Pacific mole crabs, also known as sand crabs, make their living just under the surface of the sand, where they're safe from breaking waves and hungry birds. Some very special physics help them dig with astonishing speed.

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Among the surfers and beach-casting anglers, there’s a new visitor to San Francisco’s Ocean Beach shoreline.

Benjamin McInroe is there for only one reason -- to find Pacific mole crabs, a creature commonly known as “sand crabs” -- and the tiny animals whose burrowing causes millions of small bubbles to appear on the beach as the tide comes in and out.

McInroe is a graduate student from UC Berkeley studying biophysics. He wants to know what makes these little creatures so proficient at digging their way through the wet sand.

McInroe hopes that he can one day copy their techniques to build a new generation of digging robots.

-- What are Pacific Mole Crabs?

Pacific mole crabs, also known as sand crabs, are crustaceans, related to shrimp and lobsters. They have four pairs of legs and one pair of specialized legs in the front called uropods that look like paddles for digging in sand. Pacific mole crabs burrow through wet sand and stick their antennae out to catch bits of kelp and other debris kicked up by the breaking waves.

-- What makes those holes in the sand at the beach?

When the waves recede, mole crabs burrow down into the sand to keep from being exposed. They dig tail-first very quickly leaving holes in the wet sand. The holes bubble as water seeps into the holes and the air escapes.

-- What do birds eat in the wet beach sand?

Shore birds like seagulls rush down the beach as the waves recede to catch mole crabs that haven’t burrowed down quickly enough to escape. The birds typically run or fly away as the next wave breaks and rolls in.

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Benjamin McInroe, a Ph.D. student at UC Berkeley, studies how Pacific mole crabs burrow
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Professor Robert Full directs the Poly-PEDAL Lab at UC Berkeley, where researchers study the physics of how animals and use that knowledge to build mechanical systems like robots based on their findings.
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