal kidneys, all the rest are only maxillary ^[30] . Both pairs of kidneys are present only in the already mentioned Nebalia crustacean from the group of higher crayfish, as well as in sea shell crustaceans. The remaining crustaceans have only one of two pairs of kidneys, and in the process of ontogenesis they undergo a change: if the maxillary glands function in the larval state, then in the adult state the antennal ones. Apparently, initially, crustaceans had 2 pairs of kidneys, like Nebalia , but in the course of subsequent evolution, only one was preserved ^[31] .

Nervous system

The nervous system of crustaceans, like all arthropods, is represented by paired pharyngeal ganglia, the nerve ring and the abdominal nerve chain. In primitive gill-footed cancers, the nervous system is of the ladder type: paired ganglia in segments are widely spaced and connected by commissures. In most crustaceans, the abdominal trunks came closer, the right and left ganglia merged, commissures disappeared, and only the duality of the longitudinal ligaments between the ganglia of adjacent segments indicates the paired origin of the abdominal nerve chain ^[32] . Like most arthropods, crustaceans show a tendency to oligomerization (fusion) of ganglia from different segments, which distinguishes the abdominal nerve chain of arthropods from that of annelids ^[33] . So, in cancer, whose body consists of 18 segments, there are only 12 nerve nodes ^[34] .

The brain of crustaceans is represented by paired lobes of prototserebrum (innervation of the acron and eyes) with mushroom bodies and deutocerebrum (innervation of antennules). Usually, the ganglia of a segment carrying a second pair of antennas merge forward with the brain. In this case, the third division is isolated - trisocerebrum (innervation of antennas), in the remaining crustaceans, the antennas are controlled by the near- oropharyngeal ring ^{[33] [35]} .

Crustaceans have a well-developed sympathetic nervous system, mainly innervating the intestines. It consists of a cerebral section and an unpaired sympathetic nerve, along which several ganglia are located ^[35] .

The nervous system of crustaceans is closely related to the endocrine. The ganglia of cancers include neurosecretory cells that secrete hormones that enter the hemolymph. These hormones affect metabolic processes, molting and development. Neurosecretory cells are located in various parts of prototserebrum, tritotserebrum and ganglia of the abdominal nerve chain ^[35] . In some crustaceans, hormones from the neurosecretory cells of the optic nerves enter the special sinus gland and from there into the hemolymph. They are responsible for the mechanism of discoloration of the body described above ^[33] .

Sense organs

Organs of vision

Almost all crustaceans have well-developed eyes: simple or facet (complex); eyes are absent only in deep-sea, sessile and parasitic species. Some crustaceans (cyclops) have only simple eyes, while the majority of higher cancers have only complex eyes, and carnivores have eyes of both types ^[36] .

A simple peephole is a pigment glass, into which the visual cells are turned. It is covered with a transparent cuticle forming the lens. Light first passes through the lens, visual cells, and only then - at their light-sensitive ends. Such eyes are called inverted (i.e., inverted). Simple eyes are assembled in 2–4 and form an unpaired nauplius (nauplial) eye , characteristic of crustacean larvae — the nauplius ^[37] . In adult nauplii, the eye is located between the bases of the antennas ^[38] .

Faceted eyes consist of simple eyes - ommatidia . Each simple ocellus is a cone-shaped glass limited by pigment cells and covered with a hexagonal cornea from above. The light-refracting part of ommatidium is composed of cells of the crystal cone , and the photosensitive part is retinal cells , at the point of contact of which a photosensitive bacillus is formed - rhabdom . Crustaceans with faceted eyes have mosaic vision , that is, the general visual perception consists of parts perceived by individual ommatidia ^[39] . Complicated eyes often sit on special mobile outgrowths of the head - stalks ^[40] .

In some cancers, the visual perceptions of certain light stimuli are necessary to trigger the mechanism of body color described above ^[40] .

Balance organs

Some crustaceans have balance organs - statocysts. In crayfish, they are at the base of the antennae. During molting, the lining of the statocyst changes, and the animal loses coordination of movement ^[39] . Statocysts are characteristic of decapods and some other higher cancers ^[38] .

Other senses

The organs of touch and smell in crustaceans are numerous sensilla and tactile hairs, mainly located on antennas, limbs, and a fork ^[39] . The sense of touch is confined only to those parts of the integument where sensitive hairs are located. At the base of such hairs, bipolar neurons are located under the hypodermal epithelium. Hair with a particularly permeable cuticle, localized on the antennas, are the organs of smell ^[35] .

The reproductive system

Crustaceans are overwhelmingly dioecious animals and reproduce sexually ^[41] . However, cases of hermaphroditism are known: hermaphrodites are some representatives of the barnacle groups, remypedia ^[42] , cephalocarids ^[43] . Sexual dimorphism is often expressed, for example, in some parasitic crustaceans, males are several times smaller than females ^[39] . Some crustaceans can change sex during life ^[43] . In addition, parthenogenesis is widespread among crustaceans ^[41] . It occurs among many branchial, some shell, branched (daphnia) ^[44] , isopods, and also among some higher crustaceans, for example, in Procambarus fallax subsp. virginalis .

Sometimes in males, antennas or antennals play the role of grasping organs, and in crayfish, 1-2 pairs of abdominal legs function as copulative organs. Gonads in primitive forms, reproductive ducts and openings paired. More often, the gonads are fully or partially fused. The walls of the oviducts secrete a dense shell around the eggs. In some cases, females have testicles. In this case, fertilization occurs when the female lays eggs and sprinkles them with sperm from the openings of the testicles. Some cancers have spermatophore fertilization; when mating, males of these species glue spermatophores to the female’s body or insert them into her genital opening ^{[45] [30]} .

In crustaceans, the shape and size of spermatozoa vary widely. So, in some small shell crustaceans, the length of the sperm is 6 mm, which is 10 times longer than the animal itself. In Galathea and higher cancers, the sperm is like an hourglass. During fertilization, the sperm is attached to the egg by processes, then the tail of the sperm, swallowing moisture, swells and explodes, and the head end with the nucleus sticks into the egg ^[46] .

Most crayfish are characterized by care for offspring, although some of them simply throw eggs into the water. Often, females carry eggs glued to the genital openings in the form of egg sacs (typical for copepods) or long threads. Decapods glue eggs to the extremities of the abdomen. Peracarids, shields, gills, and many isopods of carapace and pectoral legs form a brood bag (Marsupium) ^[41] . Most of the thin-shell and krill crustaceans hatched eggs between the thoracic legs ^[41] . Females of carnivores do not bear eggs, but lay them in rows on stones and other objects ^[47] .

Cancer fertility varies ^[30] .

The eggs of some cancers (shieldworms and gills) are highly resistant: they easily tolerate drying, freezing, and are carried by the wind ^[48] .

Embryonic Development

The crushing pattern of crustaceans depends on the amount of yolk in the eggs. When the yolk is small in the egg (for example, some copepods), the crushing proceeds like crushing of annelids: it is complete, uneven, deterministic, with a teloblastic laying of the mesoderm (that is, from a teloblast cell) ^[49] .

In most cancers, the eggs are rich in yolk, and crushing becomes partial and superficial. In the course of several nuclear fissions, without dividing the cells, daughter nuclei are formed that extend to the periphery and are located there in one layer (therefore, crushing of crustaceans is called superficial ). Next, a cytoplasmic region is separated around each nucleus and a small cell is formed; the central mass of the yolk remains undivided. This stage is similar to a blastula with a blastocele filled with yolk. Then part of the blastula cells on the future ventral side goes under the outer layer, forming a multicellular plate - the germinal strip . Its outer layer is formed by the ectoderm, the deeper - the mesoderm, the deepest adjacent to the yolk - the endoderm ^[50] .

Further development of the embryo occurs mainly due to the germinal streak. It begins to segment, and from the front and most powerful section of it appear paired head ganglia, due to which complex eyes arise. Behind her, the rudiments of the acron, antennae and mandibular segments are laid. Sometimes the mesoderm is laid in the form of paired coelomic sacs, like annelids, which subsequently collapse: their cells go to build mesoderm organs (muscles, hearts, etc.), and the cavities merge with the remnants of the primary body cavity. This forms the myxocele, or mixed body cavity. In some cases, the mesoderm loses distinct segmentation, and the expressed whole does not form at all ^[51] .

Postembryonic development

Postembryonic development of most crustaceans occurs with metamorphosis. As a rule, a planktonic larva - nauplius emerges from the egg, this larva is most characteristic of crustaceans. The structure of the nauplius is characterized by the following features. The body consists of an acron, two segments of the body and the anal lobe, there are single-branching antennules and 2 pairs of bifurcated swimming legs, which are holologous to the antennas and mandibles of adult cancers. The nauplius has an intestine, kidneys (often antennal), head ganglia, and the aforementioned unpaired nauplial eye on the head lobe. In front of the anal lobe there is a growth zone where new segments are laid. The nauplius stage is followed by the metanauplius stage, which has all the head segments with limbs and the anterior thoracic segments with the jaw. Larvae undergo several molts, during which their external and internal structures reach the level of development characteristic of adult individuals ^[52] .

In higher crustaceans, the metauplius stage is followed by a special larval stage - zoea (the larva got its name when scientists considered it a separate species ^[53] ). This larva has developed head and prothoracic limbs, there are rudiments of the remaining pectoral legs, a formed abdomen with the last pair of legs. In addition, zoea has facet eyes. Further, zoea develops into a mysid larva with formed pectoral legs and primordia of all abdominal extremities. After that, the misid larva molts and transforms into an adult animal ^[52] .

Some higher cancers have differences from the life cycle described above. So, in many crabs, zoea immediately leaves the egg, and in river cancer the development is direct: a young crustacean with the full composition of segments and limbs appears from the egg, then it grows and molts, turning into an adult ^[54] .

Finally, different groups of crustaceans may have special larval stages.

Shedding

Shedding in crustaceans is best studied by the example of higher crayfish. It is accompanied by both morphological and physiological changes ^[55] .

Before molting, a number of organic (lipids, proteins, vitamins, carbohydrates, etc.) and mineral compounds accumulate in the tissues and hemolymph of the animal. Partially, they come from the old cuticle. Oxygen consumption increases, the intensity of metabolic processes increases ^[55] .

At the same time, hypodermal cells begin to secrete a new cuticle due to substances from hemolymph and tissues. The new cuticle gradually thickens, while retaining flexibility and elasticity. Finally, the old cuticular cover bursts, the animal gets out of it, leaving an empty cover - exuvium . Shedding cancer is rapidly increasing in size, but not due to the proliferation of tissues, but the accumulation of water in them. Due to cell division, tissue volume increases only between molts. Some time after dropping the exuvium, mineral salts are deposited in the new cuticle, and it quickly hardens ^[55] .

The molting process is regulated by the hormonal system. An important role is played by neurosecretory cells associated with the sinus gland mentioned above and the small endocrine gland. Its hormones start and accelerate shedding, and hormones are produced in the neurosecretory cells of the eye stalks that inhibit its activity, that is, prevent the onset of molting. Their content is especially high in the period after molting and between molting, then the activity of the head gland is activated and preparation for new molting begins. In addition to the above, other hormones also take part in the regulation of molting ^[56] .

Other life cycle features

Some crustaceans, for example, daphnia, are characterized by complex life cycles with alternating parthenogenetic and sexual reproduction. In addition, among generations of Daphnia living at different times of the year, seasonal changes occur, expressed in changes in the shape of the head, the length of the rostrum, spines, etc. ^[57] .

Distribution

В настоящее время считается, что исходным морфоадаптивным типом ракообразных были мелкие пелаго-бентосные формы, ведущие плавающий образ жизни. От них произошли группы, специализировавшиеся к планктонному, нектонному и бентосному образу жизни. Часть групп приспособилась к паразитическому образу жизни, некоторые вышли на сушу ^[58] .

Паразитические формы имеются во многих группах ракообразных: веслоногие, усоногие раки, паразитический отряд карпоеды, представители которого живут на коже рыб ^[59] . При этом они претерпевают упрощения в организации в разной степени: веслоногий рачок Ergasilus внешне напоминает циклопа, Lamproglena ещё сохранила частичную сегментацию, а паразит рыб Lernaeocera branchialis и усоногие рачки — паразиты десятиногих Sacculina и Peltogaster характеризуются столь глубоким упрощением организации, что их систематическую принадлежность удалось установить, лишь пронаблюдав историю развития ^[60] . Ещё один паразитический отряд ракообразных — мешкогрудые — паразитируют на коралловых полипах и иглокожих ^[61] . Существуют и паразитические высшие раки, например, некоторые равноногие рачки. Среди них есть временные ( Aega ) и постоянные ( Cymothoa , Livoneca ) эктопаразиты рыб ^[62] . Как видно из примеров, среди ракообразных есть как эктопаразиты, так и эндопаразиты.

В морях и океанах ракообразные распространены настолько же широко, как насекомые на суше. Ракообразные многообразны в пресных водоёмах, а некоторые жаброноги встречаются во временных лужах, остающихся после таяния снега. Другой жаброногий рачок — Artemia salina — обитает в осолоненных водоёмах в степях и полупустынях: в лиманах, солёных озёрах ^[63] .

Nutrition

Большинство планктонных ракообразных питается бактериями, а также одноклеточными организмами, детритом. Донные представители питаются частицами органических веществ, растениями или животными. Бокоплавы поедают трупы животных, способствуя тем самым очищению водоёмов ^[14] .

Был проведён ряд исследований пищевого поведения краба Portunus pelagicus , в которых исследовались реакции животного на конкретные пищевые вещества, а также сравнивались с реакциями на естественную пищу (рыбу, моллюсков). В результате было установлено, что реакция ракообразного на некоторые аминокислоты и сахариды была такой же, как на естественную пищу, причём реакции на аминокислоты и сахариды были очень похожими. Особо сильный ответ наблюдался на аланин, бетан, серин, галактозу и глюкозу. Эти данные могут быть полезными для разведения крабов в аквакультуре ^[64] .

Для щитней характерен древний тип питания, имевший место также у трилобитов: они питаются кусочками детрита и мелкими донными животными, которые захватываются жевательными отростками всех ножек и передаются затем по брюшному желобку ко рту ^[65] .

Ракообразные — важный объект промысла, включая добычу креветок , крабов , лангустов , лангустинов , раков , омаров (лобстеров), разнообразных балянусов , включая морскую уточку (или персебеса), который является самым дорогим из деликатесных ракообразных.

На рыбоводных заводах разводят рачков в качестве корма для рыб. Кроме того, мелкие рачки являются одним из основных видов пищи многих промысловых рыб. Важна роль ракообразных в биологической очистке вод, они представляют одну из самых многочисленных групп биофильтраторов и детритофагов. С другой стороны, некоторые ракообразные могут быть переносчиками разных инфекций. Сидячие формы ракообразных прикрепляются к основанию судов и замедляют их скорость. Некоторые ракообразные ведут паразитический образ жизни ( карповая вошь ).

В настоящее время известно более 73 000 видов ракообразных (включая более 5 тыс. ископаемых видов), объединяемых в 1003 семейства, более 9500 родов (Zhang, 2013) ^[3] , 42 отряда и 6 классов ^[4] :

• Класс Branchiopoda — Жаброногие
  • Подкласс Phyllopoda — Листоногие
  • Подкласс Sarsostraca — Сарсостраки
• Класс Cephalocarida — Цефалокариды
• Класс Malacostraca — Высшие раки
  • Подкласс Eumalacostraca — Эумалакостраки
  • Подкласс Hoplocarida — Гоплокариды
  • Подкласс Phyllocarida — Филлокариды
• Класс Maxillopoda — Максиллоподы
  • Подкласс Branchiura — Карповые вши
  • Подкласс Copepoda — Веслоногие
  • Подкласс Mystacocarida — Мистакокариды
  • Подкласс Pentastomida — Пятиустки
  • Подкласс Tantulocarida — Тантулокариды
  • Подкласс Thecostraca — Текостраки
• Класс Ostracoda — Ракушковые
  • Подкласс Myodocopa — Миодокоповые
  • Подкласс Podocopa — Подокоповые
• Класс Remipedia — Ремипедии

По последним данным, в состав ракообразных входят также насекомые — класс Hexapoda, которые являются сестринской группой жаброногих. В случае принятия этой концепции (концепции Pancrustacea или Tetraconata, см., например ^{[66] [67]} ) приходится изменить таксономическое положение ракообразных (например, для них уже не является общим признаком наличие двух пар антенн). В противном случае ракообразные оказываются парафилетическим таксоном .

Альтернативная классификация

Представленную выше классификацию разделяют не все систематики. Сайт World Register of Marine Species использует другую, отличающуюся прежде всего расформированием мусорного класса максиллопод и выделением двух надклассов. Классификация до подклассов включительно ^[2] :

• Класс Branchiopoda — Жаброногие
  • Подкласс Calmanostraca
  • Подкласс Diplostraca
  • Подкласс Sarsostraca — Сарсостраки
• Класс Cephalocarida — Цефалокариды
• Класс Remipedia — Ремипедии
• Надкласс Multicrustacea
  • Класс Hexanauplia
    • Подкласс Copepoda — Веслоногие
    • Подкласс Tantulocarida — Тантулокариды
    • Подкласс Thecostraca — Текостраки
  • Класс Malacostraca — Высшие раки
    • Подкласс Eumalacostraca — Эумалакостраки
    • Подкласс Hoplocarida — Гоплокариды
    • Подкласс Phyllocarida — Филлокариды
• Надкласс Oligostraca
  • Подкласс Mystacocarida — Мистакокариды
  • Класс Ichthyostraca
    • Подкласс Branchiura — Карповые вши
    • Подкласс Pentastomida — Пятиустки
  • Класс Ostracoda
    • † Подкласс Archaeocopa
    • † Подкласс Metacopa
    • Подкласс Myodocopa — Миодокоповые
    • Подкласс Palaeocopa
    • Подкласс Platycopa
    • Подкласс Podocopa — Подокоповые

• Список ракообразных, занесённых в Красную книгу России
• Список угрожаемых видов ракообразных

• Догель В. А. Зоология беспозвоночных. — 7-е изд.. — М. : Высшая школа, 1981. — 614 с.
• Пименова И. Н., Пименов А. В. Зоология беспозвоночных. Теория. Задания. Ответы. — Саратов: Лицей, 2005. — 288 с. — ISBN 5-8053-0308-6 .
• Шарова И. Х. Зоология беспозвоночных. — М. : Владос, 2002. — 592 с. — ISBN 5-691-00332-1 .
• Шевяков В. Т. Ракообразные // Энциклопедический словарь Брокгауза и Ефрона : в 86 т. (82 т. и 4 доп.). - SPb. , 1890-1907.
• Carbonnier P. L'écrevisse: mœurs — reproduction — éducation. — Paris, 1869. (фр.)
• Huys R., Boxshall GA Copepod Evolution. — The Ray Society, 1991. — 468 p. (eng.)
• McLaughlin PA Comparative Morphology of Recent Crustacea. — San Francisco: WH Freeman and Company, 1979. — 177 p. (eng.)

• «Ракообразные» — статья в Энциклопедии «Кругосвет» .
• «Ракообразные» — статья А. В. Рыбакова на сайте Дальневосточного геологического института ДВО РАН .
• Lowry JK (1999 onwards). Crustacea, the Higher Taxa: Description, Identification, and Information Retrieval. Version: 2 October 1999. (англ.)
•   Ракообразные. Учебный фильм для средней советской школы .