​7 lesser-known animals that have colourful blood and why

When we think of blood, the colour red usually comes to mind. That’s because in most animals, blood gets its red colour from hemoglobin, an iron-rich protein that carries oxygen. But not all creatures follow that rule. Some animals have evolved with blood that appears blue, green, purple, or even milky white. These unique colours are caused by different oxygen-binding molecules or other compounds in their circulatory systems.

These unusual blood colours aren’t just eye-catching, but they have their special biological purposes. Some help animals survive extreme environments, others help in camouflage, and some are simply a result of ancient evolutionary paths.

If one could ever imagine whether animals can bleed anything but red, the answer is yes. Here are seven animals whose blood is colorful and not red!

Octopuses have blue blood because of a copper-based molecule called hemocyanin, which helps transport oxygen. Unlike red-blooded creatures, which depend upon iron-rich hemoglobin, hemocyanin is more efficient in cold, low-oxygen environments that is also perfect for the deep sea. The blue tint is most visible when oxygen is bound to the copper. It also supports their survival in harsh underwater conditions.

Icefishes, which are native to the frigid waters of Antarctica, have clear or milky-white blood. They’re the only known vertebrates that don’t use hemoglobin, giving their blood an almost colourless appearance. Instead, they rely on oxygen dissolved directly in their plasma and have larger hearts and blood vessels to compensate. This special trait helps them to survive in extremely cold, oxygen-rich waters. Their unique circulatory system is one of the strangest adaptations in the animal kingdom.

Peanut worms, or sipunculids, are marine worms with deep purple blood. This colour comes from hemerythrin, an iron-based oxygen-binding molecule different from both hemoglobin and hemocyanin. Hemerythrin gives the blood its purple hue when oxygenated. These soft-bodied invertebrates live in ocean mud or sand and are rarely seen. Their purple blood may also play a role in immunity, making it even more valuable to their survival.

Some leeches have green blood due to the presence of a molecule called chlorocruorin, another iron-based protein similar to hemoglobin but with a special green tint when oxygenated. Chlorocruorin helps transport oxygen in some species of segmented worms, including leeches. This works well in low-oxygen environments, like muddy water.

Sea cucumbers possess blood containing hemocyanin, similar to octopuses and other mollusks, though their exact blood colour can vary. It often appears bluish or greenish, depending on the oxygenation level and species. These bottom-dwelling echinoderms use their strange circulatory system to live in low-oxygen environments deep underwater. Their blood chemistry, combined with their strange appearance and defense mechanisms, makes sea cucumbers among the ocean’s most bizarre and resilient animals.

Spiders don't have "blood" in the same way that humans do. They have a fluid called hemolymph, which is a pale blue color due to the presence of hemocyanin, a copper-based protein that carries oxygen. Some spider species, like tarantulas, are known to have this blue hemolymph.This copper-based molecule helps them survive in environments with varying oxygen levels. The blue colour becomes most pronounced when the blood is oxygenated

Some species of skinks, or lizards native to New Guinea, have bright green blood, muscles, and bones. This bright colour is caused by extremely high levels of biliverdin, a toxic bile pigment that would be harmful to most animals. Strangely, these skinks seem immune to its effects. Scientists believe the green colour might protect them from parasites. Their unusual blood chemistry is still being studied and offers insight into how animals can evolve resistance to substances usually considered harmful.