Are my pet clams alive?

Yesterday I bought 8 Venus clams from the market, half which I cooked while the other half I kept as pets for my aquarium. They were all tightly shut when they were first put in the saltwater tank, but today their shells are a bit open (by just a crack). I also believe I saw some frilly things attached to the sides of one or two which looked like tiny fins. The water looks fine and I don't smell anything rotting, are my clams alive or dead then?