Most crabs walk sideways but some still prefer to walk forwards, such as the lumbering spider crabs, which use the sharp tips on their legs to climb sheer rock faces, and the hermit crabs, which carry their seashell homes around on their backs.
It is thought that crabs evolved from an ancient crustacean body plan that, like those of shrimps and lobsters, included a large tail. Though this appendage enables a lobster to swim backwards quickly when faced with a predator, it is heavy and cumbersome, especially out of water.
It also means they can only walk forwards, which is not ideal – not only is it slower, but since a lobster’s long legs are arranged in two rows either side of its body, they tend to get in the way of each other.
Through evolution, crab tails have reduced dramatically. Today, they comprise just a tiny flap held against the underside of the body. This allows the animals to walk sideways, which is considerably quicker (the fastest-moving are the ghost crabs, which live on tropical beaches).
Their lightweight bodies also enable them to move quickly out of the water so they can forage on the shore, and some species have even evolved into land crabs.
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