Animals have a variety of blood colors based on the type of hemoglobin they contain. Hemoglobin is the protein in red blood cells that carries oxygen throughout the body. While most animals have red blood from the iron-rich hemoglobin, some animals have blood that appears a different color due to different hemoglobin structures or scavenged metals that affect hemoglobin colors.
Mollusks
Certain mollusks like snails, clams, and octopuses have blue, purple, or colorless blood. Their blood gets its hue from hemocyanin, a copper-rich protein, rather than the iron-rich hemoglobin found in most other animals. When oxygenated, hemocyanin turns a blue or purple color instead of the red hemoglobin turns.
| Animal | Blood Color |
|---|---|
| Octopus | Blue |
| Snails | Blue, purple, or colorless |
| Clams | Colorless, blue, or purple |
Octopuses, which are mollusks, have blue blood because their hemocyanin contains one copper atom per unit. When the copper binds to oxygen, it turns blue. Other mollusks like clams and snails can have colorless, blue, or purple blood depending on the number of copper atoms in their hemocyanin and whether it is carrying oxygen.
Horseshoe crabs
Though they are more closely related to spiders than crabs, horseshoe crabs also have blue blood like mollusks. Their blood contains hemocyanin with two copper atoms per unit, which turns their blood a distinct blue color when oxygenated.
| Animal | Blood Color |
|---|---|
| Horseshoe crab | Blue |
Horseshoe crab blood is valued in the biomedical industry because it contains amebocytes, blood cells that are very sensitive to bacteria endotoxins. Scientists use horseshoe crab blood to test medical equipment and vaccines for contamination.
Green blood
While it’s less common than blue blood from hemocyanin, some lizards and other reptiles have green blood. The green color comes from elevated levels of the toxic pigment biliverdin. High biliverdin levels help camouflage some lizards in the green forests and jungles they inhabit.
Examples of lizards with green blood include:
- Prasinohaema – “Green blood” in Greek
- Several New Guinea skinks
- Some species of chameleon
These lizards tend to have higher biliverdin levels than hemoglobin levels. Researchers think the biliverdin may suppress parasites, in addition to assisting with camouflage.
Purple blood
Very few animals have purple blood. Sea cucumbers and some marine worms contain the protein hemerythrin instead of hemoglobin. When oxygenated, hemerythrin turns a purple or pink hue. Scientists are still studying the evolutionary benefits of hemerythrin over hemoglobin in these deep sea creatures.
| Animal | Blood Color |
|---|---|
| Sea cucumber | Purple |
| Some marine worms | Purple or pink |
Purple blood is relatively rare compared to blue and green blood in the animal kingdom. Only a handful of sea worm and sea cucumber species contain the hemerythrin protein responsible for the distinctive hue.
Yellow, orange, and red blood
While most vertebrate animals have red blood, some fish and amphibians have yellow, orange, or even bright red blood. Fish like salmon and frogs contain carotenoid proteins in their blood for oxygen transport. Carotenoids contain antioxidant properties and enhance vision and immunity.
The carotenoid proteins bond with fats in the blood cells turning the blood yellow, orange, or red depending on the species. Some example animals with carotenoid-rich blood include:
- Rainbow trout – Yellow or orange blood
- Salmons – Bright red blood
- Some species of frog – Yellow or orange blood
Carotenoids are the same pigments that give carrots and autumn leaves their vibrant orange hues. Animals with carotenoid proteins tend to be brightly colored themselves for camouflage or mating purposes.
Colorless blood
While blood is never completely clear, some animals have colorless or pale colored blood when compared to the bright reds, blues, greens, and purples seen in other species. Animals with colorless or very pale blood include:
- Insects
- Crustaceans like shrimp or crayfish
- Some fish like eels and icefish
- Some worms and leeches
Instead of hemoglobin or carotenoids, these animals use plasma freely dissolved oxygen. A lack of respiratory pigments make their blood appear colorless or pale yellow. However, the blood is still vital for delivering nutrients around their bodies.
Conclusion
While red blood is the most common, animals have evolved a rainbow of blood hues thanks to different respiratory proteins and pigments: octopuses and horseshoe crabs have blue blood plasma, lizards have green blood, sea cucumbers have purple blood, and some fish and frogs have yellow, orange or red blood. The reason behind the different colors depends on the proteins and molecules used to carry oxygen through the body in each species. This variety shows the incredible diversity and adaptability of life on Earth.
