Types of crustaceans (Crustacea) and their diversity. Crayfish Crustaceans representative of the crayfish body parts
The class Crustaceans includes about 25 thousand species of animals that live mainly in marine and fresh waters. A typical representative of this class is crayfish.
External structure
The body of the cancer has a hard chitinous cover, under which there is a layer of epithelial cells. In crustaceans, the head and thorax are usually fused to form the cephalothorax. A characteristic feature of crustaceans is the transformation of the anterior body segments into the head.
On each segment, except the last, as a rule, there is a pair of limbs. Due to various functions, the shape of the limbs of crustaceans is very diverse. The limbs of the head segments usually lose motor function, turning either into part of the oral apparatus or into sensory organs.
On the front part of the cephalothorax there are 5 pairs of limbs, some of which have turned into long and short antennae that serve as organs of touch, hearing, smell, balance or chemical sense, while others are used for grinding food and chewing it. Each chest segment has a pair of legs. The 3 anterior pairs are transformed into jaws, which take part in capturing, retaining food particles and transferring them to the mouth. The other 5 pairs of thoracic legs are used for crawling (locomotor legs, also known as walking legs).
The front legs are also used to capture food, defend and attack, which is why they have claws. In hermit crabs, crabs and other related species, claws were formed only on the front pair of walking legs, in many species of shrimp - on the two front pairs of limbs, and in lobsters, crayfish and others - on the three front pairs, but on the first pair of claws significantly larger than the others. With the help of walking legs, the crayfish moves along the bottom with its head forward, and swims forward with its tail end.
Nervous system and sensory organs
The sense organs are well developed. Eyes are of two types: one simple eye in the larva, which is absent in adult higher crayfish, and a pair of complex compound eyes in adult higher crayfish. A compound eye differs from a simple eye in that it consists of individual eyes, identical in structure and consisting of the cornea, lens, pigment cells, retina, etc. It is believed that each eye sees only part of the object (mosaic vision).
Cancer's organs of touch are long antennae. There are many bristle-like appendages on the cephalothorax, which apparently perform the function of organs of chemical sense and touch. The organs of balance and hearing are located at the base of the short antennae. The organ of balance looks like a pit or sac with sensitive bristles on which grains of sand press.
Like annelids, the nervous system of crustaceans is represented by a peripharyngeal nerve ring and a ventral nerve cord with a paired ganglion in each segment. From the suprapharyngeal ganglion, nerves extend to the eyes and antennae, from the subpharyngeal ganglion to the oral organs, and from the abdominal nerve cord to all limbs and internal organs.
Digestive and excretory systems
Crayfish feed on both live and dead prey. Their digestive system begins with a mouth opening, surrounded by modified limbs (the upper jaws were formed from the first pair of legs, the lower ones - from the second and third, the maxillae - from the fourth to sixth). The crayfish grabs its prey with its claws, tears them apart, and brings pieces of it to its mouth. Then, through the pharynx and esophagus, food enters the stomach, which consists of two sections: chewing and filtering.
On the inner walls of the larger chewing section there are chitinous teeth, thanks to which food is easily ground. In the filtering section of the stomach there are plates with hairs. Through them, the crushed food is filtered and enters the intestine. Here food is digested under the influence of the secretion of the digestive gland (liver). Digestion and absorption of food can occur in the outgrowths of the liver. In addition, the liver has phagocytic cells that capture small particles of food and are digested intracellularly. The intestine ends with the anus, located on the middle blade of the caudal fin.
In spring and summer, white pebbles (millstones) consisting of lime are often found in the stomach of crayfish. Its reserves are used to soak the soft skin of crayfish after molting.
The excretory system in cancer is represented by a pair of green glands located in the head section. The excretory canals open through holes at the base of the long antennae.
Circulatory and respiratory systems
The class Crustaceans has an open circulatory system. On the dorsal side of the body there is a pentagonal heart. From the heart, blood flows into the body cavities, supplying the organs with oxygen and nutrients, then through the vessels it enters the gills and, enriched with oxygen, returns to the heart.
Crustaceans breathe using gills. They are even found in terrestrial crustaceans - wood lice, living in cellars, under stones and in other damp and shaded places.
Reproduction of crustaceans
Most crustaceans are dioecious. The gonads in both sexes are paired and located in the chest cavity. The female crayfish is noticeably different from the male; her abdomen is wider than the cephalothorax, while the male’s is narrower.
The female spawns eggs on the abdomen at the end of winter. The crustaceans hatch in early summer. From 10 to 12 days they remain under the mother’s abdomen, and then begin to lead an independent lifestyle. Since the female lays a small number of eggs, such care for the offspring contributes to the preservation of the species. The class of crustaceans is divided into 5 subclasses: cephalocarids, maxillopods, branchiopods, shellfish and higher crustaceans.
Meaning in nature
Higher crustaceans are inhabitants of marine and fresh waters. Only certain species from this class live on land (woodlouse, etc.).
Crayfish, crabs, shrimp, lobsters and others are used as food by humans. In addition, many crayfish have sanitary significance, as they clear water bodies of animal corpses.
Crustaceans - primary water animals, therefore, as respiratory organs they have special outgrowths of the limbs - gills. Representatives of this class differ from all other arthropods by the presence on the head two pairs of antennae. The limbs of crustaceans often retain a primitive two-branched type of structure.
Crayfish. Let us consider the main morphophysiological features of this class using the example of a well-known representative - crayfish.
External structure and lifestyle. Crayfish live in fresh waters: rivers, streams, lakes. The presence of crayfish in a pond indicates the purity of the water. Crayfish lead an active nocturnal lifestyle, and during the day they hide under stones, snags or in burrows. Crayfish are omnivores; they feed on both plants and animals, including their decaying remains. The size of an adult cancer reaches 20 cm or more.
On the outside, the cancer is covered with a hard chitinous shell, which serves as reliable protection from enemies. The dark greenish-brown color of the shell makes the crayfish invisible at the bottom. Like all crustaceans, the body of the crayfish consists of the head, thoracic and abdominal sections. However, its structure has some peculiarities. The external structure and size of crustaceans can be very diverse. Thus, in some primitive forms the segmentation of sections is almost homonomic, and one part of the body imperceptibly passes into another. In more highly organized species, body parts are clearly differentiated. The head of a crayfish consists of a head lobe (acron), on which the first pair of antennae is located (antennas 1, or antennules, and 4 segments (Fig. 42).
Rice. 42. Limbs of a female crayfish: 1 - antenna, 2 - antenna 11, 3 - limbs of the head, 4 - limbs of the chest, 5 - limbs of the abdomen
The limbs of the first segment represent the second pair of antennae (antennas), much longer than the antennules. The antennae are mobile and serve for touch and smell. The remaining 3 segments of the head also bear modified 4 limbs: on the second segment - the upper jaws (mandibles), on the third and fourth - two pairs of lower jaws (maxill). The jaws surround the mouth opening and form an oral apparatus that crushes food and delivers it to the mouth.
The chest consists of 8 segments. The first 3 segments are equipped with paired jaws, involved in grinding, sorting and transferring food particles to the oral apparatus. The next 5 segments are carried in pairs walking legs. Powerful claws on the first pair of walking legs are used to capture food, attack and defend. The cancer uses the remaining walking legs for movement.
The abdomen consists of six movably articulated flattened segments. The first two abdominal segments of males are equipped sexual legs, shaped like tubes. With their help, sperm is transferred to the female genital tract. In females these legs are vestigial. On the following segments there are small two-branched swimming legs. On the last, sixth segment of the abdomen, the swimming legs are greatly enlarged and, together with the wide anal lobe, form caudal fin.
It should be noted that the head of the crayfish consists of two articulated sections: the protocephalon and the gnathocephalon. The protocephalon is formed by the fusion of the head lobe and the first head segment, and the gnatocephalon by the fusion of the three subsequent head segments bearing the jaws. Moreover, the gnatocephalon fuses with the thoracic region, forming the so-called maxillary thorax (gnathothorax), covered from above and from the sides by a durable solid shell - caropax. Thus, the body of the crayfish is divided into the following sections: head - progocephalon (acron and one segment), maxillary thorax - gnathothorax (three cephalic and eight thoracic segments) and abdomen (six segments and anal lobe). Quite often in textbooks they talk about dividing the cancer body into the cephalothorax and abdomen. As you can see, this is not entirely correct, because the head lobe and the first segment of the head section are not fused with the underlying segments.
In a calm state, the crayfish moves along the bottom on its walking legs, head first. At the moment of danger, the crayfish, straightening its tail fin, sharply and often bends its abdomen and quickly swims away backwards with jerks.
COVERS. In primitive crustaceans, the integument is relatively thin and is formed by cuticle plates that cover the body on all sides. However, in crayfish and other highly organized forms, the outer integument thickens and forms a hard shell. The outer layer of the cuticle is impregnated with salts, which significantly increases the strength of the integument.
The shell reliably protects the animal's body, but does not allow it to grow. Therefore, the growth and development of crayfish occurs during periodic molts. Young crayfish grow quickly and therefore molt several times a year, adult crayfish molt much less frequently - once a year. After the old cuticle is shed, the new cuticle remains soft and easily stretchable for some time. At this moment, the crayfish become vulnerable to enemies and hide in shelters. Then the cuticle hardens, becomes saturated with lime, and the animal’s growth stops until the next molt.
Digestive system. The digestive system begins with the mouth opening, covered by cuticle outgrowths - the upper and lower lips. The foregut includes a short esophagus and stomach (Fig. 43). The cancer stomach consists of two sections: chewable And filtering (nuloric). The inner walls of the chewing section bear powerful chitinous plates, with the help of which food is finely ground. There are also white rounded calcareous thickenings - millstones. They accumulate calcium carbonate, which is necessary for the cancer to saturate the cuticle after molting. In the filtering section of the stomach, thin outgrowths of the cuticle form a sieve through which only highly crushed food is filtered. From the stomach, food is sent to the short midgut. In most crustaceans, the midgut has lateral glandular projections, not entirely correctly called liver. In crayfish, the liver is formed by two independent lobes (right and left), the ducts of which flow into the midgut. The liver produces digestive enzymes that enter the chewing stomach. Cavitary and intracellular digestion and absorption of nutrients coming from the midgut also occur in it.
Rice. 43. Internal structure of crayfish (female):
1 - antennas II, 2 - antennas 1 (antennas), 3 - eye, 4 - stomach, 5 - digestive gland, 6 - arteries, 7 - ovary, 8 - heart, 9 - abdominal nerve cord, 10 - hindgut, 11 - gills
Since the liver of crustaceans combines the functions of the liver and pancreas, zoologists prefer to call this organ simply the digestive gland. Since the liver partially performs the functions of the midgut, in the class of crustaceans there is an inverse relationship between the development of the midgut and the liver. For example, daphnia have a small liver and a long midgut, while in crayfish the midgut is a short tube, the length of which is 10 times less than the hindgut.
Undigested food remains enter the long rectum, which passes through the abdomen and opens with an opening in the anal lobe.
The foregut and hindgut, which are of ectodermal origin, are lined with a cuticle, which peels off during molting and comes out in the form of tubes. Therefore, during molting, crayfish do not feed.
Breath. Crayfish breathes with gills (see Fig. 43). They are located under the carapace in the gill chambers and are reliably protected from damage. Fresh water is constantly supplied to the chambers due to the flow of water created by the limbs. The gills are delicate, numerous thread-like outgrowths of the thorax limbs, covered with a thin cuticup, into which the body cavity extends. Gas exchange occurs through the thin coverings of the gills. Hemolymph, passing along the gill filaments, is saturated with oxygen and releases carbon dioxide.
Many small crustaceans with a thin cuticle lack gills and breathe over the entire surface of the body. Land crustaceans have special respiratory organs. Thus, woodlice have deep branching invaginations of the integument on their abdominal legs, reminiscent of trachea, in which gas exchange occurs.
Circulatory system. Circulatory system open The heart of cancer is located on the dorsal side of the chest and is a pulsating pentagonal muscle pouch with three pairs of holes (osty)(see Fig. 43). When the heart contracts, hemolymph is pushed into the branching arteries and distributed throughout the body. From the blood vessels it pours into the body cavity, washes the internal organs, gradually releases oxygen and goes to the gills. After being saturated with oxygen in the gills, the hemolymph enters the pericardium and from it through the ostia into the heart.
Excretory system. Excretory organs of crayfish - green glands, received this name for their color. They are located in the anterior part of the maxillary thorax. The internal part of the gland, which looks like a small sac, is a remnant of the coelom and opens into the body cavity. It is followed by a thin convoluted tubule consisting of several sections, the last of which expands into the bladder. A short canal extends from the bladder, which opens outwards with an excretory opening at the base of the second pair of antennae.
Nervous system. The nervous system of cancer consists of a well-developed brain connected by the peripharyngeal nerve ring to the ventral nerve cord (see Fig. 43). From the brain, nerves travel to the eyes and sensory antennae. From the peripharyngeal ring - to the oral apparatus, and from the nodes of the abdominal nerve chain to the rest of the limbs and internal organs of the body.
Sense organs. The sense organs are well developed. The organs of touch and chemical sense are located on the head antennae. At the base of the first pair of antennae are the organs of balance - statocysts.
The equilibrium organs of crayfish are located at the base of the antennules and are open sac-like invaginations of the integument, communicating with the environment. The bottom of the statocysts is lined with a thin cuticle with sensitive hairs. Sand grains entering the statocyst through its external opening act as statoliths. When the position of the crayfish body in space changes, the statoliths irritate the hairs, and corresponding nerve impulses enter the brain. During molting, the cuticular lining of the statocyst is also torn off, so during this period the crayfish loses coordination of movements.
Complex faceted eyes consist of numerous simple ocelli, each of which functions individually and perceives the image of only part of the surrounding space. As a result, the overall visual perception consists of individual fragments. This vision is called mosaic. The eyes of cancer are mobile, they sit on special outgrowths - eyestalks.
Reproduction and development. Crayfish are dioecious, with pronounced sexual dimorphism. In females, unlike males, the abdomen is wider than the thoracic segments. The first pair of abdominal limbs of the male is transformed into a copulatory organ; in females the legs are rudimentary. In the maxillary thorax there are unpaired gonads with paired reproductive ducts, opening genital openings at the bases of the third (in females) and fifth (in males) pairs of thoracic walking legs. In late autumn or winter, mating occurs, during which the males, using the first pair of abdominal legs, glue packets of sperm next to the genital openings of the females. After this, the females lay eggs, which are glued to the abdominal legs. In this case, the abdomen is pressed against the cephalothorax, forming a brood chamber. Fertilization and development of eggs occurs inside the chamber. In the spring, small crustaceans hatch from the eggs, which remain on the mother’s abdomen for some time. Then the crayfish leave the female and move on to independent life.
Crustaceans have a wide variety of shapes and sizes of male gametes. In many species, gametes are very large and completely immobile. For example, some small crustaceans, whose length is less than 1 mm, have the longest sperm of all animals - they are an order of magnitude longer than the crustacean itself and reach 6 mm! Remember that male gametes that do not have organelles of movement are called sperm. It’s the same in botany: motile gametes of spore plants are called sperm, and immobile gametes of seed plants are called sperm.
Rice. 44. Commercial crustaceans: A- Kamchatka crab; B- lobster; IN- lobster
The importance and diversity of crustaceans. Crustaceans are found in almost any body of water. Their numbers and biomass are very high, so crustaceans play an important role in aquatic ecosystems.
The plankton of fresh and marine waters is home to many small crustaceans that feed on unicellular algae. In turn, they serve as food for larger animals - from fish fry to whales. Thus, small crustaceans (cladocera and copepods, shrimp, etc.) represent an important link in the food chain of any aquatic community.
Among crustaceans there are many valuable commercial objects that humans eat: shrimp, lobsters, lobsters, Kamchatka and other crabs (Fig. 44). The crustacean fishery is widely developed and reaches 700 thousand tons per year in the world. Freshwater crayfish are not only caught in the wild, but also successfully bred on specially created farms. In fish hatcheries, small crustaceans (for example, daphnia) are grown as food for fish.
Some crayfish are loved to be consumed with beer, others are cared for in aquariums, but few people remember that these creatures managed to survive for 130 million years, practically without changing their structure. The only thing that distinguishes them from their ancient counterparts is their size. During the Jurassic period, some types of crayfish reached 3 m in length and could fend for themselves.
Today, among the ranks of crustaceans, there are about 55,000 representatives of various lengths, living in sea or fresh water, and some of them prefer to be land-dwelling.
History of the delicacy
People have been using crayfish since Antiquity, but then they were not served as a delicacy. It is obvious that the healers and healers of the ancient world knew about the beneficial properties of shells, since they made potions from them for the bites of poisonous insects.
The first mention of the fact that river crayfish are a tasty dish was recorded in the 16th century, when one of the Swedish kings accidentally tasted them. A decree was immediately issued that the peasants should catch and deliver them to the royal table, but not dare to eat them themselves under pain of the death penalty.
Imitating the king, the Swedish nobles did the same, although the poor people were perplexed by the royal decree. They did not consider crayfish to be food and were content with them only in times of famine, which happened extremely rarely in this country.
In modern Sweden there is even a national holiday, Crayfish Eating Day, when people gather in large groups, boil these arthropods and drink strong alcoholic drinks.
Today, some types of crayfish (the photo demonstrates this) are considered a delicacy and are not just served with beer, but are prepared from them into soups, salads, stewed with vegetables, sauces made from them, and even fried.
Their meat is considered one of the most environmentally friendly, despite the fact that they are sanitation workers and “orderlies” of water sources. This is due to the balanced, self-cleaning organism given to them by nature.
Stream arthropods
There are different types of crayfish, but this name is not entirely accurate, since they live in swamps, ponds, lakes, and artificial reservoirs. It is more correct to use the term “freshwater”.
All representatives of crustaceans living in fresh water have the same structure:
- their body can reach a length of 10 to 20 cm;
- the upper part of the body is called the cephalothorax;
- they have an elongated and flatter abdomen;
- the body ends with a caudal fin;
- they have 10 pectoral legs and gills.
The most famous types of freshwater crayfish are:
- The broad-toed fish (Astacus astacus) lives in the reservoirs of Western Europe and high-mountain rivers of Switzerland, preferring places with temperatures from +7 to +24 degrees Celsius.
- Thin-toed (Astacus leptodactylus) can live in both fresh running or standing water, and brackish water with maximum heating up to +30.
These types of crayfish are not suitable for keeping in aquariums, as they are very demanding in care, especially in terms of water filtration and temperature control.
Florida crayfish
Well known to many aquarists, the red Florida crayfish can actually be black, white, orange and even blue. It lives both in swamps and flowing rivers, and in flooded meadows, and as the water recedes, it “goes” into deep burrows underground.
These are the most undemanding species of crayfish in terms of composition and water quality. Their appearance is well known to residents not only of swamp Florida, but also of Europe. Its distinctive feature is the red spikes located on its claws.
This small arthropod (body length up to 12 cm) can easily tolerate water temperatures from +5 to + 30 degrees and reproduce year-round in an aquarium, laying up to 200 eggs. Incubation continues for 30 days, and during this time the temperature in the aquarium should be maintained at +20...+25 degrees.
The red swamp crayfish gets along well with fish, but you should remember that 1 pair will need an aquarium with 100 liters of water.
Blue crayfish from Cuba
Cuban blue crayfish can have other colors, as this directly depends on the natural conditions in their habitat and the color of their parents.
This tropical representative of arthropods lives in Cuba and Pinos. It has a small body up to 12 cm (excluding claws) and has a completely peaceful character, so it can be kept in aquariums with active or large fish.
The fact that this crayfish is unpretentious and reproduces well in captivity makes it a favorite of many aquarists. For 2 or 4 representatives of blue Cuban crayfish you will need a 50 liter container with good ventilation and water filtration.
The female of this species can lay up to 200 eggs at a time. For this to happen, it is better to transplant the crayfish into another smaller aquarium before mating, so that there is no interference from the “neighbors.” Incubation lasts 3 weeks, during which the water temperature should be +25 degrees.
Marine arthropod
The most popular among gourmets is lobster meat. These marine species of crayfish differ from their freshwater counterparts solely in size and weight. They have a strong chitinous shell, which young individuals change as they grow older.
The molting of a lobster takes from 2 to 4 weeks, during which it is defenseless and is forced to hide from its enemies in secluded places. The process of getting rid of tight coverage is interesting. The shell bursts on the lobster's back, like clothing cracking at the seams. To free itself, the crayfish has to step out of it with its back, removing one leg after the other.
A female lobster lays up to 4,000 eggs on her tail, after which the male fertilizes them. The incubation period lasts 9 months, during which the eggs remain on the mother’s body. Individuals that have survived 25 molts are considered ready to mate and eat.
Gourmets are well aware of the European, Norwegian and American types of lobsters. The cost of their tender, healthy, dietary meat starts at $50 per kilogram, and 100 years ago it was used as bait for fishing.
Land representative of arthropods
If you think about the question of what types of crayfish there are, few people will remember that there are unique individuals that can climb trees.
These are coconut crayfish (Birgus latro) that live on the islands of the Indian and Western Pacific Oceans. During the day, these amazing creatures hide in the foliage of palm trees, and at night they come down to pick up fallen fruits or carrion from the ground. The islanders call these hermit crabs thieves, as they often pick up anything they think is bad.
Although the coconut crayfish spends most of its life on the ground, it begins its life in bodies of water, where females lay eggs, from which small and defenseless crustaceans emerge. To survive, they are forced to look for a protective covering for their bodies, which most often becomes some kind of shell.
After the young grow up, the crayfish come out and can no longer return to the aquatic environment, since their gills atrophy and their respiratory organs become ventilated lungs.
Those who want to see these unusual creatures will have to go into the tropical jungle at night. Their meat is considered a delicacy and an aphrodisiac, but hunting for them is extremely limited.
Rare crustaceans
The rarest species of crayfish that can live in aquariums are called apricot crayfish. They live in Indonesia and can be either a soft orange color or blue, which is extremely rare.
They are small in size, males rarely grow to 10 cm, and the length of females is 8 cm. To keep them in aquariums, you should not only ensure that the temperature is kept within +25 degrees, but also the bottom is properly designed.
These crayfish love fine gravel sprinkled with bamboo, almond or oak leaves, which also serve as a good antiseptic. Numerous shelters in the form of driftwood, metal tubes and artificial houses will not hurt either. For the most part, Orange Papua New Guinea lobster is a non-aggressive vegetarian, but it is still not recommended to add small fish to it.
Largest freshwater arthropods
The largest species of crayfish that live in fresh water bodies come from Tasmania. In the rivers in the north of this Australian state, there are individuals reaching 60-80 cm in length and weighing from 3 to 6 kg.
Their favorite habitat is rivers with calm currents, good air ventilation and a water temperature of +18 degrees. Depending on which river these giants live in, lowland or mountainous, they can have a color from green and brown to blue.
Since Astacopsis gouldi live up to 40 years and are considered long-livers among their relatives, all their life processes are somewhat drawn out. For example, males are ready for reproduction only at 9 years old, and females at 14 years old, while mating occurs once every 2 years, and the incubation period lasts from autumn to summer of the next year. In this regard, it is customary for Tasmanian giants to keep a harem of females of different ages.
Heraxes
Another representative of Australian rivers is the Herax crayfish. What is surprising is that these arthropods, which have many species, include individuals of completely different sizes. So, some of them can be 40 cm in length and weigh up to 3 kg, while others grow up to 10 cm and are placed in aquariums with a volume of up to 20 liters. Another home for these freshwater species are the rivers of New Guinea.
It is not difficult to create conditions for keeping heraxes in an aquarium. They love warm water and the opportunity to dig in the soil, so if such “tenants” are present, it is better to plant the plants in pots. They don't eat them, but they can dig them up. Herax crayfish show indifference to the proximity of fish, but if you breed larger specimens with large claws, it is better to keep them in a separate container.
Unusual types of crayfish
Although arthropods are generally similar in appearance, their abilities to adapt and survive are strikingly different. For example, marbled crayfish reproduce asexually, and a similar phenomenon in nature is called parthenogenesis.
Females of this type of crayfish can clone themselves without involving males in the process. A similar phenomenon could previously be observed only in higher crustaceans, but never in small river specimens, reaching a maximum length of 8 cm.
In order for freshwater aquarium crayfish species to take root, it is necessary to constantly maintain clean water that is well enriched with oxygen.
When choosing a container for such “tenants”, you should proceed from the parameters that 1 individual 6-7 cm will require 15 liters of water. To make your pets feel at home, the bottom should be properly designed. You will need driftwood, gravel or sand, ceramic or metal cylinders where the crayfish can hide during the day.
Planting plants in a container depends on the type of cancer, as well as whether there will be fish with it. Otherwise, keeping these individuals is not a hassle; the main thing is to remember to cover the aquarium with a lid, otherwise you may find your pet on the bed.
Latin name Crustacea
Characteristics of crustaceans
The gill-breathing subphylum contains one class of crustaceans (Crustacea), which is richly represented in the modern fauna. It is very typical for them to have two pairs of head antennae: antennules and antennae.
Dimensions crustaceans range from fractions of a millimeter in microscopic planktonic forms to 80 cm in higher crustaceans. Many crustaceans, especially planktonic forms, serve as food for commercial animals - fish and whales. Other crustaceans themselves serve as commercial fish.
Body dismemberment
The body of crustaceans is segmented, but, unlike annelids, their segmentation is heteronomous. Similar segments that perform the same function are grouped into departments. In crustaceans, the body is divided into three sections: the head (cephalon), chest (thorax) and abdomen (abdomen). The head of crustaceans is formed by an acron corresponding to the head lobe - the annelid prostomium, and four body segments fused with it. Accordingly, the head section bears five pairs of head appendages, namely: 1) antennules - single-branched tactile antennae innervated from the brain (homologous to the palps of the rings); 2) antennae, or second antennae, originating from the first pair of two-branched limbs of the parapodial type; 3) mandibles, or mandibles - upper jaws; 4) first maxillae, or first pair of lower jaws; 5) second maxillae, or second pair of lower jaws.
However, not all crustaceans have the acron and the four segments that form the head, fused together. In some lower crustaceans, the acron is fused with the antennal segment, but is not fused with the independent mandibular segment, but both maxillary segments are fused together. The anterior section of the head, formed by the acron and a segment of the antennae, is called the primary head - protocephalon. In many crustaceans (in addition to the formation of the primary head - protocephalon), all jaw segments (mandibular and both maxillary) also merge to form the jaw section - gnatocephalon. This section fuses with a greater or lesser number of thoracic segments (in crayfish with three thoracic segments), forming the maxillary thorax - gnathothorax.
In many, the head consists of five completely fused parts: an acron and four body segments (scuttlefishes, cladocerans, some amphipods and isopods), and in some the head segments merge with one or two more thoracic segments (copepods, isopods, amphipods).
In many, the dorsal coverings of the head form a protrusion at the back, more or less covering the thoracic region, and sometimes the entire body. This is how the cephalothorax shield, or carapace, of crayfish and other decapods is formed, and the transverse groove on this shell indicates the boundary between the fused jaw and thoracic sections of the body. The carapace grows onto the thoracic segments. Sometimes it can be compressed from the sides, forming a gable shell that hides the entire body (shell crustaceans).
The thoracic segments, as indicated, can fuse with the head (1-3, even 4 segments), forming the cephalothorax. All thoracic segments bear limbs, the functions of which are not limited to motor and respiratory. Thus, in crayfish, the 3 first pairs of thoracic limbs turn into jaws that supply food to the mouth.
The abdominal segments are usually movably connected to each other. Only higher crustaceans have limbs on their abdominal segments; the rest have abdomen without them. The abdominal region ends in a telson, which does not bear limbs and is homologous to the pygidium of polychaetes.
While all crustaceans have the same number of head segments (5), the number of thoracic and abdominal segments is very different. Only in higher crayfish (decapods, isopods, etc.) their number is constant: pectorals - 8, abdominals - 6 (rarely 7). In the rest, the number of thoracic and abdominal segments ranges from 2 (shells) to 50 or more (shells).
Limbs
The limbs of the head are represented in five pairs. The antennules corresponding to the palps of the rings retain mainly the functions of the sense organs of touch and smell in crustaceans. The antennules of crayfish consist of main segments and two segmented branches.
The antennae are the first pair of limbs of parapodial origin. In the larvae of many crustaceans they are bibranched, and in most adult crustaceans they become single-branched or retain only a rudiment of the second branch (exopodite). Antennas perform mainly a tactile function.
The mandibles make up the upper jaws. They correspond in origin to the second pair of limbs. In most crayfish, the mandibles have been transformed into hard, jagged chewing plates (mandibles) and have completely lost their bibranched character. It is believed that the chewing plate corresponds to the main part of the limb - the protopodite. In crayfish (and some others), a small three-segmented palp sits on the chewing plate - the remnant of one of the branches of the limb.
The first and second maxillae, or first and second pairs of mandibles, are usually less reduced limbs than the mandibles. In decapods, the maxillae consist of two main segments, forming a protopodite, and a short, unbranched palp. With the help of the chewing plate of the protopodite, the maxillae perform a chewing function.
The thoracic limbs of representatives of different orders are arranged differently. In crayfish, the first three pairs of thoracic limbs are transformed into so-called maxillopods or maxillopods. The crayfish's jaws, especially the second and third pairs, retain a fairly strong two-branched structure (endopodite and exopodite). The second and third pairs also bear gills, and their movement causes water currents to flow through the gill cavity. Therefore, they perform a respiratory function. However, their main function is to hold food and move it towards the mouth. Finally, the endopodite of the third pair serves as a kind of toilet device, with the help of which the antennules and eyes are cleaned of foreign particles adhering to them.
However, in many other crustaceans, the first three pairs of thoracic limbs perform primarily a locomotor function.
A peculiar change in the thoracic limbs is their adaptation to grasping, for example, the claws of decapod crayfish. The claw is formed by two segments of the limb: the penultimate segment, which has a long outgrowth, and the last segment articulated with it, forming the other side of the claw. The fifth to eighth pairs of thoracic limbs of crayfish (and other decapods) are typical walking legs. They are single-branched, and their basal part (protopodite) and endopodite are preserved. The exopodite is completely reduced. Bibranching of the thoracic limbs is observed much more often in lower crustaceans.
Abdominal limbs, as already mentioned, are absent in many groups of crustaceans. In higher crustaceans they are usually less developed than pectorals, but more often they retain bibranching; in many crayfish they are equipped with gills, simultaneously performing a respiratory function. In crayfish, the abdominal legs - pleopods - are changed in males. Their first and second pairs represent the copulatory apparatus. In females, the first pair is vestigial. The second is the fifth pair of abdominal legs in females and the third is the fifth pair in males of the swimming type. They are bibranched and consist of a few segments, abundantly covered with hairs. The laid eggs, which they incubate, are attached to these legs of female crayfish, and then the hatched crustaceans rest on the female’s legs for some time.
The last, sixth pair of abdominal legs - uropods - is peculiarly modified in crayfish and some other crayfish. Both branches of each leg are transformed into flat swimming blades, which, together with the flat last abdominal segment - the telson - form a fan-shaped swimming apparatus.
Crabs often have an interesting protective adaptation - spontaneous throwing of their limbs, which sometimes occurs even with very slight irritation. This autotomy (self-mutilation) is associated with a strong ability to regenerate. Instead of a lost limb, a new one develops.
Skeleton and musculature
The chitinized cover is impregnated with calcium carbonate. This gives greater rigidity to the skeleton.
The mobility of the body and limbs in the presence of a hard cover is ensured by the fact that chitin covers the body and limbs with a layer of unequal thickness and hardness. Each abdominal segment of the crayfish is covered with hard plates of chitin on the dorsal and ventral sides. The dorsal shield is called the tergite, the ventral shield is called the sternite. At the boundaries between the segments, thin and soft chitin forms folds that straighten when the body is bent in the opposite direction. A similar adaptation is observed on the joints of the limbs.
The internal skeleton of the crayfish serves as an attachment site for various muscles. In many places, especially on the ventral side of the thoracic region, the skeleton forms a complex system of crossbars that grow into the body and form the so-called endophragmatic skeleton, which also serves as a site for muscle attachment.
All kinds of bristles and hairs covering the body of the crayfish and especially its limbs are outgrowths of the chitinous cover.
Digestive system
The digestive system is represented by the intestine, consisting of three main sections: the foregut, midgut and hindgut. The foregut and hindgut are of ectodermic origin and are lined from the inside with a chitinous cuticle. Crustaceans are characterized by the presence of a paired digestive gland, usually called the liver. The digestive system reaches its greatest complexity in decapods.
The foregut of crayfish is represented by the esophagus and stomach. The mouth is located on the ventral side, and a short esophagus extends from it upward, towards the dorsal side. The latter leads to the stomach, which consists of two sections - cardiac and pyloric. The cardial, or chewing, section of the stomach is lined from the inside with chitin, forming in its rear part a complex system of crossbars and protrusions equipped with teeth. This formation is called the “gastric mill”; it ensures the final grinding of food. In the front part of the cardiac section there are white rounded calcareous formations - millstones. The calcium carbonate that accumulates in them is used during molting to saturate the new chitinous cover with it. Food crushed in the cardial section of the stomach enters through a narrow passage into the second, pyloric section of the stomach, in which food particles are pressed and filtered out. This part of the stomach ensures that only highly crushed food enters the midgut and digestive gland. It must be borne in mind that in the stomach not only the mechanical grinding of food occurs, but partly its digestion, since the secretion of the digestive gland penetrates into the stomach. The remaining uncrushed larger particles of food, due to the special structure of the pyloric part of the stomach, pass directly into the hindgut, bypassing the midgut, and are excreted.
The midgut of crayfish is very short. It makes up approximately 1/20 of the entire length of the intestine. Digestion and absorption of food occurs in the midgut. Most of the liquid food from the stomach goes directly into the digestive gland (liver), which opens with two openings at the border of the midgut and the pyloric part of the stomach. Digestive enzymes that digest proteins, fats and carbohydrates are not only excreted into the midgut and stomach, but are also used in the liver tubes themselves. Liquid food penetrates these tubes, and here its final digestion and absorption occurs.
In many crustaceans, the digestive gland is much less developed (for example, in daphnia), and in some it is completely absent (in Cyclops). In such crustaceans the midgut is relatively longer.
The hindgut is a straight tube lined on the inside with chitin and opening with the anus on the ventral side of the telson.
Respiratory system
Most crustaceans have special respiratory organs - gills. By origin, the gills develop from the epipodites of the limbs and, as a rule, are located on the protopodites of the thoracic, less often, abdominal legs. In a simpler case, the gills are plates sitting on the protopodite (amphipods, etc.); in a more advanced form, the gills are a rod seated with thin gill filaments. The lacunae of the body cavity - the myxocoel - extend inside the gills. Here they form two channels, separated by a thin partition: one is inflowing, the other is outflowing.
In decapods, including crayfish, the gills are placed in special gill cavities formed by the lateral folds of the cephalothorax shield. In crayfish, the gills are arranged in three rows: the lower row is located on the protopodites of all thoracic limbs, the middle row is on the places where the limbs are attached to the cephalothorax, and the upper row is on the side wall of the body. In crayfish, 3 pairs of jaws and 5 pairs of walking legs are equipped with gills. Water constantly circulates in the gill cavities, entering through openings at the base of the limbs, in places where the folds of the cephalothorax shield loosely adhere to them, and exits at its anterior edge. The movement of water is caused by the rapid oscillatory movements of the second maxillae and partly the first pair of maxillae.
Crustaceans that have transitioned to a terrestrial existence have special adaptations that ensure breathing with atmospheric air. In land crabs these are modified gill cavities, in woodlice they are limbs penetrated by a system of air tubes.
Many small forms (copepods, etc.) do not have gills and respiration occurs through the integument of the body.
Circulatory system
Due to the presence of a mixed body cavity - myxocoel - the circulatory system is not closed and blood circulates not only through the blood vessels, but also in the sinuses, which are parts of the body cavity. The degree of development of the circulatory system varies and depends on the development of the respiratory organs. It is most developed in higher crustaceans, especially in decapods, which, in addition to the heart, have a rather complex system of arterial vessels. In other crustaceans, the vascular system is much less developed. Daphnia has no arterial vessels at all and the circulatory system is represented only by the heart in the form of a vesicle. Finally, copepods and shellfish also lack a heart.
The heart of crustaceans, tubular or sac-shaped, is located on the dorsal side of the body in the pericardial cavity - the pericardium (the pericardium of crustaceans is not associated with the coelom, but is a section of the myxocoel). Blood enters the pericardium from the gills, sufficiently enriched with oxygen. The heart communicates with the pericardium through paired slit-like openings equipped with valves - ostia. Crayfish have 3 pairs of ostia; crayfish with a tubular heart can have many pairs. When the heart expands (diastole), blood enters it through the ostia from the pericardium. During contraction (systole) of the heart, the valves of the ostia close and blood is driven from the heart through the arterial vessels to various parts of the body. Thus, the pericardial portion of the myxocoel performs the function of the atrium.
In crayfish, the arterial vascular system is quite developed. Three vessels extend forward from the heart to the head and antennae. Back from the heart there is one vessel carrying blood to the abdomen, and two arteries flowing into the lower abdominal vessels. These vessels branch into smaller ones, and eventually the blood enters the myxocoel sinuses. Having given oxygen to the tissues and received carbon dioxide, the blood is collected in the abdominal venous sinus, from where it is sent through the afferent vessels to the gills, and from the gills through the efferent vessels to the pericardial region of the myxocoel.
Excretory system
The excretory organs of crustaceans are modified metanephridia. In crayfish and other higher crustaceans, the excretory organs are represented by one pair of glands located in the head of the body and opening outward through openings at the base of the antennae. They are called antennal glands. The gland is a complex convoluted capsule with glandular walls, consisting of three sections: white, transparent and green. At one end the canal is closed by a small coelomic sac, which is a remnant of the coelom. At the other end, the channel expands into the bladder and then opens outwards. The excretory glands of crayfish are also called green glands due to their greenish color. Substances released from the blood diffuse into the walls of the canal, accumulate in the bladder and are released out.
Other crustaceans also have one pair of excretory glands of a similar structure, but they open outward not at the base of the antennae, but at the base of the second pair of maxillae. Therefore they are called maxillary glands. In crustacean larvae developing with metamorphosis, the location of the excretory organs is reversed, namely: the larvae of higher crustaceans have maxillary glands, and the remaining larvae have antennal glands. Apparently, this is explained by the fact that initially the ancestors of crustaceans had two pairs of excretory organs - both antennal and maxillary. Subsequently, the evolution of crayfish followed different paths and led to the fact that in higher crustaceans only the antennal glands were preserved, and in the rest only the maxillary glands. Proof of the correctness of this point of view is the presence in some crustaceans, namely, in marine crustaceans, nebalia from primitive higher crustaceans, as well as in barnacles from lower crustaceans, two pairs of excretory glands.
Nervous system
The central nervous system of most crustaceans is represented by the ventral nerve cord and is very close to the nervous system of annelids. It consists of the suprapharyngeal ganglion (paired in origin), forming the brain, connected to the subpharyngeal ganglion by peripharyngeal connectives. From the subpharyngeal ganglion comes a double abdominal nerve trunk, forming a pair of contiguous ganglia in each segment.
In higher crustaceans, the nervous system reaches a relatively high level of development (brain structure), while in other groups of crustaceans it is more primitive. An example of the most primitive structure is the nervous system of branchiopods, which have a cephalic ganglion, peripharyngeal connectives and two relatively widely spaced nerve trunks extending from them. On the trunks in each segment there are small ganglion thickenings connected by double transverse commissures. In other words, the nervous system of these crayfish is built according to the ladder type.
In most crustaceans, the longitudinal nerve trunks converge, the paired ganglia of which merge together. In addition, as a result of the fusion of segments and the formation of body parts, their ganglia merge.
This process is primarily associated with the formation of the head (cephalization). Thus, the brain of crayfish (and other decapods) is formed by the cephalic ganglion itself with two sections - the antennular and the antennal attached to it (the first pair of ganglia of the abdominal nerve chain, innervating the antennae). The subpharyngeal ganglion was formed by the fusion of the following 6 pairs of ganglia of the ventral nerve chain: the ganglia innervating the mandibles, two pairs of maxillae and three pairs of maxillae. This is followed by 11 pairs of ganglia of the abdominal chain - 5 thoracic and 6 abdominal.
On the other hand, fusion of ganglia may also occur due to shortening of the body or small size in a particular group of crustaceans. Particularly interesting in this regard is the fusion of all the ganglia of the ventral chain into one large node observed in crabs.
Sense organs
Crustaceans have organs of touch, organs of chemical sense (smell), organs of balance and organs of vision.
Reproduction
With rare exceptions (barnacles), all crustaceans are dioecious, and many have quite pronounced sexual dimorphism. Thus, the female crayfish is distinguished by a noticeably wider abdomen and, as we know, by the structure of the first and second pairs of abdominal legs. In many lower crustaceans, males are significantly smaller than females.
Crustaceans reproduce exclusively sexually. In a number of groups of lower crustaceans (scutellites, cladocerans, shellfishes) parthenogenesis and alternation of parthenogenetic and bisexual generations take place.
Associated with its habitat. This is a typical inhabitant of fresh water bodies that breathes using gills. This article will look at crayfish. The structure, pictures, habitat and features of life are presented to your attention below.
Characteristic signs of crustaceans
Like all arthropods, the structure of the crayfish (the figure below demonstrates it) is represented by a segmented body and limbs. These are the head, chest and abdomen. The body segments bear paired limbs consisting of individual segments. They are capable of performing quite complex movements. The jointed limbs are usually attached to the thoracic segments of the body. The external structure of crayfish fully corresponds to the characteristic features of arthropods.
Habitat
Crayfish can be found in fresh water bodies. Moreover, their presence can be considered an indicator of its environmental safety. After all, these animals prefer reservoirs with clean water and high oxygen content. The structure of crayfish determines their ability to crawl with the help of walking legs or swim. During the day they are in natural shelters. At night they crawl out of holes, from under stones and logs. At this time they are looking for food. In this regard, crayfish are not picky. They are generally omnivores. Worms, fry, tadpoles, mollusks, algae - all of them will appeal to crayfish. They do not disdain dead organic matter. If you decide to keep this animal in your home aquarium, not only special food is suitable for food, but also meat, vegetables and bread. However, it will be quite difficult to maintain the purity of the water.
External structure of crayfish
The body of crayfish consists of two parts. These are the cephalothorax and abdomen. The front part is covered with the so-called shell. The abdomen consists of separate segments, on top of which are small scutes. The cephalothorax also contains two pairs of antennae, mouthparts, and five pairs of walking legs. Each of them performs specific functions. For example, the first pair ends with powerful claws, used to grab food, tear it into pieces and protect it from enemies.
Six pairs of limbs are attached to the abdomen. The last pair of legs is expanded and, together with the anal plate, forms the caudal fin. In appearance it resembles a fan. With the help of the anal fin, crayfish swim quite quickly with their back end first. Collectively they have 19 limbs.
Coverings of the body
The structural features of crayfish are primarily determined by its cover. Like all arthropods, it is represented by a cuticle, which forms a powerful exoskeleton. It is given additional hardness by the calcium carbonate with which it is impregnated.
Since the cuticle is not capable of stretching, the growth of crayfish is accompanied by periodic molting. This period is critical in the life of these aquatic inhabitants. A few days before this, the crayfish become restless, stop feeding, and spend all their time looking for shelter. With the help of intense movements of the body and limbs, they get rid of the old cover, from which they crawl out through a gap at the border of the cephalothorax and abdomen. The crayfish remain in their safe shelter for up to ten days until the new cuticle hardens.
Internal structure of crayfish
During embryonic development, a secondary body cavity is formed in all arthropods. But it does not persist throughout the animal’s life. In the process of individual development, this structure is destroyed, merging with the remains of the primary one and forming a mixed cavity. The spaces between the organs are filled with a fatty body in crustaceans. This is a type of loose connective tissue that performs important functions: storing nutrients, forming blood cells, protecting against mechanical damage.
Musculoskeletal system
The structure of crayfish differs significantly from that of coelenterates. In particular, they do not have a skin-muscle sac. Underneath the strong one there are bundles of striated muscles that can contract quickly.
Major organ systems
The internal structure of crayfish is quite complex. Digestive - end-to-end, with the presence of a liver and salivary glands that secrete enzymes that break down nutrients. The end products of metabolism are removed from the body using the Malpighian vessels.
Crayfish are determined by their aquatic habitat, these are gills. The circulatory system is closely connected with them. She is not closed. Blood vessels open into the body cavity, mix with its fluid, forming hemolymph. It transports oxygen, carbon dioxide, nutrients, and metabolic products.
The most important function of hemolymph is protective. It contains specialized cells that carry out amoeboid movements, capture pathogenic microorganisms with pseudopods and digest them. The movement of hemolymph throughout the body is ensured by a pulsating thickened vessel - the heart. Since the blood mixes with the cavity fluid and is not divided into arterial and venous, crayfish is a cold-blooded animal. This means that his body temperature decreases as the environment gets colder.
At the end of autumn, crayfish begin to reproduce. These are dioecious animals with direct development and external fertilization. The male has a testis and two vas deferens, the female has an ovary and paired oviducts. After fertilization, the eggs are located on the abdominal legs of the female. This is how she shows her maternal instinct, taking care of her future offspring. At the end of spring or early summer, young crustaceans emerge from them, which are an exact copy of the adults.
The nervous system is also quite complex. It consists of differentiated sections: anterior, middle and posterior. The first regulates the functioning of the eyes, provides complex behavioral reactions of these animals, the rest innervate the antennae. The brain is anatomically connected to the ventral nerve cord, from which individual nerve fibers extend throughout the body.
Meaning in nature and human life
Young crayfish form plankton in fresh water bodies - an important link in the food chain. By using dead animals as food, they cleanse their habitat. Recently, due to the negative influence of humans, the number of crayfish populations has sharply decreased. In dirty water, the offspring of crayfish will inevitably die. This is also due to the important commercial significance of this representative of arthropods. After all, crayfish meat is a valuable food product, and in some regions even a delicacy. It is rich in proteins, vitamins and microelements. Crayfish is the largest representative of the class that lives in fresh water bodies. To preserve this species in nature, their fishing is officially prohibited until mid-summer.
The structure of crayfish is largely determined by its habitat and determines its importance in nature and human life.
