It is quite natural for a healthy clam (or mussel or oyster) to open its shell if it feels relaxed as it rests on a bed of ice or in a bucket of cold water. However, it is unnatural for one of these bivalves not to clamp its twin shells tightly shut if you frighten it - say, by giving it a firm flick with your finger. Closing its shell in response to such an external stimulus is the shellfish's only meaningful defense. Should your threat not evoke the safety mechanisms designed by nature, the bivalve is dead or dying and may therefore be contaminated with pathogenic microorganisms or toxins.
** Asian Recipes