Here is a
link to an NPR segment on the effect that increasing CO2 in the oceans is having on coral reefs. I have listened and read some other articles on this topic. The carbonic acid will prevent the shells from forming in the first place once it's at a critical concentration.
The most stunning part, in my mind, is the fact that the oceans have been absorbing excess CO2 (above what occurs naturally without human intervention) for a long while and this absorption has softened the apparent effects of our global emissions. As the waters reach saturation, we will see more in the atmosphere and the shell-bearing creatures will still be out of luck.