A horseshoe crab’s blood is blue, while an Antarctic icefish’s is so pale it looks almost white. The colors may seem strange, but they point to different ways animals move oxygen through their bodies—and, in some cases, to lives shaped by cold seas or unusual chemistry.
Human blood gets its familiar red color from hemoglobin, an oxygen-carrying protein in red blood cells. Hemoglobin contains iron: oxygen-bound blood looks bright red, while blood carrying less oxygen appears a darker, muddier red. Across the animal kingdom, however, other molecules can do the job, and their chemistry gives blood a very different look.
Horseshoe crabs, along with octopuses, squid, shrimp and other crustaceans, use hemocyanin. Instead of iron, this protein contains copper, which turns blue when it binds to oxygen. Hemocyanin does not carry oxygen as efficiently as hemoglobin, but it works especially well in cold conditions, including those of the deep ocean. Researchers also think it may help with other tasks, such as immune defense and wound healing.
Purple blood has a different source. Brachiopods, clam-like marine animals, and peanut worms that live on the seafloor use hemerythrin, an iron-rich pigment that carries oxygen and gives their blood its violet color.
Some sea squirts have greenish-yellow blood, though scientists have not settled on why. These soft, potato-shaped ocean animals can concentrate vanadium in their blood, making it one possible source of the color. Another idea links the hue to carotenoids, yellow, orange and red pigments produced as the animals digest marine phytoplankton.
Several lizard species in New Guinea take a more striking route to green. They use hemoglobin to carry oxygen, then break it down into biliverdin, a green pigment. Most vertebrates process biliverdin into other compounds. These lizards instead let it build up in their blood and tissues until the green overwhelms hemoglobin’s red.
The most unusual case may be Antarctic blackfin icefish. They are the only known vertebrates that make no hemoglobin and have no red blood cells. Their blood is milky and translucent. “They really are kind of ghostly,” says John Postlethwait, a developmental geneticist at the University of Oregon.
Icefish do not replace hemoglobin with another oxygen-carrying protein. Oxygen dissolves directly into their blood, which their larger hearts and capillaries circulate in greater quantities than in other Antarctic fish. Their patient, ambush-style hunting also means they use less energy—and need less oxygen—than fish that dart after prey.
Why icefish lost the genes for making hemoglobin remains unclear. Postlethwait and colleagues have studied the genomes of several fish species, but it is still a mystery whether the fish lost their red blood cells or their hemoglobin first. One possibility is that the cold, oxygen-rich waters of the Southern Ocean made hemoglobin less necessary. The colors of animal blood, then, are more than a visual surprise: they are clues to how different species meet the same basic need to deliver oxygen.