Crabs are some of the most famous arthropods that people around the world love to eat. Unlike other decipedes (such as shrimp, lobsters, and crayfish), the tails of crabs are curved under the rib cage. Here are some more interesting facts about crabs:
Crabs have a hard and strong exoskeleton
This shell is typical of all representatives of the genus Brachyura. The head and thorax are merged into a wide and flattened cephalic thorax. The shell of crabs is divided by more or less distinct grooves into several sections. One to three anterior thoracic appendages turn into jaw-facial appendages, which are used in feeding.
The circulatory system in large species may be highly developed and widely venous, although still considered open. The nervous system is very centralized. The brain is connected to several ganglia by a paired ventral nerve cord that runs the length of the body.
Crabs have a wide, flattened body and a short, symmetrical abdomen.
Both of these adaptations allow them to squeeze under rocks and in crevices for feeding and shelter. Most of the crab’s body is occupied by the abdominal cavity. Attached to the abdomen is a small head with short eye stems that fit into special sockets on the shell. There are also several pairs of antennae of varying lengths and leg jaws. The first pair of walking legs are larger compared to the rest of the body and end with pinching pincers. They are commonly referred to as chelipeds. In most species, the tips of the other four pairs of legs end in pointed tips.
Some crab species are active predators
Like many other crustaceans, crabs are often omnivorous and behave as scavengers, but many are predators and some are vegetarians. Carnivorous species feed on small fish, mollusks, worms, and other crustaceans. During feeding, they grab the food with their chelipeds, tear it apart and transfer it in small portions to their jaws, and from there it is pushed down to the throat.
Others feed on detritus, scooping up large volumes of silt with their chelipeds like a shovel. They then filter out the food particles and discard the inedible material. Some species of crabs that hide in the soft seabed dig burrows, creating a flow of water and filtering out food particles. Their chelipids are also edged with tiny bristly structures that help extract large materials from the water flow and leave small nutrient particles behind.
Although no crab is a true parasite, some have a similar lifestyle. One example is the tiny soft-bodied spiny crab (Pinnotheridae), which lives in the shells of mussels and many other mollusks, and shares food with its hosts. Many spider crabs (Majidae) cover their shells with growing algae, zoophytes and sponges, which give them a very effective camouflage.
When moving on land or the seafloor, crabs usually move sideways
Walking or crawling is a common way for crabs to move, and the famous askew gait of the common shore crab is characteristic of most members of this group. Crabs can walk in all directions. When walking, the leading legs pull the body forward, while those on the opposite side help push off. Some crab species may use only two or three pairs of legs when walking quickly, stopping occasionally to turn around and change the order of the legs.
Contrary to popular belief, few crabs swim. The floating crab group (family Portunidae) has an oval body shape, and the last pair of walking legs are flattened and act as oars. Examples include the blue crab (Callinectes sapidus), the European green crab (Carcinides maenas), and the chintz crab (Ovalipes ocellatus).
Crabs exhibit several sexual dimorphies
In all species, males have a narrow abdomen, and the first pair of anterior abdominal appendages are converted into reproductive structures designed to aid in sperm delivery. The abdomen of adult females is broadly rounded, its width is almost equal to the width of the ventral surface of the body. Pleopods (abdominal appendages) are used to retain fertilized eggs during the incubation of offspring. Immature females have a wider abdomen than males, but are narrower than mature females. In many species, the claws of males are much larger than those of females. Males use their enlarged claws for display, defense, or combat, as well as for handling food. Often one claw is larger than the other; this can occur in both sexes. The size, shape, ornamentation, and color of the body can make a difference in determining species.
Mating is usually preceded by a short courtship period
Shortly after mating, female crabs lay 1,000 to 2,000 eggs at a time. These remain on the female’s body until larvae emerge.
As with most crustaceans, the young of almost all crabs, having just hatched from their eggs, are very different from their parents. The larval stage, known as the zoea, is a tiny transparent organism with a legless, rounded body that swims and feeds on plankton. After several stages of molting, the enlarging crab transitions to a stage known as the megalopa or postlarva, in which the body and limbs become more crab-like. Further development leads to the immature and mature adult form.
There are a few crabs, especially those that live in freshwater, that do not go through a series of free-living larval stages but instead leave their eggshells as miniature adults.
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Crabs are among the most common marine invertebrates
Crabs are one of the most diverse groups of crustaceans, with about 7,000 described species from 98 families in marine, freshwater, and terrestrial habitats. Representatives of most species prefer the marine environment. Although even terrestrial individuals, of which there are many in tropical countries, usually enter the sea from time to time and undergo their early developmental stages there.
As a rule, crabs breathe with their gills, which are located in a pair of cavities under the shell walls, but true land crabs have enlarged and modified cavities that act as lungs. Removal of nitrogenous wastes probably also occurs through the gills or the body wall itself.
Crabs are also considered the most common introduced and invasive species. Several introduced species are already found in one or more countries in Northern Europe, and several more introduced species may spread from neighboring countries.
Crabs can reach enormous sizes
The Japanese deep-sea spider crab (Macrocheira kaempferi) and the giant Tasmanian crab (Pseudocarcinus gigas) are the two largest crabs in the world. The first can reach a size of up to 4 meters when fully extended. The Tasmanian crab, can weigh more than 9 kg. Its legs are much shorter and stronger; the main claw can be 43 cm long; the body or shell of a very large specimen can reach 46 cm across. In comparison, the world’s smallest crab, the pea crab, grows from 1.02 to 1.52 cm in diameter.
5 facts about crabs
- The annals of crab fossils date back to the Jurassic period, which is 200 million years ago.
- Crab meat is rich in vitamin B12 and omega-3, and 60-85 grams of crab meat is enough to meet an adult’s daily vitamin B12 requirements.
- Crab accounts for over 20% of the marine crustaceans grown and caught. About 1.5 million tons of crabs are sold worldwide each year.
- Kamchatka and horseshoe crabs are false crabs. They come from the order Anomura. They have a very small last set of legs, an elongated abdomen and a tail like shrimp or lobsters.
- Small crabs have an average life span of about 3-4 years, but larger species, such as the giant Japanese spider crab, can live up to 100 years.