"Donax clams live in the intertidal zone on wave-swept beaches, where the sand is constantly kept wet.  They lie buried just an inch or so from the surface, with the ends of their two siphons extending slightly above the surface.  Waves flush them out of the sand; then they move either up or down with the wash and quickly bury themselves again with their powerful foot."—from Harald A. Rehder in The Audobon Society Field Guide to North American Seashells, Alfred A. Knopt, 1981, p.779