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Shore Scene: Burrowing Bug of the Beach

By Mia Davidson and Jan Sattler

Pacific mole crab

Pacific mole crab

Colored a light sandy gray with an oval shaped body, Pacific mole crabs (Emerita analoga) also called Pacific sand crabs are found in great abundance in Laguna Beach.  They are located in the exposed surf zone between the lowest and highest reach of the waves, buried in the shifting sand.   Depending on tide direction, the Pacific mole crab uses wave action to carry them higher or lower on the beach. Unlike other crabs, their multi-jointed legs are directly under their bodies enabling them to burrow very quickly into the wet sand.  In fact, all swimming, crawling and digging of this crustacean is made backwards.

Pacific mole crabs are invertebrates (animals lacking a backbone) and can reach one to two inches in length.  The female is significantly larger than the male.  Like other crabs, they have an exoskeleton (external skeleton), which they must molt in order to grow.  While buried in the sand, the crabs gather food by extending their feathery antennae into the waves to strain plankton and microscopic detritus suspended in the water.  Another set of projections, called the antennules, have eyes at the end. They live two to three years. They mate in the spring and summer and the female carries thousands of bright orange eggs for about 30 days under her body. Upon hatching, the larvae drift for several months in ocean currents before they return to the beach.

One of the most important invertebrates of the beach, they are considered a keystone species by scientists who use them to monitor coastal contaminants and consider them an indicator of the health of a beach. Predators include shorebirds, certain fish such as corbina and surfperch as well as fishermen and children who eagerly dig them up and deposit them into pails, now illegal in Laguna Beach. Easily damaged, the Pacific mole crab can be found by looking for a quarter inch hole in the wet sand.

 

Mia Davidson and Jan Sattler are residents of Laguna Beach, year-round ocean swimmers and board members of Laguna Ocean Foundation, which is celebrating 10 years of preserving Laguna’s coastal treasures.   For more information, go to www.lagunaoceanfoundation.org

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