There’s a shift happening in the way scientists are thinking about how ocean acidification affects marine creatures. Originally, when researchers in the Southern Ocean watched the shells of tiny marine snails dissolve in high-carbon dioxide water, they suspected that similar animals with calcium carbonate shells and skeletons would most likely be harmed by ocean acidification. After all, this made intuitive sense: Ocean acidification means there is more carbon dioxide in the water, which lowers the water’s pH. All of this decreases the amount of carbonate ions in the ocean—the chemical building blocks found in animals’ shells. Wouldn’t decreases in these building blocks rob animals of the very things they need to build their shells?
Ocean acidification biological research has looked at this “building blocks” hypothesis for a while. Many excellent studies have shown that time after time, decreases in seawater carbonate ion levels are associated with decreases in shell building by corals, plankton, oysters, and more. But that clear relationship doesn’t hold for crabs and lobsters, even though they too have calcium carbonate in their shells. And different shell formers respond to different degrees of change. What’s going on?