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Saturday, February 25, 2012

Probing for prey

Sanderlings (Calidris alba) are one of the more familiar shorebirds on outer coast sandy beaches.  They spend the winter here, and then fly to the high-Arctic tundra to nest during the summer.  You'll encounter them running along the edges of the waves, then following the receding water out and probing very rapidly in the sand.

Can you identify the type of prey captured by this Sanderling?

It's a mole crab, or sand crab (Emerita analoga).  Mole crabs are active burrowers in the swash zone.  They have smooth, rounded bodies (up to ~35 mm long) and specialized appendages for rapid digging.  Instead of claws (mole crabs are filter feeders), the terminal segment of the first leg formally called a dactylopodite is a large, oval, paddle-like appendage.  

Their eyes are on long stalks that are sometimes visible above the sand, even when the rest of the crab is below the surface.  Check out the eyes and the digging paddles in the photos below.  (These images were taken at Asilomar Beach in Pacific Grove, but the same species is found on Salmon Creek Beach and Doran Beach).

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