Blood, a fluid often associated with the color red, plays a central role in the sustenance of many life forms. Its familiar hue in humans and most vertebrates comes from hemoglobin, a protein responsible for efficiently carrying oxygen throughout the body. This protein’s iron content binds to oxygen, giving blood its characteristic red appearance as it circulates. The effective transport of oxygen is a fundamental process, enabling cellular functions and supporting the metabolic needs of organisms.
The Chemistry of Blue Blood
Some animals possess blood that appears blue, a distinct coloration resulting from a different oxygen-carrying protein known as hemocyanin. Unlike hemoglobin, which relies on iron atoms, hemocyanin incorporates copper atoms to bind oxygen. When hemocyanin is oxygenated, these copper atoms impart a blue tint to the blood. Conversely, when oxygen is released, the deoxygenated hemocyanin becomes colorless.
This copper-based protein is found dissolved directly in the hemolymph, the circulatory fluid, rather than within specialized blood cells. Hemocyanin molecules are larger than hemoglobin, a structural difference contributing to blue blood’s unique properties.
Creatures of Copper Blood
A variety of invertebrates across different groups utilize hemocyanin, leading to their blue-colored blood. Horseshoe crabs, prominent marine arthropods, are examples whose blood flows through an open circulatory system. Octopuses and squids, intelligent marine cephalopods, also possess blue blood and complex circulatory systems with three hearts.
Other creatures with copper-based blood include various mollusks such as snails, clams, and scallops. Many arthropods, beyond horseshoe crabs, also exhibit this characteristic, including lobsters, crabs, and some arachnids like spiders and scorpions.
The Significance of Blue Blood
The presence of hemocyanin offers specific advantages for animals living in certain conditions. Hemocyanin is particularly effective at binding oxygen in cold or low-oxygen environments, such as deep-sea habitats. This allows animals like octopuses and squids to thrive where oxygen might be scarce.
However, hemocyanin has a lower oxygen transport capacity than hemoglobin, making it less efficient for large body sizes or high metabolic rates. Beyond its biological function, horseshoe crab blood has significant applications in medicine. An extract from their blood cells, known as Limulus Amebocyte Lysate (LAL), is used to detect bacterial endotoxins. This LAL test helps ensure the safety of injectable drugs, vaccines, and medical devices by identifying contaminants. Approved by the U.S. Food and Drug Administration in 1977, the LAL test replaced less efficient methods for detecting bacterial contamination.