Crab is an invertebrate that belongs to the crustacean family.
There are over 4,500 species of crabs.
They live in all the world’s oceans, in fresh water, and on land. But most crab species are found in the shallower ocean waters where the crabs tend to inhabit rocky pools and coral reefs.
The lifespan of a crab varies according to species. A blue crab lives one to eight years, while a Japanese Spider Crab lives 50 to 100 years. Crab life expectancies also vary according to habitat. Crabs kept as pets can live longer than crabs in the wild when they are properly cared for.
Crabs greatly vary in size.
The pea crab (Pinnotheres pisum) is the smallest known species reaches from 6 to 13 millimeters (0.24 to 0.5 inches).
The largest species is the Japanese Spider Crab (Macrocheira kaempferi) reaching 5.5 meters (18 feet) from claw to claw.
Crabs have a thick external skeleton called an exoskeleton. It is a shell made of calcium carbonate and provides protection for the soft tissue underneath.
Crabs are also known as decapods because they have 10 legs. First pair of legs is modified into claws, called chelae which the crab uses to catch its prey.
Crabs have their eyes on the stalks, which enables them to see around even when they are under water or a rock, or in their burrow. Their eyes are made of hundreds of little lenses.
Most crabs have flat bodies that enable them to squeeze into very narrow crevices.
Crabs typically walk sideways because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs can walk forwards or backwards, and some are capable of swimming.
A crab may lose a claw or leg in a fight. In time (about a year), the claw or leg grows back.
The vast majority of crabs have gills, much like fish, which extract oxygen dissolved in water. Even when they are on land, the gills can still absorb oxygen as long as they are kept moist. There is also a small selection of land crabs that have dual-circulatory systems, meaning they have lungs as well as gills.
Crabs are omnivorous, meaning that they will eat both plants and other animals for sustenance. Some feed primarily on algae, others feed on mollusks, worms, fungi, bacteria and even other crustaceans, such as shrimp or barnacles. In times of desperation, they have been known to eat the offal from the sea floor, which can include dead and decaying animals.
Most crabs have very soft feathery mouths, so they need a way to break up all the food they ingest. Instead of chewing and breaking up food in their mouths, they do this is their stomachs. A crab’s stomach is split into two parts – the first houses three strong teeth to grind up the food they eat, strong enough to grind up hard and gritty material, including shells.
Crabs are mostly active animals with complex behaviour patterns. They can communicate by drumming or waving their pincers.
Some species of crabs are solitary, while other live in the group. Collective name for the group of crabs is “cast”.
Male and female crabs can be distinguished by looking at their abdomens. In most male crabs, the form of the abdomen (pleon) is narrow and triangular, while the females have a broader, rounder abdomen. Additionally, female crabs have smaller claws than male crabs.
Crabs attract a mate through chemical (pheromones), visual, acoustic, or vibratory means.
All species of crabs reproduce by laying eggs, but the females and males still need to mate to fertilize the eggs. When fertilisation has taken place, the eggs are released onto the female’s abdomen, below the tail flap, secured with a sticky material. In this location, they are protected during embryonic development. When development is complete, the female releases the newly hatched larvae into the water. The crab larvae undergo many molting stages in the process of developing into an adult crab.
Crabs cannot grow in a linear fashion like most animals. Because they have a hard outer shell (the exoskeleton) that does not grow, they must shed their shells, a process called molting. Prior to molting, a crab reabsorbs some of the calcium carbonate from the old exoskeleton, then secretes enzymes to separate the old shell from the underlying skin. Then, the epidermis secretes a new, soft, paper-like shell beneath the old one. This process can take several weeks.
Most crabs have many natural enemies, or predators. Birds attack smaller crabs, sometimes carrying them high into the air and then dropping them on boulders to crack their shells. Fish with powerful, shell-crunching jaws also eat crabs. Many kinds of mammals, from seals to raccoons, also look forward to a crab dinner.
Organisms bearing similar names such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice are not true crabs.
Crabs are widely eaten by humans, making up 20 percent of all marine crustaceans that are farmed or caught around the world.
The most consumed species of crab in the world is the Japanese Blue Crab.
The crabs can grow back their claws after losing it. So, in some species, for human consumption their claws are manually squeezed and pulled off returning the live crab in water.
Crabs are often boiled alive. In 2005, Norwegian scientists concluded that crustaceans could not feel pain. However, a study by Bob Elwood and Mirjam Appel of Queens University in Belfast, found that hermit crabs reacted to electric shocks. This may indicate that some crustaceans are able to feel and remember pain.
Crabs are prepared and eaten as a dish in several different ways all over the world. Some species are eaten whole, including the shell, such as soft-shell crab; with other species, just the claws and/or legs are eaten.
Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as a crab.