How do copper-containing hemocyanins transport oxygen in some animals?

Hemocyanins are copper-containing respiratory pigments that facilitate oxygen transport in the circulatory fluid of various invertebrates, including many mollusks (such as cephalopods, gastropods, and some bivalves) and arthropods (such as many crustaceans, arachnids, and the horseshoe crab).1

Unlike iron-based pigments, hemocyanins do not contain heme groups. Instead, the copper atoms are bound directly to the protein. These pigments function by binding oxygen reversibly in a specific ratio of one oxygen molecule to two copper atoms.21

When deoxygenated, hemocyanin is colorless; however, it turns blue upon oxygenation, forming a compound known as oxyhemocyanin. By increasing the oxygen-carrying capacity of the blood, these pigments allow active animals to meet higher metabolic demands, while also serving secondary roles such as acting as pH buffers and providing temporary oxygen storage during periods of hypoxia.13