On the half shell

I was reading this article when I came upon an interesting saying which I can not find on this site... "Ironically, the best dietary source of zinc - oysters - is also a source of cadmium. But you also get vitamin D and omega-3 fatty acids from oysters, so you probably come out on the plus side with a dozen on the half shell."

Anyone have an idea what that means?

Oysters are a bivalve which means their shells have two parts. Oysters are often served "on the half shell" which means using one of the two halves of the shell as a little dish.