Blood, a fluid often synonymous with the color red, circulates throughout the bodies of many animals, delivering essential substances and removing waste. This familiar reddish hue in humans and most vertebrates stems from a specific protein. However, the animal kingdom showcases a surprising diversity in blood coloration, with some creatures possessing blue, green, purple, or even clear blood. These fascinating deviations from the norm are directly linked to the unique biological adaptations and the chemistry of the oxygen-carrying molecules within their circulatory systems.
Animals with Blue Blood
Several invertebrates are notable for their distinctive blue blood. This includes a variety of arthropods and molluscs, such as horseshoe crabs, octopuses, squids, and some spiders. The blue coloration in these animals is due to the presence of a copper-containing protein called hemocyanin, which transports oxygen throughout their bodies.
Horseshoe crabs possess bright blue blood. Their hemocyanin-rich blood helps them survive. Octopuses and squids also exhibit blue blood, which is particularly effective for oxygen transport in their cold, low-oxygen marine environments. Some spiders and crustaceans like lobsters and crabs similarly utilize hemocyanin, resulting in their blue hemolymph, the invertebrate equivalent of blood.
Green, Purple, and Clear Blood
Beyond blue, other animals display an even wider spectrum of blood colors, including green, purple, and even clear. These variations are also tied to specialized oxygen-transporting pigments or their absence.
Green blood is observed in some specific animals, such as certain skinks from New Guinea and various segmented worms. The green-blooded skinks, belonging to the genus Prasinohaema, have high levels of a green bile pigment called biliverdin in their blood. This pigment is so concentrated that it overrides the red color of their hemoglobin, giving their muscles, bones, and even tongues a lime-green hue. Certain segmented worms, like leeches and some marine worms, have green blood due to chlorocruorin, another iron-containing respiratory pigment.
Purple blood is characteristic of marine worms such as peanut worms (sipunculids) and brachiopods. Their blood contains hemerythrin, an iron-based protein that appears violet-pink or purple when oxygenated.
Animals with clear or transparent blood typically lack significant concentrations of respiratory pigments. Many insects, including grasshoppers and butterflies, have hemolymph that is usually colorless, pale yellow, or green. Their respiratory system relies on a network of tracheal tubes for direct oxygen delivery to tissues, reducing the need for oxygen-carrying pigments in their hemolymph. Antarctic icefish also possess clear blood, an adaptation to their extremely cold, oxygen-rich habitats. These fish do not have hemoglobin or red blood cells, relying instead on the high solubility of oxygen in cold water to supply their bodies.
The Chemistry of Blood Colors
The diverse colors of animal blood are fundamentally determined by the specific respiratory pigments present and the metal ions they contain. These pigments bind to oxygen and change color depending on their oxygenation state.
Hemoglobin, the most common respiratory pigment, is responsible for the red color of blood in humans and most vertebrates. This protein contains iron atoms within a heme group, which bind to oxygen. When oxygenated, hemoglobin appears bright red, while deoxygenated blood is a darker red.
Hemocyanin, found in blue-blooded animals like horseshoe crabs and octopuses, uses copper atoms to bind oxygen. In its deoxygenated state, hemocyanin is colorless, but when it binds to oxygen, the copper ions react, causing the blood to turn blue.
Chlorocruorin, an iron-containing pigment similar in structure to hemoglobin, gives some annelid worms green blood. While it contains iron like hemoglobin, its molecular structure leads to a green color when oxygenated. The green blood in Prasinohaema skinks, however, results from an accumulation of biliverdin, a green bile pigment, which is a breakdown product of hemoglobin that is not fully processed.
Hemerythrin, the pigment responsible for purple blood in certain marine worms, also contains iron. Unlike hemoglobin, hemerythrin binds oxygen directly to its iron atoms. This interaction results in a violet-pink or purple hue when the blood is oxygenated, and it is colorless when deoxygenated. Lastly, clear blood in animals like insects and icefish occurs because they either lack these respiratory pigments entirely or have them in very low concentrations, relying on other mechanisms for oxygen transport.