Crab facts to give you claws for thought

Contrary to popular belief, crabs don’t always walk sideways. Although the way most crabs’ legs are hinged means they can move much faster by walking sideways than forward, most species can walk forward if they choose.

The Japanese spider crab, which also holds the title of the largest living crab species, is widely recognised as a crab that tends to walk forwards as its long, spindly legs allows it the range of motion to do so.

Another way to tell true and 'false' crabs apart is their tails. True crabs tuck their tails under their carapace, while false crabs tend to have a visible fanned tail.

In the case of hermit crabs - a type of 'false' crab - they have a long, curled tail or abdomen they use to hold themselves inside the shell. Hermit crabs are actually more closely related to lobsters than crabs.

Crabs have, over time, learnt different ways to get around that don’t rely on walking. Some crabs, like the Atlantic blue crab, whose Latin name means ‘beautiful savoury swimmer’, have developed paddle-like back legs it uses to swim.

Other crabs such as the Columbus crab have created much more extravagant ways of travel, like by hitchhiking on floating debris. Some even catch a ride on the back of sea creatures such loggerhead turtles, where they can travel vast distances and feed on algae and small organisms found on the turtles' shells.

Perhaps the most impressive are crabs such as mangrove tree crabs which have learnt to climb trees!

True crabs evolved around 150 million years ago in the Jurassic period, meaning their ancestors would have seen the rise and fall of the most famous dinosaurs that ever roamed the planet. However, it's unlikely that these giants would have given them much notice due to their small size. Instead, these crabs would most likely have feared being hunted by marine reptiles such as plesiosaurs and ichthyosaurs, or by flying reptiles such as pterosaurs.

Similar to other animals, some crabs have an innate sense of navigation. While this isn’t fully understood, it's thought that it could be due to the presence of magnetite particles in their body acting as a form of compass, or that they can actually see the Earth’s magnetic field due to a specific protein found in their eyes. This, along with their other senses, like vision and chemical detection, help crabs navigate their way around the seas and shore.

This skill is particularly useful to species like the Christmas Island red crabs, which may use it to find their way to the coast during their annual mass migration.

We might think that crabs spend most of their time near beaches, but crabs have actually found many environments to flourish in. The endangered purple land crab, found in the Caribbean, spends most of its life on land but returns to the coast to lay eggs. And the Chinese mitten crab is only found in freshwater rivers.

Some species of crabs have adapted to more difficult environments as well. The Yeti crab, which has distinct hairy pincers, lives in the deep sea near hydrothermal vents, where many creatures could not survive. While the deep, dark depths of the ocean such as the Mariana Trench are still not fully explored, it's thought that more species of crab live there, too.

Many crabs have the ability to regrow lost or damaged limbs, but it takes a long time. Several moulting seasons (when crabs shed their old exoskeletons to allow for a new exoskeleton to be formed) must pass before the new limb can be used, and it's often not as functional as its original.

This ability of crabs is of great interest to regenerative medicine. Although it's unlikely that we'll be able to regenerate our limbs in the same way, the phenomena could hold the key to improving how we heal damaged tissue or treat wounds so they heal more quickly.

As the name suggests, the pea crab is roughly the same size as a pea. They're found in the shells of bivalve molluscs such as oysters and clams, using their hosts as protection and for food the mollusc has filtered - a bit like parasites do. Because they rely on clams and oysters, pea crabs are found anywhere that these bivalve molluscs live, including around the UK coast.

Most crabs are omnivorous scavengers, meaning they'll eat almost anything they can find. Their diets usually consist of algae, plankton and molluscs, but they'll also eat decaying plant material and other organic matter such as dead animals.

Crabs are also an abundant food source for many predators such as birds, cuttlefish and some rays. This makes crabs incredibly important to their environments as they help maintain the population and growth of other species. The presence of crabs can therefore be used as a bioindicator for balanced and thriving ecosystems.

Some crabs also have a mutually beneficial relationship with other species; as its name suggests, the black coral crab lives on coral, helping to clean it while receiving shelter in return.

Unfortunately, threats such as pollution, overfishing, and climate change are affecting crustaceans. This, in turn, is having a major effect on the health of marine habitats that rely on them for stability as well as the delicate balance of the food chain.