One surprising variation lies within the fluid that sustains life: blood. While the familiar red hue of human blood is common across many species, a fascinating group of animals circulates blood in shades of blue, revealing nature’s diverse solutions for essential biological functions. This unexpected coloration highlights the incredible adaptability and complexity of life on Earth.
The Common Thread: Invertebrates
The majority of animals possessing blue blood are found within the kingdom of invertebrates. Invertebrates are animals that do not have a backbone or spinal column, a characteristic that distinguishes them from vertebrates like fish, birds, mammals, and reptiles. This group includes a wide range of creatures, from insects and spiders to mollusks and crustaceans.
Their circulatory systems often differ significantly from those of vertebrates, sometimes being “open” systems where blood, or a similar fluid called hemolymph, flows freely within body cavities rather than being confined solely to vessels. This fundamental difference in anatomy allows for a variety of biochemical approaches to oxygen transport, leading to the unique blue coloration.
Why Their Blood Isn’t Red
The blue color of these animals’ blood stems from a different oxygen-carrying protein than the one found in red-blooded creatures. In humans and many other vertebrates, blood is red due to hemoglobin, an iron-based protein that binds to oxygen. Hemoglobin contains iron atoms within a structure that, when oxygenated, reflects red light.
In contrast, animals with blue blood use a copper-based protein called hemocyanin for oxygen transport. Hemocyanin contains two copper atoms that reversibly bind a single oxygen molecule. When hemocyanin binds to oxygen, the copper within its structure causes it to turn blue, or sometimes a blue-greenish tint. When deoxygenated, hemocyanin is typically colorless or a very pale yellow. This copper-based system is particularly effective in cold, low-oxygen environments, which are common habitats for many blue-blooded species.
Meet the Blue-Blooded Animals
Horseshoe crabs, often referred to as “living fossils,” are well-known for their blue blood. Their copper-rich blood is highly valued in the biomedical industry because it contains a substance called Limulus Amebocyte Lysate (LAL), which is used to detect bacterial endotoxins in medical products like vaccines.
Octopuses are another example of blue-blooded animals. These intelligent cephalopods have three hearts, two of which pump blood through their gills and one that circulates oxygenated blood to the rest of their body. The efficiency of hemocyanin in low-oxygen, cold deep-sea environments helps octopuses thrive in their marine habitats.
Squid, closely related to octopuses, also possess blue blood due to hemocyanin. Like octopuses, many squid species also have three hearts, which aids in circulating their hemocyanin-rich blood. This adaptation supports their active, high-energy lifestyles in oceanic depths. Certain snails, particularly marine species, also have blue-tinged blood containing hemocyanin. Crustaceans such as lobsters and crabs also fall into this category, utilizing copper-based blood to survive in various aquatic conditions.