crab (Brachyura) belongs to crustaceans. There are more than 6.700 known species that are divided into 93 different groups. They are found in waters around the world, both in salt water, fresh water or on land. They are usually covered by a thick exoskeleton and have a pair of forceps.

There are many other species that have a similar name but are not true crabs such as the hermit crab, king crab, porcelain crab, horseshoe crab, and crab louse.

Crab on the beach

Crab on the beach


Generally, the crab is not more than 40 cm in length with exceptions such as the Japanese spider crab that has a leg that is more than 4 meters long. The smallest is the pea crab which is only a few millimeters long.

They are covered by an exoskeleton composed of highly mineralized chitin that protects the crab from immediate danger. They also have a pair of front claws to catch their prey or fight with other males during mating season.

To hunt they hide in a dark hole and wait until a potential prey swims past them. This allows them to feed effectively and stay hidden from predators.

Differences between males and females

Crabs present some differences between males and females. Normally, males possess larger claws, especially notable in the fiddler crab species of the genus Uca (Ocypodidae) where males possess a claw larger than another that they use for communication and defense.

Another difference is found in the abdomen, which in most males is narrow and triangular in shape, while females have a wider and rounded abdomen to favor breeding.


Crabs are known to walk sideways, because the lateral configuration of their legs forces them to walk that way to be more efficient. However, there are some species capable of walking forwards and backwards, such as the Raninidae, Libinia emarginata and Mictyris platycheles. Species such as the Portunidae and the Matutidae are able to swim by flattening their legs, turning them into a kind of oars.

They communicate by playing drums or waving their claws and are quite aggressive towards each other, very often fighting to mate with females. They also fight for the best holes, especially the fiddler crab that burrows in the ground (sand or mud) to rest, hide, mate and protect themselves from predators.

During the mating stage the crabs cooperate to provide food and protection for their family. They also help to find a suitable place to find a good place to release the eggs.

Large claw crabs

Large claw crabs


Most of the species inhabit the waters of the shallow oceans where they find refuge among rocky pools and coral reefs. Although there are other species that are found in fresh water such as lagos o rivers and they spend a lot of time on the land and even climb trees or palm trees like the coconut crab, but they will always stay close to the coast at a maximum distance of 5 km.


The crab is a omnivorous animal that feeds on algae and other invertebrates such as mollusks, worms, crustaceans, fungi, bacteria and detritus. Although the diet varies depending on the availability of the food and the species.

For most crabs, mixed vegetation provides them with rapid growth and great fitness, but other species are stricter in their diets consuming plankton, shellfish such as clams, and even catching small fish.


Due to their size, crabs have a high number of predators. They are normally hunted by marine animals such as octopuses, dogfish, otters, sea turtles and some species of crabs.

When it comes to land crabs, birds are their main enemy such as seagulls, eagles and hawks, they can even be eaten by cats and dogs. During the molting of the shell, being soft they are easy prey for fish such as drumfish or sharks.

However, the most voracious predator is humans who are hunted for fishing nets and large quantities to sell in the market.

Bird hunting a crab

Bird hunting a crab


The crab attracts its mate using different visual, acoustic, vibratory and chemical methods such as phoromonas. The aquatic crab will use pheromones and the terrestrial or semi-terrestrial crabs use visual methods, such as the fiddler crab that uses its large claw to attract females. But the vast majority of crabs mate belly to belly.

Females can store sperm for a long time before fertilizing their eggs. With the fertilization carried out, the eggs are released into the abdomen (below the flap of the tail) secured with a sticky material. Females that carry eggs are called "berries" because of the similarity of the eggs to berries.

When the development is complete, the female releases the larvae, already hatched, into the sea where they will form part of the plankton. The larvae are known as zoea and in some species they are wrapped in yolk with which they will feed while rubbing through the sea, others will have to look for food.

The crab must go through several zoeal stages separated by molts before it reaches the megalopa stage which is similar to an adult crab but with a tail behind. When you reach this stage, you have the most important stage where you will stop floating to move to firm ground.

Land crab

In land crabs the reproduction stage happens in the same way, with the difference that it must go to the ocean to release its larvae, in some cases the crabs must travel long distances. After living for a time in the ocean, the juveniles must do the same stage as their parents, in reverse, towards land.

In many cases, long migrations end in death because they often have to cross roads.

The molting of the crab

The juvenile crab will need to keep molting to become an adult crab. Being covered by a hard shell to grow, you need to get rid of it so that it does not impede growth.

When molting begins, the old shell softens and begins to partially deteriorate, while underneath a new rudimentary shell begins to form. During the molt, the crab will take a lot of water to expand to open the old shell along the back edge of the shell.

This is where a difficult part begins, as you will have to remove the old peel from all of your limbs, mouthparts, eyes, and even the lining of the front and back of the digestive tract. It will take you many hours and if you get it wrong getting caught up in the process you will die.

After breaking free from the old shell the crab is extremely soft and must hide until its new shell has hardened. While the shell is still soft, it can be expanded for future growth.

State of conservation

The crab is part of many species, but in general, it is threatened due to hunting by humans and the deterioration of the reefs where they live or the construction of roads near the coast, in the case of land crabs. . However, the state of conservation of the crab can vary according to the species.

Popular culture

The crab appears in the constellation Cancer and the astrological sign Cancer which is represented as a crab. In 1848, William Parsons (3rd Earl of Rosse) drew the Crangejo Nebula and noted its similitude with the animal, the Crab Pulsar is located in the center of the nebula.

In various cultures the crab has been worshiped, such as the Moche people of ancient Peru who represented them in their art. In Greek mythology, Karkinos It was a crab that came to the aid of the Hydra Lernaica in their fight against Heracles.

In literature, we can find «The Crab that Played with the Sea"(The crab that played with the sea) by Rudyard Kipling that tells the story of a giant crab that was capable of rising and falling waters, like tides.

In animated films like «The little Mermaid»One of the characters that accompany the protagonist, Ariel, it's the crab Sebastian who is a composer of the royal court.

Image of the crab Sebastian

Image of the crab Sebastian

We also have another very famous crab, called Eugene H. Crab also know as Mister crab in Spain or Mr. Crab in Latin America (Eugene H. Krabs) Of sponge Bob (SpongeBob SquarePants), a Nickelodeon series released on July 17, 1999.

He is the owner of the Krusty Krab (In Spain), or Krabby Crustacean (In Latin America) his personality is quite rough and he has only one real interest in life: money. He's SpongeBob's employer and Calamardo. SpongeBob's best friend is a starfish named Patrick.

Mr. Krabs, or Mr. Krabs, is a character from SpongeBob SquarePants.

Mr. Krabs, or Mr. Krabs, is a character from SpongeBob SquarePants.

List of other interesting animals