Monday, May 23, 2011

Pacific Mole Crab

Walking along the slash and backwash of La Jolla Shores is an enjoyable activity alone or with someone special. Have you noticed hundreds of little critters moving under your feet while on this walk? These critters are called the Pacific Mole Crab, or the sand crab. Sand crabs make a living in the sandy, intertidal zone of the Pacific Coast. It is an unusual crab because it does not have claws or spines… and it moves backwards!

Sand crabs are found in large patches that are unevenly spaced along the shore. This may be a method of predator avoidance, or for mating advantages. They are found in these patches in very large numbers in every month, except for winter. In the winter, large, forceful waves carry mole crabs offshore with the sand into sandbars. When this sand resurfaces in the spring, so do the sand crabs. They move up and down the beach according to tides and always stay in the swash zone.

Males are smaller than females. Females carry eggs, which are yellow or orange, and can produce one clutch of up to 45,000 eggs per month. It spends most of its time buried in the sand, protruding its primary antennae into a V-shape, allowing water to flow freely between the antennae. This funnels oxygen in the ocean water towards and into the gills. The secondary antenna has cross crossing bristles, looking more or less like a feather, that catch dinoflagellates and other phytoplankton as it flows by. The sand crab then pulls the antennae into its mouth and scrapes off the nutrients.

Scoop up sand where the sand crabs are congregating and feel them burrowing down into your skin. To actually take a good look at one, you may need to free all the sand from your hand; these babies can completely bury themselves into the sand in 1 to 5 seconds, depending on age. Cool, right?

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