From my reading yesterday, they say eggshells are not enough calcium. If you're worried about the mercury, then I would try and find the limestone.
I believe they hunt critters for their tasty meat. Hehe probably not much else going thru their head.
Not sure really about the mercury, and hadn't heard of that being a big problem. Where'd you read that? Not sure the mercury would transfer to the shells as much as much as the oyster itself.