In the pictures, we have a hermit crab, a crab, and a nautilus.
All of these organisms have shells, even if the hermit crab doesn't make its own.
All the shells of these organisms are made of calcium carbonate, although the shell of the crab is also composed of chitin.
Calcium carbonate can be diluted in acid, so the acidification of water makes the shells weaker and can make these organisms more vulnerable to damage and predators, especially in the cases of mollusks.
