The shells fall open, but usually only after the flesh has mostly rotten away (meaning they've already dumped a ton of
ammonia into your tank).
If you can get them to live, they're pretty neat. They filter extra stuff out of the water, generally keeping the tanks cleaner.
There is some concern that the young larvae can harm fish in an enclosed space, because they attach to the gills. A particularly large spawning could potentially clog the fishes' gills.
They need a very well established tank, so that there's lots of stuff in the water column for them to eat.
Lastly, I would not put wild clams into my tank. Because they filter garbage out of the water, they have the potential to be very polluted (including truly toxic waste like mercury if the water source is so polluted), and can dump this stuff into your tank when introduced or when they die.