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Why do some mollusks and crustaceans have blue blood?

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A periodic table element box for copper shows its symbol Cu, atomic number 29, atomic weight 63.546, electron configuration, and name. Icons indicate copper is a solid, weakly basic transition metal with face-centered cubic crystal structure. A key explains the symbols and color coding. — copper Properties of copper.

Some mollusks and crustaceans have blue blood because they utilize a copper-containing respiratory pigment called hemocyanin to transport oxygen throughout their bodies.12

Unlike the iron-based hemoglobin found in vertebrates, which appears red when oxygenated, hemocyanin is colorless when deoxygenated but turns blue when it binds with oxygen. In this process, oxygen combines reversibly with the protein in a ratio of one oxygen molecule to two copper atoms, forming oxyhemocyanin. These pigments are essential for active animals that require a higher oxygen-carrying capacity than plasma alone can provide.123