These are Atlantic Jackknife Clams, a kind of Razor Clam, and I found that they actually have 3 ways of escaping danger. First, they are incredibly fast diggers, easily digging their way through the sand to safety. Unlike most bivalves, they are also excellent swimmers, pumping their powerful foot muscle to force water through their shell for "jet propulsion." They can also use that strong foot muscle to "leap", extending it along the outside of the shell, and then flexing it quickly to flip the clam quite a distance.