Hemoglobin is a key ingredient in the circulatory systems of nearly all vertebrate animals. Yet many spineless creatures use an alternative protein: Hemocyanin.
Both are capable of binding to and transporting oxygen. But whereas hemoglobin contains iron atoms, hemocyanin incorporates copper. As a result, blood containing the latter protein looks markedly different from our human blood. When hemocyanin-rich blood becomes oxygenated, the copper turns it blue.
The list of invertebrates that rely on hemocyanin instead of hemoglobin is a long one. Crustaceans utilize the protein in their bloodstreams, as do spiders and scorpions. The roster also includes certain mollusks like everyone's favorite multi-armed brainiacs, the octopuses. Yes friends, octopuses — or if you prefer, "octopi" — have bluish blood. To make things even stranger, they've got three hearts with which to pump this liquid.
In oxygen-poor deep-sea environments, hemocyanin is better than hemoglobin at carrying precious oxygen through an animal's veins. Octopuses use the copper-laden protein to stay alive in some deep, cold and thoroughly anoxic waters. Additionally, hemocyanin helps the tentacled critters regulate the salt content of their blood so it matches that of the water they're swimming in.
The setup is not without its drawbacks. Octopuses have a hard time adapting to fluctuations in water acidity. Scientists use the pH scale to determine how basic or acidic a given water sample is. Research has shown that even a small change in the local pH level can weaken the ability of hemocyanin to bind with oxygen in octopus bloodstreams. The consequences may be fatal.