i went to the supermarket today and my mom bought a bunch of clams. could i put one of these clams into my saltwater tank?

Are these clams for eating??? If so you could clean a pice of one and put in the tank to kick start the cycle....but you may not need to if you are still planing on getting some live rock....

I believe the clams you get from the supermarket aren't tropical species but come from temperate mudflats... which is another thing that puts them out of favor for placement in an aquarium. Intertidal animals need the daily rise and fall of the tides and partial exposure to air as their adaptational features (such as their filtration method of feeding and "covering up" at low tide) are suited to living by this routine. Also, an aquarium generally lacks the necessary concentration of zooplankton on which majority of clams feed. (Tridacnid clams are photosynthetic animals so at least they get by with proper tank lighting)

Yes, Man Yu...you are right, they wouldn't do well as an addition to the salt water reef tank...but these clams are for eating, already dead I believe...and he is just needing something to kick start his cycle....some organic dead critter to give off ammonia.... Not the best choice...but it will do in a pinch.... Live Rock would be the best choice to start the process right!!!

I thought he was asking about putting live ones in a tank for maintenance

Yep....I figured that is what you were thinking!!!