I had always understood that D. variabilis "migrated"up and down beaches to
remain in the ideal wash zone.  This requires that the shell remain free of
growths like algae or barnicles. When collecting on one of the Sea islands
in Georgia though I observed D. variabilis that had algae growing on the
posterior end of the shell.  The algae is visible above the sand surface
but it appeared to form a growth too thick to allow the clam to move about.
 It might serve to catch food that is then pulled in by the siphon but I am
guessing.
Has anyone else observed this same sort of algal growth on D. varabilis?
Any ideas as to whether it helps or hurts the clam's survival and how?