This crab’s blood doesn’t contain hemoglobin. Instead, they use hemocyanin, which makes it blue. When pathogens or bacteria get in it, the blood clots very quickly. This quality is very valuable in pharmacology for the detection of endotoxins in medical applications. Horseshoe crabs are set free after their blood has been drawn.

The majority of the medications that we use today were tested using horseshoe crabs’ blood.