Crabs are one of the most common creatures in the rock pools. There are lots of different species, but with a little help they are easy to identify.
Learn more about how to hold a crab and keep it (and you!) safe here.
Ready to identify Your Crab? Just answer these questions:
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Does it have 10 legs?
Yes? Go to step 2. All ‘true’ crabs have 10 legs (four pairs of walking legs plus a pair of pincers). They do often lose and re-grow limbs so look carefully.
No? If your crab has 8 legs with none missing (three pairs of walking legs and one pair of pincers) it is a Porcelain crab (a close relative of the ‘true’ crabs). There are two species: the Broad-clawed porcelain crab and the Long-clawed porcelain crab. They’re only a few centimetres across and cling to the underside of rocks.
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Does it have flattened back legs and red eyes?
Yes? You have a Velvet swimming crab. These crabs are feisty and don’t hesitate to use their pincers. If you can get close enough, the shell feels soft and velvety and has up to ten very sharp teeth down each side and more between the eyes. There are other species of swimming crabs, which also have flattened back legs, but without the red eyes.
No? Keep going.
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Does it have one claw bigger than the other and bristly hair all over?
Yes? You have a Hairy crab. These aren’t very common but are easily recognised.
No? It’s something else – keep reading…
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Does its shell have five pointy ‘teeth’ down the side and three bumps between the eyes?
Yes? This sounds like a Green shore crab. This is the species of crab you are likely to catch with crab nets and is very adaptable, living in estuaries and on most areas of the shore. Young crabs are hugely variable in colour so the shell shape is the best guide to identification. Mature adults have a green tinge to their shell.
No? If your shell has a different shape read on…
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Is the shell oval and crimped like a pasty?
Yes? You have an Edible crab. These crabs have green eyes and strong claws, but tend to be gentle. The name is unfortunate as these crabs are no more edible than any other and they need many years to grow to maturity, so please take care to return them safely to where you found them.
No? Keep going…
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Does your crab’s shell have five blunt teeth/bumps down the side and a wide flat section (with a small notch in the middle) between the eyes?
Yes? You have a Xantho’s crab. There are two species, so put your crab in water and look at the back legs.
If the back legs of your crab are densely fringed with hairs you have a Xantho pilipes (pilipes means ‘hairy legged’). These crabs vary a lot in colour, often having a marbled appearance.
If the back legs aren’t hairy, you have a Xantho hydrophilus. These crabs are sometimes called ‘Furrowed crabs’ because their shells tend to have a furrowed look.
No? Keep reading…
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Does your crab have a spiny shell?
Yes? It’s a Spider crab. These crabs have long spindly legs giving them their name.
Large spider crabs come close inshore to breed in the early summer and are often found on the shore.
There are other, much smaller species that can be seen on shore all year round the Decorator crabs. These often adorn themselves with seaweed or sponges, making them very hard to identify on the shore.
No? Go to 8 for some ideas.
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If you haven’t identified your crab yet it might be one of these…
- Is it living inside the shell of a sea snail? You have a hermit crab. In the most common species (Pagurus bernhardus) the right claw is bigger than the left. There are several other less common species including the St Piran’s Crab.
- Does it look like a mini-lobster with long claws stretched out to the front? This is likely to be a squat lobster. Lobsters can also occasionally be found on the shore.
- Does it have two long antennae sticking straight up from the top of its head? This is a Masked crab. They hide in the sand with just their antennae sticking up to catch prey.
- Does your crab have a small, heart-shaped shell with a sandy pattern? This is probably a Pennant’s swimming crab (Portumnus latipes). It has flattened back legs like the Velvet swimming crab.
Still not sure? There are other species not covered here. Take a photo if possible or send me a description, I’d love to help.
Discover the secret world of crabs and other rock pool creatures with my book Rock Pool: Extraordinary Encounters Between the Tides.
Have your own beach adventures and see the underwater world as never before with my hands-on children’s activity book, Beach Explorer: 50 Things to See and Discover.
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Cornish Rock Pools 