Haven’t we all spent time at the beach turning over rocks to watch the scurrying tiny shore crabs? Some of us are fearless enough of tiny claws to pluck them quickly up and let them scramble over our palms, before they drop back to the sandy muck or pebbles below, in spite of our best efforts to retain them. In sizes from minuscule to simply small, and in colors from deep purple to drab gray or yellow-green, I’ve wondered how many species and types are represented on our Puget Sound beaches.
During a recent visit to Harper Estuary shore crabs were in abundance, and were not limited to their under-rock hiding places. My companion, my teenage daughter, was as enchanted as a little girl with the responsive crabs, alternately running sideways in search of shelter or raising their claws in defense at her intruding, peering face. I myself had my eye on the creatures underwater, crab-walking back and forth without discernible purpose in the slowly moving tidal channel.
I was unable to determine from observation if there was a seasonal or otherwise exceptional aspect to their behavior. Research at home revealed there was something going on, even if too tiny to see – hatching! From May–June, the eggs of both of our native and common shore crabs, the purple shore crab (Hemigrapsus nudus) and the hairy shore crab (Hemigrapsus oregonensis, also commonly called yellow, green, mud-flat, and Oregon shore crab) are hatching from the eggs glued to underside of the bodies of the female crabs. The tiny larvae float to the surface and spend approximately five weeks as plankton, before settling to the bottom to metamorphose into their first true crab stage, at a truly minuscule size of about 1.6 mm – about the width of the tip of a bold ballpoint pen.
Jacoby CA. 1981. Behavior of the Purple Shore Crab Hemigrapsus nudus Dana, 1851. Journal of Crustacean Biology 1, 531–544.
Oliver J, Schmelter A. 1997. Life History of the native shore crabs Hemigrapsus oregonensis and Hemigrapsus nudus and their distribution, relative abundance and size frequency distribution at four sites in Yaquina Bay, Oregon [Internet]. Corvallis (OR): Oregon State University. [Accessed 2016 May 30]. Available from http://people.oregonstate.edu/~yamadas/crab/ch5.htm.