The blue blood of Horshoe Crabs

While peering into the tanks at the LFS, I usually happen upon a horseshoe crab rooting around in the sand. There are many strange creatures in the sea, yet that smooth, shiny helmet with eyes on top seems especially wierd. Take a look at those faces.

Everything about the horseshoe crab is unusual: its anatomy, history, and lifecycle. Of special note is its blood. First of all, it’s blue. Why is horseshoe crab blood blue?

The oxygen-carrying pigment in horseshoe brab blood is a protein called hemocyanin. It is very similar to the hemoglobinmolecule we have in our blood. Hemoglobin gets it’s red color (which makes our blood red) from the iron molecule in the center of the protein. Hemocyanin contains a copper molecule which results in a blue color.

Blood that uses a hemocyanin carrier isn’t uncommon, actually. Many arthropods and gastropods use hemocyanin. Where the horsehoe crab’s blood distinguishes itself is in its ability to detect bacterial toxins. Horseshoe crab blood clots quite quickly and visibly when in the presence of bacterial endotoxins. This ability has made horeshoe crab blood an invaluable tool in ensuring that medical drugs and equipment are sterile and bacteria free. This blue blood is the basis of an entire industry. That strange looking helmet bulldozing through the sand at the fish store is a medical marvel.