Shrimps and lobsters, lobsters and crabs. The amazing world of creatures that appeared more than five hundred million years ago. They will be discussed in the article. You will also see a giant crustacean in the photo, the span of which extends up to four meters.
When the Crustacean class is studied in the classroom, a table of the main traits of these creatures is often given at home. After reading our article, any student will be able to easily compile it.
What are crustaceans
One of the largest groups of the biological type Arthropods is Crustacean. It includes more than seventy three thousand species. And the researchers call the shield beetle the oldest representative of this group. Its modern structure is completely identical to the petrified remains, which are more than 200 million years old.
The peculiarity of this subtype is that it also includes a transitional stage from plants to the animal world. These are sea ducks and sea acorns. They do not move at all. Among other crustaceans it is worth noting woodlice and some crabs that live not on water but on land. Amphipods live in the moist soil of the tropical zone of our planet. There is even a parasitic species - taxa.
Thus, this biological subtype has mastered almost all variants of the Earth's water bodies, from the seabed to river banks.
Next, we will consider the main representatives. You will see a giant crustacean and learn about the smallest crustacean, whose body size is only 0.1 millimeter.
All types of living creatures of our planet are studied by biology. Crustaceans are managed by a discipline such as carcinology.
Among the important distinguishing features of this subtype is the shell, or chitinous exoskeleton. These are solid parts of the body of crustaceans that protect them from external mechanical stress. Since the exoskeleton does not increase in size, animals have to drop it several times in life in order to continue their growth.
They also have two pairs of antennae and they breathe using the gills, which are located on the legs.
We will talk more about the external and internal structure of crustaceans later. Now it is important to note one more thing. This biological subtype is an essential link in the food chain. People, for example, eat a lot of shrimp. Therefore, nature provides for a large number of representatives of this class.
Krill, for example, and copepods, which we will discuss at the end of the article, have the largest biomass among all living species on earth.
So, let's get acquainted with the structure of crustaceans.
External structure
As a subtype of the biological type, Arthropods, crustaceans have an external chitinous (or calcareous) carapace, as well as a segmented body surface with a different number of paired limbs.
The smallest group are tantulocarides. Their value varies from 0.1 to 0.3 millimeters. This also includes the parasite - the smallest crustacean Stygotantulus stocki, which was mentioned above. Its size fluctuates around 94 micrometers (less than 0.1 millimeters).
The largest representatives reach a weight of twenty kilograms and a span of the forelimbs of 3.5 - 3.8 meters. There is a riddle about this type of crustacean: “which crab can hug a hippopotamus?” This is a Japanese crab spider, which we will talk about later.
The external structure of crustaceans is approximately the same in all species, but differs in the number of segments and the appearance of legs at different stages of the life cycle.
So, the head, abdominal and thoracic regions are present in all representatives of this subtype. True, in some underdeveloped crustaceans, the segmentation of the last two sections is homogeneous. That is, the surface of the body is divided into parts of the same size.
In higher cancers, which we will talk about a little later, segmentation is constant. It consists of four parts: the abdomen, which consists of six segments, the cephalothorax, in which there are four head and eight thoracic segments, and the acron (a special section of the head near the mouth, only arthropods have).
The limbs of crustaceans are located on separate segments of the body, in pairs. Speaking in scientific language, the “leg” consists of a protopodite (the base of the limb), which includes coke feed (gills are here) and basipodite (connecting part), and two extensions - exopodite and endopodite.
The functions of the limbs are different, mainly they vary depending on the type of crustacean. For some, they are for breathing, for others, for movement or eating, while for the higher ones, all functions are used without exception.
Internal structure
The internal structure of crustaceans consists of five systems, muscles and sensory organs. We will begin to study it with muscles.
So, as in the representatives of the biological type Arthropods, in crustaceans, the muscles are represented by striated muscle tissue. They have no common sac, and the muscles are arranged as if in separate bundles. They usually serve as a link between different segments of the body surface.
The circulatory system of this subtype is open. That is, blood and lymph are combined and move along the sinuses of the mycelium and the vessels. It is noteworthy that the heart is always located near the respiratory system. It turns out that it is different in individual representatives of crustaceans. In some, this organ is located above the intestines, in others it is in the form of a tube over the entire length of the body. In each compartment of the latter there are special openings for the distribution of blood throughout the segment. There are hearts in the form of a barrel with spikes. Thus, this organ in different representatives of the subtype is long or shortened.
The nervous system is different in primitive and more developed crustaceans. For the former, it is of the “ladder type”, for the latter it is more integral, many sectors of which merged together. The first type is characterized by spaced apart ganglia that are connected by commissures. The brain of crustaceans consists of deutero-cerebrum, which has a connection with antennals, and proto-cerebrum, which is responsible for the eyes, acron and antennas.
The nervous system in some species is tightly connected with the endocrine. Due to this, certain types of crustaceans can change body color and better adapt to the environment.
The respiratory system also varies depending on the level of evolution of the species. So, the least developed crustaceans breathe the entire surface of the body, passing water through a special system. Land residents have acquired a special body - pseudotracheas - but they need only moist air for life. The respiratory system of the bulk of crustaceans consists of epipodites, special gills, which are located on the front or abdominal extremities.
The digestive system looks like a tube and consists of three intestines - front, middle and back. Grinding of substances occurs in the anterior section, absorption and digestion - on average, and output - through the back.
The excretory system consists of one pair of kidneys. In general, these organs are of two types - maxillary and antennal. Some crustaceans are born with the first type of kidney, replacing it with a second type in the process of life. And vice versa. All four organs have only one type of higher cancers - Nebalia.
The sense organs are represented by perfectly developed eyes, statocysts (balance organs) and special hairs on the antennas, which provide the sense of smell and touch.
Life cycle
Further, our characterization of crustaceans will continue with a description of their life cycle. It begins with the embryonic period. It all depends on the amount of yolk in the laid eggs.
If it is small, which is the norm for the underdeveloped representatives of the subtype, then the division occurs in exactly the same way as in annelids. That is, all parts receive an equal amount of substances and at the end of embryonic development the same creatures appear.
Otherwise, in higher crustaceans, there is a lot of yolk in the eggs, so the separation is superficial. They form an embryonic streak, which further forms the whole process of development.
After this, hatching from the egg occurs. The crustacean larva is called “nauplius”. He has an acron, antennae, a pair of swimming limbs and a two-segment body. The following stages are found only in higher crustaceans.
Zoea is a period of development when the larva has eyes, an abdomen and limbs on the head and front of the chest.
The next step is called the misid larva. She completely forms all the limbs of crustaceans and sense organs with body systems. In the process of increasing size, it sheds several times, dropping the cuticle. We will talk more about this metamorphosis later.
Shedding is regulated at the level of hormones. In the process of the creature’s growth, a certain stage begins when the old carapace interferes with further development. Due to the signal from the hypodermis, the body begins to form an increased supply of nutrients. With its help, a new layer of cuticle is formed. After the old one bursts and falls off, the young surface of the crustaceans quickly hardens due to mineral salts.
It is noteworthy that cancer growth occurs in two stages. Before molting, this is an increase in the number of cells, and after - a set of water in the tissue.
There are also some types of crustaceans that change the size, length of limbs and organs depending on the seasons.
Ecological connection
Further, our characterization of crustaceans will affect lifestyle and distribution.
Scientists compare representatives of this subtype in ponds with insects on land. There are also many types, shapes, sizes, and the amount of their biomass is simply huge.
In the process of evolution, there was a division into the simplest and highest species of crustaceans. Some became parasites, some were able to adapt to life on land and developed specific respiratory organs.
As for parasitic forms, they exist in the most diverse groups of this subtype. The main feature of this part of crustaceans is the maximum simplification of different organs and systems. Since they live off of others, many parts of the body simply atrophy as unnecessary. There are those that parasitize inside various living things, and there are those that attach themselves from the outside.
It is noteworthy that representatives of this subtype can be found everywhere. Small species live in puddles, melting snow, salt marshes. Large crustaceans are found both in depth and on the coast of the sea, lake or river.
Small representatives of this subtype, mainly related to plankton, feed on bacteria and protozoa. Other crustaceans that live at the bottom of water bodies feed on carrion. Pieces of flesh of dead animals from higher layers settle down to them. Amphibians eat dead bodies located on the surface of the water or at a shallow depth.
In addition, crustaceans are an important target. Shrimps, lobsters, lobsters, crabs, crayfish - these are just some of the names of the species that people eat. For example, a sea duck, or bloody fingers, cost up to 150 euros per kilogram on the Iberian Peninsula.
The second use of crustaceans is in the form of feed for fish and birds that are farmed. Aquarists also use them in dried fish food.
The ability of crustaceans to absorb carrion is also used. They are used for the natural cleaning of water bodies from pollution.
Higher crayfish
In biology classes, the Crustacean class is usually studied. Table of their distribution, structure, life cycle. You can easily answer all these questions based on the previous part of the article.
Now let's move on to a more developed group of these creatures. Next we will visit the world of these amazing creatures, get acquainted with its giants and dwarfs. In the meantime, it’s worthwhile to dwell on general information about this class.
Higher cancers include more than thirty-five thousand species of living things. The first representatives of this class appeared in the Cambrian period. And that was about five hundred and forty million years ago. It includes crabs, amphipods, crayfish, wood lice and shrimp. These creatures live in sea and fresh water, as well as on land.
The structure of higher cancers is somewhat different from less developed counterparts. A syncephalon is formed on their head due to the fusion of the first three segments. The forelimbs turn into the jaw to make it easier to get food. In addition, they have a bicameral stomach.
Now let's get to know more closely the individual representatives of this class. Next, you will learn the most common names for crustaceans, habitats, structure and methods of their use by humans.
You will also be presented with a crustacean of gigantic proportions, the scope of the forelimbs, which reaches three and a half meters.
So what is this giant crab?
The largest representatives
The “giant crab” is known in many cultures of the peoples of the world. Today we’ll talk about real, not legendary, representatives of this class.
So, the first giant crustacean in our list is the Tasmanian crab. Individuals of this species reach a weight of thirteen kilograms. Their carapace is up to half a meter wide. He lives on the South Australian shelf, at a depth of one hundred to three hundred meters. It feeds on everything that moves slower than it. Starfish and gastropods form the bulk of the diet.
The next giant is the Kamchatka crab. This is a craboid from hermit crabs. Previously found only in the Far East. But in the seventies of the twentieth century, they managed to resettle him in the Barents Sea. Now these crabs are found off the coast of Norway and Svalbard.
After they took root in a new place, Kamchatka crabs began to multiply rapidly and destroy the local fauna. In addition, they are quite large. The length of the span of the limbs reaches one and a half meters, and the weight of the males is up to eight kilograms. These two factors influenced the fact that these crabs became the subject of fishing. More than twelve thousand tons of them are caught in Russia annually.
But the record holder, which the Crustacean class undoubtedly boasts, is the Japanese crab spider. Its shell, like the previous ones, is about half a meter wide. But the range of limbs reaches three - three and a half meters. The weight of the largest individuals can be up to twenty kilograms. Estimated lifespan is about a century.
This giant is found off the coast of Japan, at a depth of up to three hundred meters. Although they saw individual individuals and eight hundred meters under water.
This type of crustacean is used not only for food, but also for decorative as well as scientific purposes.
Freshwater crayfish
In addition to marine representatives of the class, such as crab, spiny lobster and lobster, a crustacean creature can also live in fresh waters. Among the most common species it is worth noting the following: river wide-toed and American signal cancer.
The first of them was, until recently, very common in almost all water bodies in Europe. But due to the plague of the crayfish and the importation of an American species, it began to quickly disappear.
The size of wide-toed crayfish varies about twenty centimeters. Color is from green to brown and brown with a blue tint. It is noteworthy that they settle only in clean reservoirs. If they are not in the nearest river or lake, then the area is chemically contaminated.
The Crustacean class also has a more adapted appearance. American signal cancer is slightly smaller than its European counterpart, but is distinguished by its survivability and better adaptation.
Crayfish feed on detritus. These are the remains of dead living creatures that have settled on the bottom of the reservoir, as well as their secretions.
American signal cancer is found today in more than twenty-five European countries. In Russia, it is known only in the Kaliningrad region.
The problem of the coexistence of these two species is that American cancers are not susceptible to the fungus that causes the plague of the dog, but they themselves are carriers of another infection. Therefore, with their appearance in the reservoir, the probability of survival of broad-toed cancer in it tends to zero.
Crabs, Lobsters, Shrimps
Marine crustaceans, in contrast to river crustaceans, are more diverse. The most popular in the fishery are crabs, lobsters, lobsters, shrimps and other species.
Now we will talk about them a little more.
Lobster is a crustacean belonging to the family of decapods. In appearance, it is very similar to crayfish. But its forelimbs are much larger, as is the length of the body. The largest lobster was caught off the coast of Scotland. Its weight was 20 kilograms and 150 grams.
Spiny lobsters are similar to lobsters, but their claws are much smaller. The body length reaches sixty centimeters. The meat of these animals is considered a delicacy.
Crabs have a second name - short-tailed crayfish. They live in almost all climatic zones. The main feature of them is that the abdomen is practically combined with the jawbone. And the abdominal limbs serve to carry eggs.
Crabs occupy a fifth of the catch of all anglers who are interested in crustaceans. Examples of this can be found in almost all countries. They are caught by hand, nets, special traps. The sizes of representatives of this class range from babies a few centimeters long to a giant Japanese crab.
Shrimp are also among the decapods. The size of their body ranges from two to thirty centimeters. Like crabs, they occupy a huge part of the seafood market.
