A crab is a crustacean and arthropod animal that belongs to the order of the decapods. To know its main characteristics, as can be seen, it is necessary to define several terms.
An arthropod is an invertebrate being whose body presents bilateral symmetry, is made up of several segments, has appendages with articulated pieces and has a cuticle as a covering. In this group we can find crustaceans: animals that breathe through gills and have appendages, antennae and a calcified shell. The decapod, on the other hand, are crustaceans that have ten legs.
Crabs, therefore, are ten-legged animals with a calcified carapace, antennae, and appendages that develop gill respiration. Thanks to their clamps they can fight against other animals and manipulate food.
There are about four thousand species of crabs: regardless of the differences, they all share the peculiarity of living in or near water. Crabs, however, are not characterized by their ability to swim, but they tend to walk along the bottom of the body of water.
According to DigoPaul, the crab is an edible animal. One of the most popular dishes is crab soup, which can be made with the whole body or with the meat of the crustacean.
The hermit crabs are very popular, thanks to the fact that use snail shells to cover her abdomen, consisting of a tissue less rigid than other crabs, and so protect it. Its scientific name is paguroid, and it is a superfamily (the Paguroidea) of crustaceans that have ten legs.
The kind of relationship that the hermit crab maintains with the shells of other mollusks is called thanatocresis and there are not many species that practice it. It is important to note that the crab does not kill to get the shell, but takes it from the carcasses. Around the world there are approximately 500 species of paguroid and almost all of them are aquatic.
The front of the hermit crab exhibits a rigid exoskeleton, which is why it only needs to increase the protection of its rear. To facilitate the task of getting inside a foreign shell, its abdomen is curled in a spiral and its extremities cover the gaps to prevent other animals from sneaking in.
As expected, the hermit crab must change shells more than once in its life, as a consequence of its natural growth. The selection process is meticulous: he does not settle for just any house, but he approaches the ones that interest him and inspects them carefully using his tweezers until he finds the one he likes best; At that moment, he proceeds to carry out the move as quickly as possible, since every second outside is in great danger.
Since not having a shell to protect its abdomen is practically surrendering itself to certain death, the hermit crab cannot allow another to snatch a shell from it if it is the only one it has found; for this reason, it is very common for him to face his opponents by force.
With regard to food, the hermit crab is primarily a scavenger, although it also feeds on plants, larvae, small crustaceans, mussels, snails, and worms if needed.
The concept of crab is used in other ways as well. In the nautical world, a perch is called a crab which, with a semicircular mouth at one end, adjusts to the pole to rotate around it.
In armor, the crab is the piece made of sheets to protect those sectors of the body that need to flex.