oysters alive when eaten?

Hi, I've looked and looked and can't find any information as to whether
oysters are alive when they are eaten.

I know that if they die naturally their shells open up, but that doesn't
necessarily mean that when you pry their shells open they instantly die.
AFAIK it's just a muscle that hold it closed.

Does anybody know this information -- namely how long it takes them to die
after they've been shucked?

Thanks!