Thin Time for Shelled Creatures: Ocean acidification caused by climate change is making it harder for creatures from clams to sea urchins to grow their shells scientists said today. A thinning of the protective cases of mussels, oysters, lobsters and crabs is likely to disrupt marine food chains by making the creatures more vulnerable to predators, which could reduce human sources of seafood. Human emissions of greenhouse gases include carbon dioxide from burning fossil fuels, and some of that carbon dioxide ends up in the oceans, where it dissolves to form acid. The ocean acidification makes it harder for creatures to extract calcium carbonate - vital to grow skeletons and shells according to the study in the journal Global Change Biology.