Oct 26, 2026 09:29 PM
https://knowablemagazine.org/content/art...ent-colors
EXCERPTS: But red-bloodedness isn’t universal in the animal kingdom. In some creatures, the vital fluid is purple, in others it’s yellow, green or even white.
[...] The rich red of vertebrate blood comes from hemoglobin, a protein in red blood cells that contains iron atoms. Hemoglobin’s job is to deliver oxygen, and its iron atoms, which grab onto oxygen, make our blood look red. When iron atoms are attached to oxygen, blood is a bright cherry red; when less oxygen is bound it’s a muddier brown-red.
But many creatures don’t use hemoglobin. Octopus and squid, for example, and crustaceans such as shrimp and crabs, rely on a protein called hemocyanin to ferry oxygen around. Hemocyanin has copper atoms — which look blue when bound to oxygen — instead of hemoglobin’s iron.
Hemocyanin doesn’t bind to oxygen as efficiently as hemoglobin does. But it functions especially well in cold temperatures, such as those found in the deep ocean. And it can have more jobs than carrying oxygen in the creatures that use it — research suggests that hemocyanins can play an immunity role, aiding in healing wounds, for example.
Other marine creatures, such as clam-like brachiopods and the seafloor dwelling peanut worm, bleed violet — but not because of hemocyanin. Instead, these creatures use an iron-rich, purple-colored protein pigment called hemerythrin to shuttle oxygen throughout their blood.
Some sea squirts — squishy, potato-shaped ocean-living animals — bleed greenish yellow. They do not produce hemoglobin, and there’s some debate about what imparts the yellowish hue. One candidate is the element vanadium, which many sea squirts concentrate in their blood. Another theory points to diet. When sea squirts metabolize their staple meal, marine phytoplankton, they create carotenoids, pigments that are yellow, red or orange.
Different mechanisms can make animals bleed full-on green. Several lizard species in New Guinea use hemoglobin to transport oxygen around, like vertebrates typically do. But then they break down the hemoglobin into a naturally green pigment called biliverdin (the same molecule, when present in high concentrations, causes the yellow-green appearance of jaundice in people). And unlike most vertebrates that metabolize biliverdin into other, duller compounds, these New Guinea lizards accumulate the pigment in their blood to the point that its green hue overwhelms the red of hemoglobin in the bloodstream and tissues.
Antarctic blackfin icefish of the family Channichthyidae are the only known vertebrates that don’t make hemoglobin and do not have red blood cells. Their blood is a milky, translucent white. “They really are kind of ghostly,” says Postlethwait.
Instead of hemoglobin or any equivalent oxygen-binding protein, the icefish rely on oxygen dissolving directly into their bloodstream. They also have larger hearts and capillaries compared with other Antarctic fish, and these can pump and circulate greater quantities of blood. Icefish are patient ambush predators — they lie in wait rather than darting around to catch their meals, expending less energy and requiring less oxygen use... (MORE - missing details)
EXCERPTS: But red-bloodedness isn’t universal in the animal kingdom. In some creatures, the vital fluid is purple, in others it’s yellow, green or even white.
[...] The rich red of vertebrate blood comes from hemoglobin, a protein in red blood cells that contains iron atoms. Hemoglobin’s job is to deliver oxygen, and its iron atoms, which grab onto oxygen, make our blood look red. When iron atoms are attached to oxygen, blood is a bright cherry red; when less oxygen is bound it’s a muddier brown-red.
But many creatures don’t use hemoglobin. Octopus and squid, for example, and crustaceans such as shrimp and crabs, rely on a protein called hemocyanin to ferry oxygen around. Hemocyanin has copper atoms — which look blue when bound to oxygen — instead of hemoglobin’s iron.
Hemocyanin doesn’t bind to oxygen as efficiently as hemoglobin does. But it functions especially well in cold temperatures, such as those found in the deep ocean. And it can have more jobs than carrying oxygen in the creatures that use it — research suggests that hemocyanins can play an immunity role, aiding in healing wounds, for example.
Other marine creatures, such as clam-like brachiopods and the seafloor dwelling peanut worm, bleed violet — but not because of hemocyanin. Instead, these creatures use an iron-rich, purple-colored protein pigment called hemerythrin to shuttle oxygen throughout their blood.
Some sea squirts — squishy, potato-shaped ocean-living animals — bleed greenish yellow. They do not produce hemoglobin, and there’s some debate about what imparts the yellowish hue. One candidate is the element vanadium, which many sea squirts concentrate in their blood. Another theory points to diet. When sea squirts metabolize their staple meal, marine phytoplankton, they create carotenoids, pigments that are yellow, red or orange.
Different mechanisms can make animals bleed full-on green. Several lizard species in New Guinea use hemoglobin to transport oxygen around, like vertebrates typically do. But then they break down the hemoglobin into a naturally green pigment called biliverdin (the same molecule, when present in high concentrations, causes the yellow-green appearance of jaundice in people). And unlike most vertebrates that metabolize biliverdin into other, duller compounds, these New Guinea lizards accumulate the pigment in their blood to the point that its green hue overwhelms the red of hemoglobin in the bloodstream and tissues.
Antarctic blackfin icefish of the family Channichthyidae are the only known vertebrates that don’t make hemoglobin and do not have red blood cells. Their blood is a milky, translucent white. “They really are kind of ghostly,” says Postlethwait.
Instead of hemoglobin or any equivalent oxygen-binding protein, the icefish rely on oxygen dissolving directly into their bloodstream. They also have larger hearts and capillaries compared with other Antarctic fish, and these can pump and circulate greater quantities of blood. Icefish are patient ambush predators — they lie in wait rather than darting around to catch their meals, expending less energy and requiring less oxygen use... (MORE - missing details)