Unlike the clam, oyster shells are much more aggressive; sharp and jagged, round or long banana shaped with a flat or deep cup and tend to be solid gray, brown, white or green. Some oysters can produce pearls, try and have a clam pull that trick out of its shell. Because oysters spend almost there entire life in one place you will notice barnacles, seaweed and allgie growing on the shells witch you wouldnt typically find on clams. Clams have what is called a foot, this foot is a muscle that gives it the ability to become mobile, the foot also allows itself to dig in the mud and create what is called a bed. The oyster has a foot but only for a few weeks, in its first stages of life, and doesnt use it in the same way as the clam. Instead of walking with it, it uses the foot to swim until it finds a suitable, safe spot to anchor itself and stays there for its entire life. Even though the two shares the same diet plankton, both bivalves and filter feed, the action of the filtering is completely different.
The oyster sucks in about four gallons of water per hour allowing the water to flow over its gills and collects on mucus-covered cilia that the mantle produces to catch the plankton. The clam design uses a tubed siphon, which acts like a snorkel that water is sucked through then passes over the gills, which filter out the plankton also using cilia; clams dont produce mucus, than expels its waste through a discharge siphon. Both convert oxygen into carbon dioxide while filtering the water they consume. The culinary uses for these amazingly critters are just about endless. The clam is pretty striate forward, with a much stronger flavor profile compared to the oyster, which is complex and subtle, and depending on its origin can have flavors of melon, berries or cucumber which you wont find in clams.
Clams and oysters date back hundreds of years, the romans would soak them in wine and feed them pastries yes this is true to fatten them up and consume them as aphrodisiac. Abraham Lincoln would through dinner parties serving nothing but oysters. Native Americans used clamshells for currency; called wampum. The national, Marine and Fisheries in Milford, CT, pioneered the clam farming circa in 1930. Im not sure of how many clams are consumed in the U.S. annually but I do know we consume about one hundred million pounds of oysters, and the clam is preferred over the oyster, so they say. But I am a professional sucker touching thousands of clams and oysters every week for years, and the oyster is preferred over the clam all day, at least in New England.