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.