What Makes Sand Crabs Such Expert Diggers?

Pacific mole crabs, or sand crabs, live in the 'swash' zone and have perfected the art of digging. . KQED Science

If you watch a wave leave the shore of a West Coast beach, you'll notice dozens if not hundred of bubbles coming up from the sand. This is air escaping the burrows of Pacific mole crabs, commonly called sand crabs.

They're some of the most extraordinary diggers on the shore — and they have to be. It's a dig-or-die world for them as they dodge foraging sea birds and follow the tide line up and down the beach to stay just at the edge of every wave that washes up.

Practice makes perfect for these little inch-long crabs, but what makes them such lightning fast diggers? U.C. Berkeley Ph.D. student Benjamin McInroe took a closer look at their physiology to find out. Turns out, they create a sandy situation similar to what happens in an earthquake, called liquefaction.

KQED reports:

[M]ole crabs dig too quickly for the human eye to observe their special techniques. So McInroe brings specimens back to Berkeley to test them in a laboratory ... It turns out that mole crabs actually dig backward, using their pointy rumps to push past the sand grains. They vigorously beat the wet sand with their tails, whipping it into a semi-liquid state."They make the sand into a slurry," McInroe said. Then the mole crabs hand up the grains toward the surface, using their legs. A pair of modified legs at the front look like paddles. They’re called uropods and they do a great job of moving sand. "A real-world example of that is during an earthquake, when the sand is vibrated around a building foundation," he said. "It can cause the building to sink."

Except in this situation, it's the sand crab that sinks. Amazing, right? Check out the little diggers in action and spot the unique digging technique:

Why all the interest? Well, they might provide the biomimicry inspiration needed for robots. According to KQED, McInroe studies biophysics and "hopes that he can one day copy their techniques to build a new generation of digging robots."

His robots would be able to burrow to find out about underground conditions, from stability for foundations to soil conditions for farming.

Though these little grey crabs may look like nothing more than pebbles, their unique skills are inspiring new technology!