10 Curiosities About Crabs

Crabs are decapod crustaceans that have successfully colonized the marine environment. Here are 10 curiosities about these beautiful invertebrates.
10 Curiosities About Crabs
Samuel Sanchez

Written and verified by the biologist Samuel Sanchez.

Last update: 21 December, 2022

Surely you’ve seen these friendly invertebrates with pincers running busily through the sand at a beach several times. Crustaceans are abundant in coastal areas and are food for many species of birds, but they also keep human economies afloat thanks to fishing. Would you like to know some curiosities about crabs? Keep reading!

With their showy claws, the periodic shedding of their exoskeleton, the courtship methods exhibited by some species, and their fascinating life cycle, crabs will never leave you indifferent. Read on if you want to find out more about these fearful, yet fearless aquatic invertebrates.

1. Curiosities about crabs: there are thousands of species

The term “crab” has no taxonomic relevance, as it doesn’t refer to a specific species or group of living beings. When we talk about these arthropods, we’re surely referring to specimens belonging to the Brachyura infraorder, which includes 98 families and 7000 species. As studies indicate, it’s the most diverse group of crustaceans in the world.

The “true crabs” are included in the order Decapoda. For this reason, they’re direct relatives of spider crabs, lobsters, lobsters, crayfish, shrimp, and prawns, among others. Many of the decapod species are of commercial interest and sustain the economy of the world’s fishing regions.

King crabs and porcelain crabs don’t belong to the brachyura group, but they do have a body shape that’s very similar to typical crab species.

2. All crabs have a common body shape

As indicated by their taxonomic designation, crustaceans are decapods, which means that they have 10 legs distributed in 5 pairs. The pair closest to the mouth structure is modified in the form of pincers, which are used to cut up the food, carry out courtship, and as a defense method against possible predators.

Beyond this, crabs are characterized by having a very reduced “tail” (abdomen), gathered under the thorax, which occupies most of their body. In addition, they have a very hard and resistant chitinous exoskeleton, which they must molt from time to time in order to grow.

Some crabs are just a few millimeters long, while others can grow to a staggering 4 meters from leg to leg.

A crab on a sand-type substrate.

3. They have a varied distribution

Most crabs are associated with marine environments, especially coastal, stony areas with abundant sand and organic matter. However, there are about 850 types of crabs that inhabit fresh waters, especially in tropical regions. An exceptional case is the coconut crab (Birgus latro), which doesn’t go near the water again once it becomes an adult.

4. Crabs need to shed their exoskeleton

One of the most famous curiosities about crabs is that they need to change their “armor” from time to time in order to continue their development. This process is called ecdysis and is typical of arthropods in general, whether they’re crustaceans or not. In general, it’s estimated that females molt 18-20 times before they reach sexual maturity, while males molt 21-23 times.

Molting is mediated by hormonal compounds and is the most delicate moment in the crab’s life. Therefore, before the process begins, the decapod usually takes shelter under the ground or between rocks. The exoskeleton “breaks” between the carapace (thorax) and the abdomen, thus allowing the crab to leave with its new, very soft, and delicate armor.

The process can easily take 2-3 hours. Crabs are very delicate right after this process, as their new exoskeleton still isn’t hardened.

5. Crabs have blue blood

These arthropods have blood with a blue hue, but not because of any link to royalty! This is due to hemocyanin, a pigment analogous to hemoglobin that binds to oxygen to transport it through the animal’s body. Unlike vertebrates, these animals don’t have a special cell type for the process and O2 binds directly to hemocyanin.

6. A clear sexual dimorphism

Another of the curiosities about crabs is that they usually show a very clear sexual dimorphism. Males have longer and more pronounced claws, something that is especially evident in fiddler crabs (see the video below), which have exceptionally long claws.

In addition, females have a much more pronounced and larger abdomen than males, with a kind of “fold” attached to the body. This fold or bag is useful for carrying the eggs during the mating season.

7. Crabs are hairy!

When analyzing a crab, you’ve surely noticed that it has a hairy appearance, especially in certain parts of its body. As indicated by the Crab Street Journal portal, these external hairs are called setae. They’re special structures that perform many functions, but are especially useful for detecting chemicals and movements.

8. In its reproduction, quantity matters more than quality

Some species of crabs (such as fiddlers) exhibit intricate courtship routines, while with others the males simply try to search for an available newly molted female. For example, the male green crab (Carcinus maenas) is limited to locating a pregnant female and fertilizing the eggs that she has under her shell, in the abdomen.

A female crab can lay up to 100,000 eggs in a single laying, but the vast majority of larvae won’t reach the adult stage. Newborn larvae are part of the aquatic plankton and serve as food for many filter species.

9. A dangerous migration

Most crabs live in coastal areas, but females must venture out into the water to release the larvae once the eggs have been fertilized. In some tropical areas where terrestrial species exist, the timing of migration to the sea can be very dangerous.

In some regions, up to 30% of a population’s land crabs die each year crossing roads to reach the sea and lay their eggs.

10. Animals of great importance to humans

The last of the curiosities about crabs is that these decapods bring immense benefits to our species, both nutritionally and economically. In general, these Brachyura decapods account for up to 20% of all crustaceans caught worldwide, which is equivalent to 1.5 million tons of food annually.

Due to their availability in the aquatic environment, these animals have been prepared in very different ways depending on the cultures in the kitchen. In general, they’re usually served in the form of soups and stews, or they are consumed cooked directly.

A crab.

So, what do you think of all the curiosities about crabs that we’ve shown you? These wonderful invertebrates stand out for their speed and appearance, but they have many other interesting aspects apart from their large claws and their shell. Respecting them is essential, as their populations suffer enough damage from overfishing and from being run over.

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Types of Crabs: Their Characteristics, Behavior and Reproduction
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Types of Crabs: Their Characteristics, Behavior and Reproduction

Crabs are cosmopolitan decapod crustaceans that have colonized many ecosystems around the world. There are many types of crabs.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Tsang, L. M., Schubart, C. D., Ahyong, S. T., Lai, J. C., Au, E. Y., Chan, T. Y., … & Chu, K. H. (2014). Evolutionary history of true crabs (Crustacea: Decapoda: Brachyura) and the origin of freshwater crabs. Molecular Biology and Evolution, 31(5), 1173-1187.
  • Hidalgo, F. J., Baron, P. J., & Orensanz, J. M. L. (2005). A prediction come true: the green crab invades the Patagonian coast. Biological Invasions, 7(3), 547-552.
  • Porter, H. J. (1960). Zoeal stages of the stone crab, Menippe mercenaria Say. Chesapeake Science, 1(3), 168-177.
  • Lam, S. C. (1969). Abbreviated development of a non‐marine crab, Sesarma (Geosesarma) perracae (Brachyura; Grapsidae), from Singapore. Journal of Zoology, 158(3), 357-370.

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