The ocean’s shell-building animals are most vulnerable to the effects of ocean acidification.3 Many ocean creatures, like snails, clams, oysters, and other mollusks, are equipped to pull dissolved calcium carbonate out of seawater to form protective shells through a process known as calcification. As human-generated carbon dioxide continues to dissolve into the ocean, the amount of calcium carbonate available for these shell-building animals dwindles. When the amount of dissolved calcium carbonate becomes particularly low, the situation becomes significantly worse for these shell-dependent creatures; their shells start to dissolve. Simply put, the ocean becomes so deprived of calcium carbonate that it is driven to take some back.
One of the most well-studied marine calcifiers is the pteropod, a swimming relative of the snail. In some parts of the ocean, pteropod populations can reach over 1,000 individuals in a single square meter.4 These animals live throughout the ocean where they have an important role in the ecosystem as a source of food for larger animals. However, pteropods have protective shells threatened by ocean acidification’s dissolving effect. Aragonite, the form of calcium carbonate pteropods use to form their shells, is approximately 50% more soluble, or dissolvable, than other forms of calcium carbonate, making pteropods particularly susceptible to ocean acidification.5
Some mollusks are equipped with means to hold on to their shells in the face of an acidifying ocean’s dissolving pull. For example, clam-like animals known as brachiopods have been shown to compensate for the ocean’s dissolving effect by creating thicker shells.3 Other shell-building animals, like the common periwinkle and the blue mussel, can adjust the type of calcium carbonate they use to form their shells to prefer a less soluble, more rigid form.6 For the many marine animals that cannot compensate, ocean acidification is expected to lead to thinner, weaker shells.
Unfortunately, even these compensation strategies come at a cost to the animals that have them. To fight against the ocean’s dissolving effect while grasping on to a limited supply of calcium carbonate building blocks, these animals must dedicate more energy to shell-building to survive. As more energy is used for defense, less remains for these animals to perform other essential tasks, like eating and reproducing. While a lot of uncertainty remains around the ultimate effect ocean acidification will have on the ocean’s mollusks, it’s clear the impacts will be devastating.
While crabs also use calcium carbonate to build their shells, the effects of ocean acidification on crab gills may be most important to this animal. Crab gills serve a variety of functions for the animal including the excretion of carbon dioxide produced through breathing. As the surrounding seawater becomes full of excess carbon dioxide from the atmosphere, it becomes more difficult for crabs to add their carbon dioxide to the mix. Instead, crabs accumulate carbon dioxide in their hemolymph, the crab-version of blood, which instead changes the acidity within the crab. Crabs best suited to regulating their internal body chemistry are expected to fare best as the oceans become more acidic.