These crab facts provide information about the importance of crabs in the marine ecosystem.
Crabs are highly adaptable and able to survive in various environments, including fresh and saltwater. For example, true crabs have a solid exoskeleton to defend against predators and unique sideways locomotion. By learning more crab facts, we can understand these creatures better.
Related read: Check out the various types of jellyfish!
15 Compelling Crab Facts
1. There are over 4,500 crab species in the world.
The crab family is categorized under the Decapoda order, specific to a group called brachyurans. Interestingly, "false crabs" is a phrase used to identify species that resemble crabs. However, they do not belong to the true crab classification.
Over 4,500 known species of crabs are on the planet, inhabiting beaches, the ocean's cold depths, freshwater rivers, and even dry land. They have various names and sizes, from king crabs, porcelain crabs, blue crabs, and horseshoe crabs. For instance, the pea crab is tiny, while the Japanese spider crab has 12-foot-long legs.
Moreover, male and female crabs also come in different sizes. However, don't confuse crab lice with real crabs, as the former is a parasite that lives on humans.
Interestingly, the horseshoe crab has hardly changed in over 445 million years and is among the Earth's oldest creatures.
2. Crabs can live everywhere, even on land.
Crabs live in various habitats, from sandy beaches to the ocean's depths. Crab species are diverse, too; some inhabit the Arctic's icy waters, and others, like the blue crab, dwell in tropical regions.
Besides the water, several species of land-dwelling crabs, such as the Coconut Crab, also exist. This unique creature breathes air through specialized gills that retain moisture. Moreover, the Coconut Crab only ventures into the ocean to lay eggs.
Another terrestrial crab is the Christmas Island Red Crab, which prefers forested areas and only migrates to the sea for reproduction.
3. Crabs walk sideways.
Walking sideways trait is a survival strategy that has evolved for male and female crabs over the centuries. Since crabs have a specialized leg arrangement and a jointed exoskeleton, moving horizontally is easier.
Moreover, sideways walking gives crabs the advantage in various terrains; they can easily navigate rugged surfaces to outmaneuver predators. Additionally, this walking style gives them a broader field of view, making it easier to detect threats and locate food.
Crabs continue to walk sideways even in empty environments since this behavior is instinctual and hardwired into their system. For example, some crab species swim sideways.
Fun fact: Astoundingly, the Japanese blue crab has been known to march up to 150 kilometers during its yearly migration - that's no small journey for these tenacious creatures!
4. Crabs are curious creatures.
Most crabs are naturally curious beings. Most divers meet a handful of these curious crustaceans underwater. One example of a curious crab is the Fiddler Crab, which uses its curiosity to explore new territories and meet female counterparts. Fiddler crabs wave their claws to attract a female crab and intimidate rivals. This wave helps the crab survive, enabling it to protect its territory and reproduce.
Meanwhile, the Hermit Crab continually searches for new homes in the sea. Moreover, they don't choose any available option but patiently evaluate their possible new homes. This behavior shows that these crabs are curious about new environments and are not interested in simply outgrowing their current shell.
5. Crabs communicate with their claws.
Crabs use their claws to convey messages through flicking, drumming, or snapping. For example, male crabs tend to warn intruders, attract female crabs, or assert themselves against other crabs through these claw movements1.
Other instances include crabs extending their claws to look bigger. However, certain species, such as the Ghost Crab, rely on audio signals. They tap their claws against their shells to produce sounds that deter predators.
6. Crabs are masters of disguise.
Crabs blend in with their surroundings–the ocean floor, coral reefs, and beaches–through physical and behavioral adaptations. For example, some crabs' textured or colored shells mimic the terrain, while others use nearby materials to camouflage themselves.
The Decorator Crab is a prime example of a crab using active camouflage. In disguise, they attach seaweed, sponges, and other organisms to their shells. Meanwhile, Ghost Crabs can change color, becoming more transparent at night to blend in with low-light conditions.
The beautiful yet elusive colorful crab, the Sally Lightfoot, uses its vibrant shades of red, orange, and yellow to camouflage amongst the coral reefs and rocky shores.
Stone Crabs also have rough, craggy shells that turn them nearly invisible against rocky seabeds and coral reefs. While the ingeniously camouflaged mud crab can remain expertly concealed in muddy terrains, evading predators.
7. Crabs can fit into tight spaces.
Crabs can squeeze through extremely narrow spaces, helping them survive the ever-changing environments of the sea and shore. Their flat bodies help them achieve this feat, combining natural design and evolutionary adaptation.
Instinctively, crabs can sense danger and retreat into their burrows, an integral part of their daily life. The burrows are in unexpected places, such as under some sea anemones or rocks, built out of the cracks and crevices on the sea floor. Crabs dig out dens in other terrains and create secure homes wherever they are.
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8. Crabs shed their shells
Molting is a critical process in a crab's life cycle that allows for growth. When crabs molt, they shed their hard exoskeleton to reveal a softer shell underneath.
During this time, the crab's body is able to expand before the new shell hardens. This practical and essential process is a direct adaptation to living in an environment where growth would otherwise be restricted by a rigid external skeleton.
9. Crabs eat anything, even other crabs.
While crabs are omnivores, they tend to favor algae, which supplies them with essential nutrients. The specific type of algae consumed by crabs can vary depending on their environment. Besides algae, crabs feed on clams, oysters, mussels, bacteria, other crustaceans, and fungi.
Curiously, crabs exercise cannibalism, particularly during molting phases when the shells of their kin are still soft and more palatable.
Read more: What do Crab Eat?
10. Many crabs have teeth in their stomach.
Besides a shield-like exoskeleton and antennae, many crab species have teeth in their stomachs, forming part of a "gastric mill." Well-known species like hermit, blue, and brown crabs all have these teeth.
These teeth are made of chitin, a material similar to their exoskeleton. After shredding food with claws, the crab's gastric mill reduces the food to a smooth consistency, turning it into an easy-to-digest paste.
11. Crabs can live for 100 years.
What causes crabs to live for 100 years, surpassing many fish species? Some species grow up extremely slowly, like the Red King Crab. Moreover, they live in the deep sea with few natural predators, so they practically have all the time in the world to grow. Crabs in cold water also age slowly, thanks to their slow metabolism.
Another reason for their longevity is that crabs can regenerate lost limbs, allowing them to recover from otherwise fatal injuries.
12. Hermit crabs swap shells.
The hermit crab's life involves constantly changing from one shell to another since they don't create their shells. So, they must leave old shells behind and find a larger one. Usually, they pick old shells discarded by sea snails3.
However, Hermit Crabs must choose the shell with the right weight, shape, and size for their bodies, including the shell's opening. Most importantly, the shell must have enough room for them and be light enough to carry around.
13. The Japanese spider crab is the biggest species
The Giant Japanese Spider Crab (Macrocheira kaempferi) is Earth's biggest crab, whose legs stretch up to 12 feet long from claw to claw2. This length is equivalent to the height of a single-story building.
Japanese spider crabs can also carry up to 44 pounds despite looking frail. It can move quickly across the rocky seafloor, 490 to 980 feet below sea level. Moreover, it can go as low as 2,000 feet below the surface.
14. The pea crab is the smallest crab species.
The Pea Crab (Pinnotheres pisum) is the tiniest in the world, measuring around a centimeter wide. Thanks to its peculiar lifestyle, it can survive in the ocean's vastness thanks to its peculiar lifestyle.
Pea Crabs are parasitic crabs that live inside the shells of mussels and oysters, giving them protection and reliable food. Since their size prevents them from hunting or foraging, they seek out a host creature to avoid danger and enjoy a constant food supply. Pea Crabs live in both the Atlantic and Pacific Oceans.
15. Many species face threats.
Human activities threaten the survival of many crabs, such as King Crabs, Horseshoe Crabs, and Blue Crabs. For example, coastal development, deforestation, and land-use changes destroy the mangroves, seagrass beds, and coral reefs, where crabs forage, shelter, and reproduce.
Besides habitat loss, pollution threatens crabs. Plastic waste, heavy metals, and harmful chemicals dumped into rivers, lakes, and oceans kill crabs. Even harmless plastic bags can kill them. Likewise, oil spills and chemical exposure make crab habitats uninhabitable.
Finally, overfishing is also helping reduce crab populations around the world. Commercial fishers unintentionally catch them; crabbers hunt them for crab meat. These factors are ruining their habitats, making survival increasingly difficult.
We hope you enjoyed this list of fascinating facts about crabs!
How, M. J., Hemmi, J. M., Zeil, J., & Peters, R. (2008). Claw waving display changes with receiver distance in fiddler crabs, Uca perplexa. Animal Behaviour, 75(3), 1015-1022.
Hartnoll, R. G. (1982). Growth, sexual maturity and reproductive output. In D. E. Bliss & L. H. Mantel (Eds.), The biology of Crustacea: Embryology, morphology, and genetics (Vol. 2, pp. 101–128). Academic Press.
Hazlett, B. A. (1981). The Behavioral Ecology of Hermit Crabs. Annual Review of Ecology and Systematics, 12(1), 1-22.