Atlantic Mole Crabs

     Except for gulls and little groups of sanderlings, which are a kind of sandpiper, the Atlantic Ocean beaches seem devoid of life.  But there is a species of crab, a kind of crustacean, adapted to living in the forever shifting surf zone where the ocean water constantly slides up sandy beaches and retreats again to the ocean from New York to Mexico-Atlantic mole crabs.  These are the little critters we feel wiggling in the sand with our feet.  These animals don't bite or sting us, but most people don't like the idea of something moving against them unseen and unidentified.  But mole crabs are another example of an animal adapting to a special niche.  Everything about their bodily structure allows them to live in the breaking waves and intertidal part of a sandy, ocean beach.
     If we dug them up, we would notice mole crabs are about an inch and a half long, compactly built, and the color of the sand, which camouflages them.  Their shells, known as exoskeletons, are smooth and tapered at both ends for stream-lining in the sand and water.  Their eight limbs, which are tucked compactly against their bodies, are adapted to swimming, and burrowing into the sand for shelter.    
     Mole crabs get food when wavelets slide up the beaches.  Each crab comes out of the sand, swims backward frantically to another spot and quickly digs tail first and backwards into the sand for protection, leaving two pairs of antennae protruding above the sand.  The first pair of antennae take oxygen from the water.  And the feather-like second pair waves in the water to filter plankton and detritus from it as it  moves back to the ocean.  Those antennae transport that food to the crab's mouth.  All the mole crabs gather food in that way, time after time, most of each day, every day through their life span of two to three years.    
     Mole crabs have natural predators on them, which makes them part of several food chains.  A variety of fish eat some of the swimming mole crabs.  Gulls pull them out of the sand to consume them.  Larger shorebirds, including willets and oyster-catchers also take mole crabs from the sand and ingest them as well. 
     But sanderlings are the kind of sandpiper I like to watch feeding from the beaches as the waves roll in and out.  When a wave comes up a beach, the sanderling group runs ahead of it, their little, black legs twinkling rapidly with the speed of their running.  But the instant the water rolls back down the beach, the sanderlings follow it and snatch invertebrates, including younger mole crabs, from the wet sand before those animals can dig into it.  Except for periods of rest, sanderlings continually feed in that way, all day, every day they are on ocean beaches to get food.
     Female mole crabs carry many, orange eggs under their bodies, but release the young into the water.  Those youngsters travel with the ocean currents, often for long distances, and feed on plankton in the ocean.  After floating in the ocean for about four months, each larva settles on an ocean beach where it matures in the sand under the surf.
     If you're on an ocean beach and feel something wiggling against your feet, don't be alarmed.  It's probably a harmless Atlantic mole crab.  They are interesting to see, and to know of their life history in a niche that is hard to imagine living in.  But they are perfectly adapted in bodily structures and habits for that particular niche.