Crabs belong to a group of animals called ‘Decapods’ – Meaning ‘10 legs’. This group also includes lobsters, shrimps, and prawns. Crabs are encased in a hard, protective shell (exoskeleton) which acts like a suit of armour often with spines or teeth. They have a pair of claws which they use to catch, chop and crush prey. The claws are also used to fight or communicate. There are more than 62 species of crab found in British waters and approximately 4,500 known types of crab worldwide.
As crabs grow they moult their hard shell after a new, soft shell has developed underneath. After moulting, the soft shell needs to harden. During this time, crabs are at great risk of getting eaten, but this is also the time when mating happens.Male crabs fertilise eggs held by the female. Once the eggs hatch the first larvae stage form of the life cycle is called the nauplius and is planktonic. As the larvae grows, it changes shape, through several different forms in a complex life cycle before developing into an adult.
It is easy to tell the difference between female and male crabs. If you turn the crab over you will see a flap or tail known as the abdominal flap. Males generally have narrow, thin flaps, whilst females have wider, more rounded flaps used to carry and protect eggs. The female common shore crab carries around 180,000 eggs under her abdomen flap.
- A group of crabs is called a ‘cast’.
- A Crab’s blood is blue due to the copper it contains.
- Crabs are covered in many tiny ‘hairs’ called setae, to detect chemicals, touch and movement.
“How do they move?”
Crabs have ten paired legs of which the front two are usually claws. Swimming crabs such as the flying crab have two flattened, back legs used as paddles for swimming. Many crabs can only walk sideways, but not all of them. In British waters, the common spider crab is one of several types which can walk forwards or backwards.
“What do they eat?”
Most British crabs are omnivorous which means they feed on several types of food. They are usually scavengers, feeding on dead animals and plant matter, but will also graze seaweeds and prey on small animals when they can.
“How do they feed”
Crabs will often use one pincer to crush and hold onto their prey while the other is used to tear small pieces off and move them into their mouths. Some crabs filter feed by using thin ‘hairs’ (called setae), which they waft though the water creating a small water current. Food particles from the water are then filtered and passed into the mouth.
“What is the biggest?”
The largest is the Japanese Spider Crab which can grow up to 3.7 m across (Around the size of a small car!). The smallest crabs are pea crabs, the males of some species do not grow larger than a few millimetres across!
“Where do they live?”
Crabs are found in almost all marine environments around the world from the coast to the deep sea and from polar waters to the tropics. Not all crabs live in the sea, in fact there are around 850 types which live on land or freshwater (mostly in the tropics), returning, sometimes undertaking long migrations to the sea to breed.
Some species commonly found around the British Isles
Common Shore Crab (Carcinus maenas) – One of the most common crabs in British waters, it can be found in most marine habitats and has become an invasive species in many parts of the world.
Edible Crab (Cancer pagurus) – Called the edible crab due to its importance as food for humans, it is a large crab, which can be distinguished from other British species by its pie crust shell.
Velvet Swimming Crab (Necora puber) – A fairly large swimming crab, with bright red eyes, which defends itself aggressively, leading to its other common name the devil crab.
Masked crab (Corystes cassivelaunus) – A crab which lives buried in sand. It makes a ‘snorkel’ from its antennae, allowing it to circulate freshwater when buried.
Angular crab (Goneplax rhomboides) – A strangely shaped crab with very long claws. It lives in burrows beneath the mud.
Chinese mitten crab (Eriocheir sinensis) – An invasive species from Asia, it is now found in many rivers and estuaries around Great Britain and elsewhere in Europe and is spreading fast. It is the only freshwater crab in the British Isles and migrates long distances up and down rivers before and after breeding. If you spot a Chinese Mitten Crab report your sightings here.