Crabs are amazing animals whose lives are full of interesting facts, many of which people do not know

Crabs are extraordinary beings who have been shaped by millions of years of evolution, turning into some of the most fascinating inhabitants of our planet’s waters and shores. Sadly, though, they are among the most exploited crustaceans on Earth. Tens of billions of them are killed every year for flesh eaten by humans, and millions more are now confined in cruel crab farms. Species such as the blue crab (Callinectes sapidus), the snow crab (Chionoecetes opilio), the king crab (Paralithodes camtschaticus and related species), the Japanese blue or gazami crab (Portunus trituberculatus), and the European edible crab (Cancer pagurus) are treated merely as “seafood”, their individuality and sentience entirely ignored. It is disheartening that a being capable of such complexity is viewed as a commodity rather than a person. Let’s set aside this human‑centred view and rediscover who crabs truly are.

1. Their shape has evolved multiple times

Crabs are not all closely related to each other. What we recognise as the “crab shape”, a broad, flattened body with a tucked‑under tail and powerful claws, — has evolved at least five times independently in nature. This process, known as carcinisation, suggests that the crab‑like form provides evolutionary advantages, such as efficient mobility, protection, and adaptability. Even creatures that were not originally crabs, like some lobsters, have “evolved” into crab‑like shapes. Nature seems to appreciate the crab design so much that it keeps reinventing it.

For example, the king crab (Paralithodes camtschaticus and related lithodids) is not a “true crab” at all, but a distant relative of hermit crabs (superfamily Paguroidea) that lost its shell‑carrying habit. Similarly, porcelain crabs (family Porcellanidae), which look like delicate miniature crabs, actually belong to the squat lobster group within the infraorder Anomura. This repeated evolution of the crab form is so striking that scientists now use carcinisation as a textbook example of convergent evolution (nature’s tendency to arrive at the same design again and again when faced with similar challenges).

2. They are legally recognised as sentient beings

Science has finally caught up with what some of us have long known: crabs feel and are capable of suffering as we are. They can experience pain and distress, and they show learning, memory, and preferences when encountering different options in their environment. Because of this, several jurisdictions, including the UK, Switzerland, and Norway, now recognise decapod crustaceans (order Decapoda, which includes crabs, lobsters, and shrimps) as sentient beings in law. This legal recognition means they deserve protection from harm. 

This recognition was largely prompted by growing behavioural evidence. Scientific studies have shown that shore crabs (Carcinus maenas), for instance, learn to avoid painful stimuli — such as mild electric shocks — and remember which shelters are safe. In experiments, these crabs have been observed rubbing or guarding injured parts, just as vertebrates do when hurt. The UK’s Animal Welfare (Sentience) Act 2022 finally accepted these facts, including decapod crustaceans among the species whose well-being must be considered in policy decisions.

3. Some species live in trees

Although most people associate crabs with beaches and the seabed, some species have taken an arboreal path. Tree‑associated crabs — such as the coconut crab (Birgus latro) and certain mangrove crabs — have adapted to life above ground, clinging to trunks and branches, and even stealing fruit or coconuts to eat. The coconut crab, in particular, spends most of its adult life on land, relying on its remarkable climbing abilities and keen sense of smell to find food.

The mangrove tree crab (Aratus pisonii) lives among the mangrove roots and branches of tropical coasts in the Americas. They spend their days basking on leaves and their nights feeding on detritus, algae, and sometimes even insects. Their presence helps control the mangrove ecosystem’s balance, as their leaf clipping accelerates decomposition and nutrient cycling. 

4. They can grow to the size of a person

When we think of crabs, we often picture small, scuttling beings on a rock, but some species can reach astonishing sizes. The Japanese spider crab (Macrocheira kaempferi), for instance, has a leg span that can stretch over 3.5 metres — wider than most humans are tall. Their bodies alone can weigh up to around 20 kilograms. These gentle giants inhabit the deep sea, where they move slowly and gracefully, far from the beaches that humans frequent.

Another spectacular example is the coconut crab, the largest land‑living arthropod on Earth, which can reach a weight of about 4 kilograms and a span of nearly one metre. These huge beings can live for more than 60 years and are known for their strength and curiosity. Unfortunately, their impressive size and slow reproduction make them prime targets for illegal hunting and consumption in several Pacific regions.

5. Their claws are incredibly powerful

A crab’s claws, technically called chelae, are not just for defence or eating; they are their main tools for survival. Certain species, like the coconut crab, have claws strong enough to crack coconuts, while others can deliver pinches powerful enough to break human bones. The strength of their claws makes them exceptional builders, diggers, and defenders, giving them an edge in the evolutionary race for survival.

For instance, the massive claws of the male fiddler crab (Uca and related genera, now often placed in the genus Leptuca and others) are used not only to fight but also to court, waving them in elaborate displays to attract mates. The coconut crab’s grip has been measured at thousands of newtons of force, roughly equivalent to the bite of large terrestrial predators. 

6. They communicate and perform rituals

Crabs are social beings who use a sophisticated range of signals to communicate. Many wave their claws in rhythmic patterns to warn off rivals or attract mates. Some species produce sounds by drumming or rubbing body parts together, a behaviour known as stridulation. These rituals and displays reveal a level of social complexity we rarely consider when we see them trapped in a pot or tank.

Among ghost crabs (genus Ocypode), for example, males stand at the entrance of their burrows, waving and drumming to advertise their homes to passing females. Some species even create “sand chimneys” as markers (impressive little towers built from pellets) as part of their social dances. Hermit crabs (superfamily Paguroidea), including species such as the Caribbean hermit crab (Coenobita clypeatus) or the common marine hermit crabs in genera like Pagurus, engage in unique “shell exchange” negotiations, forming orderly queues to trade shells by size. These behaviours show not just intelligence but also cooperation.

7. They play a vital ecological role

Crabs are essential to many ecosystems. They aerate and mix sediments, recycle nutrients, and even clean beaches by feeding on decaying matter. Mangrove crabs — including species such as Aratus pisonii and various Ucides and Sesarma crabs — help maintain their forests by pruning leaves and controlling pest populations. In coral reefs, their constant foraging prevents algae overgrowth, supporting the health of countless other aquatic species. Without crabs, entire marine systems would collapse.

In seagrass meadows and mudflats, the digging activities of small burrowing crabs like Helice crassa oxygenate the soil and promote biodiversity. In the polar seas, snow crabs (Chionoecetes opilio) recycle organic matter and keep detritus from accumulating, functioning like underwater gardeners. The crab’s ecological service is so profound that scientists sometimes call them “ecosystem engineers”. 

If we recognise that crabs are sentient beings who feel pain, have complex social lives, and play vital ecological roles, the most impactful thing we can do for them is to stop consuming them and their fellow aquatic beings. Choosing to become vegan directly reduces the demand for crab fishing and farming, and it aligns our daily choices with the recognition of their sentience, now enshrined in law in places like the UK. By avoiding crab flesh, crab “stock”, and any products that use crabs or other crustaceans, we withdraw our financial support from industries built on their suffering. You can also support campaigns that call for stronger protections for decapod crustaceans, such as bans on boiling them alive, restrictions on declawing and live dismemberment, and inclusion of crabs in general animal welfare laws rather than only in symbolic “sentience” acts. Organisations working specifically on crustacean protection, as well as broader animal rights and environmental groups, depend on public support, so signing petitions, donating, or volunteering can amplify their impact.