Octopuses have blue blood. To survive in the deep ocean, octopuses evolved a copper rather than iron-based blood called hemocyanin, which turns its blood blue. This copper base is more efficient at transporting oxygen than hemoglobin when water temperature is very low and not much oxygen is around. But this system also causes them to be extremely sensitive to changes in acidity. If the surrounding water’s pH dips too low, octopuses can’t circulate enough oxygen. Accordingly, researchers worry about what will happen to the animals as a result of climate change-induced ocean acidification.