Shellfish as Food and Medicine Types of Mollusks Mollusks have adapted to an amazing range of environments. Clams and snails can be found living on mountains, in lakes and ponds, marshes, bays and estuaries, along sandy seashores, floating on the sea surface and living at the bottom of the sea, near hot springs. There are clams that burrow into wood and limestone rock, that live attached to the undersides of sea urchins and the gills of fish. Snails can be found living in high trees, in the intestines of sea puddings, and within the arms of starfish.
Seashells occur in a great range of shapes, colors and sizes.
One of the smallest seashells is the Pythina clam, a tiny, smooth translucent clam the size of a rice grain, that lives attached to the underside of shrimp and crayfish. This monster's shell consists of two attached valves which are four foot long and weigh 500 pounds. Mollusks first made their appearance 500 million years ago, during the Cambrian period. 100 million years later, the six classes of mollusks living today could be found in the fossil record. They have one shell, which is often coiled.