This group of animals is ideally suited to the intertidal zone as they are able to extract oxygen from both air and seawater. Crabs have five pairs of legs with the front pair modified into claws. They tend to be more active at night when torchlight will reveal them scuttling over the rocky shore. Crabs moult several times during their lifetime. A crab will grow until it reaches a certain size and cannot grow further because of its rigid shell. By this time the crab will have grown a new but very soft shell. It sucks in water to expand its body, cracking apart the old shell. The crab then wriggles out of the old shell, leaving it behind, together with the external coverings of the legs and gills. The new shell is still very soft so the crab has to hide itself from predators until it hardens.
If you want to hold a crab without getting nipped, first stop it from moving by pushing gently down on the shell. You can then pick it up by putting a thumb and forefinger on either side of the shell.
You can see many types of crab at Trevaunance:
- Edible crab: Smaller specimens (juveniles) live in the rock pools. This crab isn’t aggressive but has big claws. The claws of large specimens can crush walnut shells! Fishermen catch edible crabs using baited traps. The edible crab is also called the ‘pasty crab’ as its rounded shell resembles a pasty with crimped pastry along the edge.
The edible crab is also called the ‘pasty crab’ as its rounded shell resembles a pasty with crimped pastry along the edge.
- Shore crab: You’re very likely to spot this common crab, which is green-ish in colour but can also be orange, brown or red. Shore crabs are often quite small and are non-aggressive. They have three rounded lobes between the eyes.
Shore crabs yield little meat due to their small size but they are eaten in Portugal, Italy and Spain.
- Montagu’s crab: This non-aggressive crab looks chunky and the males have massive claws. They were first spotted in Cornish waters about 25 years ago, having shifted northwards from Spain and Portugal.
Montagu’s crabs have furrowed grooves between their eyes. [Photo by Matt Slater]
- Velvet swimming crab: These red-eyed crabs are really quite aggressive so need careful handling. When disturbed they will throw their claws up into the air and their pinch can be painful. Their hindmost legs are flattened paddles, which make them good swimmers.
The shell is covered in short soft hairs with a velvet-like texture.
- Spider crab: You will sometimes find small specimens in the rock pools (the bigger ones are pot-caught further out to sea and are prized for their edible flesh). They have a red spiny shell.
Spider crabs often decorate themselves with seaweed, using their legs to attach living seaweed to hooked bristles on their shells. This acts as a camouflage. When they moult, they carefully transfer these decorations from the old shell to the new one.
- Common hermit crab: These small crabs have soft reddish bodies but cannot grow their own shells. This makes them vulnerable to predators so they curl themselves up inside empty shells once occupied by whelks and periwinkles.
When it grows too large, a hermit crab will seek out another empty shell. Hermit crabs sometimes fight for possession of an empty shell. [Photo by Matt Slater]
- St Piran’s hermit crab: This crab disappeared from UK waters in the 1980s but has recently made a comeback. They, too, make use of empty periwinkle and whelk shells. They have stunning red-and-blue colouration.
St Piran’s crabs are more cautious than the common hermit crab and will wait in their shells until danger has gone. [Photo by Matt Slater]
