Blood is typically red due to hemoglobin, but some animals have distinctly colored blood. Certain species stand out for circulating green blood, a rare biological phenomenon. This unusual coloration in their circulatory systems points to unique biochemical adaptations and evolutionary paths.
Animals Known for Green Blood
The Prasinohaema skinks, a genus of lizards from New Guinea rainforests, are striking examples of green-blooded animals. These small, agile reptiles exhibit vivid green blood, muscles, bones, and even tongues. Their skin typically ranges from dark brown to black, contrasting sharply with their internal greenness.
Other animal groups also display green blood, often for different biochemical reasons. Certain marine worms, particularly polychaetes, are known to have green blood. Some leeches, a type of segmented worm, also exhibit green blood. These diverse creatures highlight that green blood, though uncommon, has evolved independently in different lineages.
The Science Behind Green Blood
The distinctive green color in Prasinohaema skink blood results from extremely high concentrations of biliverdin, a green bile pigment. Biliverdin is a byproduct of hemoglobin breakdown, typically converted to bilirubin and excreted. In these skinks, biliverdin accumulates in the blood plasma at levels toxic to most other vertebrates, reaching concentrations over 40 times higher than in humans with severe jaundice. This intense green pigment overwhelms the red color normally imparted by hemoglobin.
While biliverdin causes the green hue, skinks still use hemoglobin in their red blood cells for oxygen transport, much like humans. The green pigment dissolves in the plasma, not as part of the oxygen-carrying molecule. Some marine worms use chlorocruorin, a respiratory pigment similar to hemoglobin, which appears green when deoxygenated and light red when oxygenated. This demonstrates different biochemical mechanisms can lead to green blood in various species.
Function and Evolutionary Significance
The extreme accumulation of biliverdin in Prasinohaema skinks raises questions about its purpose, given its toxicity to most other animals. One theory suggests high biliverdin levels may protect against malaria parasites. Studies show biliverdin can inhibit Plasmodium parasites, which cause malaria and are common in the skinks’ tropical environment. This could represent a unique resistance to a widespread pathogen.
Another hypothesis suggests biliverdin acts as an antioxidant, helping to neutralize harmful free radicals. This detoxifying property could be beneficial in managing metabolic stress or environmental toxins. It is also possible that green blood is simply a metabolic byproduct with no direct adaptive function, and the skinks have evolved a remarkable tolerance to its high concentrations. The precise evolutionary pressures that led to this unique trait are still subjects of ongoing scientific investigation.