When you think of blood, chances are you think of it as being the color red. Animal blood comes in a variety of other colors, however, including yellow, blue, green, and purple. You can trace each to the unique plasma protein molecules that carry essential nutrients throughout their bodies. Different proteins produce different colors, yet it is sometimes only revealed when blood has been exposed to oxygen.
Let’s take a look at each blood color.
Species: Humans, most vertebrates
Humans have higher iron hemoglobin concentrations, making their blood red when oxygenated. You may wonder why blood is red when veins look blue — because blue light doesn’t penetrate the skin deeply, that is the color reflected back to our eyes.
Species: Beetles, sea squirts, sea cucumbers
Blood with a high concentration of vanabin contains the chemical vanadium and turns yellow when oxygenated. While scientists have yet to unravel its mystery and purpose, they have discovered it doesn’t aid in the transport of oxygen throughout the body.
Species: Spiders, octopus, squids, lobsters, crabs
Usually, these creatures have colorless blood. When the copper in their blood protein is exposed to oxygen, however, it turns blue. The protein floats freely in plasma instead of being carried by red blood cells.
Species: Some lizards, most annelids, including marine worms, segmented worms, leeches
These creatures have a unique protein in their blood which gives them light-to-dark-green blood. Their blood is chemically similar to hemoglobin.
Species: Peanut worms, brachiopods
Most marine invertebrates transport oxygen throughout their bodies using a blood protein called hemerythrin. Once exposed to oxygen, hemerythrin turns from purple to fuchsia.
Species: Blackfin icefish
Scientists have located a species of fish living deep under the coastlines of Antarctica that doesn’t need a protein to transport oxygen. These fish can dissolve the required oxygen directly into their plasma. Blackfin icefish have evolved from their relative — the marbled rockcod fish — to withstand deep waters by eliminating their need for red blood cells that can freeze.