If being a “blue blood” is a sign of royalty, then horseshoe crabs may be the kings and queens of the sea. In fact, their unique blood is one reason they have survived for more than 450 million years.
Horseshoe crab blood is actually gray-white to pale yellow most of the time because it rarely carries much oxygen. But when oxygenated, horseshoe crab blood is blue because it contains the copper-based compound, hemocyanin, instead of the iron-containing hemoglobin that gives most other creatures red blood.
The remarkable properties of this blue blood protect the horseshoe crab from infections so well that scientists have begun using it for everything from medical sterilization to searches for alien life in space.
The blood contains special cells called amoebocytes and helpful enzymes that together form a powerful immune system. The enzymes recognize invading bacteria and the amoebocytes immediately surround them, coagulating into a miniature fortress that keeps the bacteria from harming the horseshoe crab. These enzymes are so good at recognizing any intruder that they’ve been used for decades by the pharmaceutical and medical industries to test how well medical equipment and intravenous drugs have been sterilized.
More recently, NASA has used this blood to test surfaces for bacteria and fungi, making sure no microbes hitchhike on outbound spacecraft. And because horseshoe crab blood so sensitively responds to microbes, NASA is also preparing the blood for use in the search for life on other planets.
These well-adapted arthropods may not be the biggest or fastest creatures in the sea, but their blue blood “rules” when it comes to fighting off microscopic trespassers.