The bright red color of human blood, a familiar sight across the animal kingdom, often leads to the idea that all blood is fundamentally the same. Blood’s primary function is to deliver oxygen from the respiratory organs to the body’s tissues, relying on specialized molecules for transport. In most vertebrates, the protein hemoglobin utilizes iron atoms to bind oxygen, which is the source of the ubiquitous red hue. However, nature has evolved alternative solutions for oxygen transport, and the color of blood changes when a different metal is employed as the oxygen carrier.
Who Has Blue Blood
The animals that possess blue blood are primarily found within two major invertebrate phyla: Mollusca and Arthropoda. These creatures, many of which live in marine or low-oxygen environments, rely on this unique blood to circulate oxygen effectively throughout their bodies.
Examples from these phyla include:
- Cephalopods (octopuses, squid, and cuttlefish)
- Snails and bivalves (clams and scallops)
- Arachnids (spiders and scorpions)
- Crustaceans (lobsters and crabs), including the horseshoe crab.
Hemocyanin: The Copper Core
The substance responsible for the striking blue color is a respiratory pigment known as hemocyanin. Unlike iron-containing hemoglobin, hemocyanin uses two copper atoms at its core to bind a single oxygen molecule. These copper atoms are suspended directly in the hemolymph, the invertebrate equivalent of blood, rather than being contained within cells.
The color change results from the chemical reaction during oxygen transport. In its deoxygenated state, the copper is colorless (cuprous ion form, Cu(I)), making the hemolymph appear clear or a pale gray-yellow. When oxygen is taken up, the copper is oxidized to the blue cupric ion form, Cu(II), causing the hemolymph to turn distinct blue.
Iron vs. Copper: Efficiency and Environment
The difference in metal core translates directly to differences in oxygen transport efficiency and suitability for certain environments. Hemocyanin is generally less efficient at transporting oxygen than hemoglobin under normal terrestrial conditions.
However, hemocyanin offers a distinct advantage in specific habitats, particularly those characterized by cold temperatures and low oxygen pressure, such as the deep ocean. The structure of hemocyanin allows it to bind and release oxygen more effectively in these challenging marine conditions. For creatures like deep-sea octopuses, this copper-based system provides a superior mechanism for survival where oxygen is scarce and temperatures are low.
The molecule’s affinity for oxygen is highly sensitive to changes in temperature and pH, which aids adaptation to fluctuations in the surrounding water. Another element is that while hemoglobin is packed into red blood cells, hemocyanin is often found freely dissolved in the hemolymph. This feature may aid in circulation through narrow vessels in these invertebrates.