Clams that do not open their mouths cannot be eaten. Not opening the mouth means that the clam is dead. Shrimp, crab and shellfish marine organisms, because the water as a medium leads to the body surface and the body of many microorganisms, alive, the body of the metabolism of the strong can inhibit the growth and reproduction of these organisms. And after its death, these microorganisms will rapidly multiply and produce many harmful substances, so can not eat.