Although human blood appears in shades of red due to iron atoms within hemoglobin, this color is not universal across the animal kingdom. Various creatures possess vital fluids that are purple, yellow, green, or white, depending on the specific proteins they use to transport oxygen. In many marine animals like octopuses, squid, shrimp, and crabs, a protein called hemocyanin replaces hemoglobin. Hemocyanin contains copper atoms instead of iron, which gives the blood a blue hue when bound to oxygen. While less efficient at binding oxygen than hemoglobin, this protein functions exceptionally well in the cold temperatures of the deep ocean and may also play a role in immunity and wound healing.
Other marine organisms exhibit different colors through distinct mechanisms. Brachiopods and peanut worms bleed violet because they utilize hemerythrin, an iron-rich purple pigment, to shuttle oxygen. Certain sea squirts display a greenish-yellow blood color, potentially caused by the concentration of vanadium or the production of carotenoid pigments from metabolizing marine phytoplankton. On land, several lizard species in New Guinea have bright green blood. These lizards use standard hemoglobin but break it down into biliverdin, a naturally green pigment. Unlike most vertebrates that further metabolize this compound, these lizards accumulate biliverdin to such high levels that its green color overwhelms the red of the hemoglobin.
Perhaps the most striking deviation is found in Antarctic blackfin icefish, the only known vertebrates that do not produce hemoglobin or possess red blood cells. Their blood is a milky, translucent white, leading developmental geneticist John Postlethwait to describe them as "ghostly." Instead of relying on oxygen-binding proteins, these fish depend on oxygen dissolving directly into their bloodstream. To compensate, they have evolved larger hearts and capillaries to circulate greater quantities of blood. As patient ambush predators that expend less energy, they require less oxygen, a strategy supported by the oxygen-rich cold waters of the Southern Ocean. Additionally, these fish possess antifreeze proteins to survive in sub-freezing temperatures.
The diversity of blood also includes adaptations for high-altitude environments where oxygen is scarce. Species such as the deer mouse, Andean hummingbirds, and black-spotted frogs have evolved hemoglobin that forms extra-tight bonds with oxygen to cope with thin air. However, ecophysiologist Zachary Cheviron notes that these adaptations come with tradeoffs; for instance, high-elevation deer mice struggle to release oxygen from their hemoglobin into tissues but compensate by having higher quantities of mitochondria in their muscle fibers. Scientists believe studying these variations could inform human health, offering potential therapies for hypoxia-inducing diseases like chronic obstructive pulmonary disease or new treatments for anemia affecting nearly two billion people.
Sources: Smithsonian Magazine.