Despite appearances, that's actually the clam's "foot," a muscly appendage used for surveying, burrowing into, and transporting itself across the sandy beaches of its natural habitat. This particular clam seems to be getting an uncertain feel for its surroundings. As Scripps Institution of Oceanography biologist Miriam Goldstein put it to us, "It's probably trying to find a place to dig. I doubt the salt has anything to do with it."
For reference, below you'll find a video of a clam using its foot to make a speedy, if clumsy, getaway.
So how do clams actually eat? Clams are suspension feeders, meaning they eat by filtering dissolved and suspended food particles through gills located inside their shells. Captured food is transported to the mouth by way of another internal structure known as the palp. Some clam anatomy helps paint a clearer picture of where foot, gills and palp are situated relative to a clam's various other organs (its heart, for example).
Flee, clam, flee! Flee as fast as your shockingly large, vaguely tentacle-pornish foot can carry you!
Bam. Clam anatomy.
[Spotted on kottke]