Why Do Crabs Walk Sideways? How Crab Legs and Joints Work

How crab legs work

A crab’s body is wide and flat, with its tail folded tightly underneath. Ten legs are attached along the sides of this broad shell: one pair of claws at the front and four pairs of walking legs behind them. Each leg is a stiff tube of shell divided into segments, joined by flexible joints.

Most of these joints work like simple hinges, bending in only one plane. In a typical crab, the big joints in the middle of each leg, the equivalent of our knees, swing the lower leg outward and inward, across the body rather than along it. When a crab extends and flexes its legs, the natural direction of movement is therefore sideways.

To walk forward, a crab would have to swing each leg forward and back at the base, close to the body. That movement exists, but its range is small, and with four walking legs packed closely on each side, the legs soon bump into one another. Walking sideways avoids the problem completely.

Why walking sideways works so well

When a crab moves to the right, the legs on its right side reach out and pull, while the legs on its left side push. The two sides work together, a little like one person pulling a sled while another pushes from behind. The legs move in alternating groups, so the crab always has several feet on the ground and stays stable.

This sideways gait has several advantages:

• Long strides. Each leg can swing through its full range of motion, which makes sideways walking faster than forward walking.
• Easy changes of direction. A crab can switch from going left to going right without turning around.
• Stability. A low, wide body with legs spread to the sides is hard to knock over, a big advantage on wave-washed rocks and beaches.
• Defense. The claws and stalked eyes stay pointed toward a threat while the crab retreats.
• Hiding. A flat crab moving sideways can slip straight into a narrow crack between rocks.

Ghost crabs, the pale crabs of sandy beaches, show how fast this can be. Studies of their running found that they change gait as they speed up. At walking speeds they use all four pairs of walking legs; at higher speeds they lift the last pair off the ground, and at top speed they run on only the first two pairs, sprinting sideways across the sand to their burrows.

Do all crabs walk sideways?

No. Sideways walking is typical of true crabs, but not universal.

• Soldier crabs (Mictyris), small blue crabs of beaches from the Bay of Bengal to Australia, are among the few crabs adapted to walk forward. At low tide, large groups of males march across the sand like armies.
• Spider crabs, with long, thin legs that point in many directions, often walk forward or diagonally. The largest of them, the Japanese spider crab, picks its way slowly over the deep seabed.
• Hermit crabs, which are only distant relatives of true crabs, walk forward while carrying their borrowed shells.

Even ordinary crabs can walk forward or backward when they need to, for example while feeding or backing into a hole. They are simply slower and clumsier going that way.

Swimming crabs: sideways in the water too

Some crabs leave the bottom altogether. Swimming crabs, family Portunidae, have their last pair of legs flattened into oval paddles. The blue crab of the Atlantic coast sculls these paddles rapidly, a little like oars, and swims sideways, just as it walks.

The paddles also help swimming crabs dig. A blue crab can shuffle backward into soft sand or mud until only its eyes and the front of its shell show, a good hiding place from fish and birds.

Escape tricks of a sideways walker

Walking sideways is only one part of a crab’s escape kit. Depending on the species, a crab under attack may:

• Dash sideways to a burrow or into a crack between rocks.
• Turn to face the attacker and spread its claws wide, a threat display that makes it look bigger.
• Bury itself in sand or mud.
• Pinch hard with its claws.
• Drop a leg that has been grabbed and run off without it, then grow a new one later, as explained in can crabs regrow legs?

How the crab shape evolved

The crab body plan, a broad, flat shell with the tail tucked underneath, has evolved several separate times among decapod crustaceans, a process biologists call carcinization. True crabs first appear in rocks from the Early Jurassic, more than 170 million years ago, and other groups such as king crabs and porcelain crabs became crab-shaped independently.

Scientists think this shape is so successful because it protects the soft abdomen, lowers the center of gravity and suits life in crevices and on wave-swept shores. Sideways movement goes hand in hand with it: a short, wide body with legs along the sides is ideal for scuttling to the side. Long-tailed relatives such as lobsters move very differently, as shown in crab vs lobster.