Horseshoe crabs have existed in essentially the same form for the past 135 million years. Their blood is blue in colour and provides a valuable test that protect us humans from the toxins that cause septic shocks.
Unlike mammals, horseshoe crabs do not have haemoglobin in their blood, but instead use haemocyanin to carry oxygen. This protein is rich in copper which tints the blood blue.
Their blood also contains amebocytes, which play a role similar to white blood cells for vertebrates in defending the organism against toxic bacteria. This reaction is valuable to the medical industry, as it is the most effective and least expensive way of approving any drug, product or device from containing endotoxins that could threaten the immune system of any living organism.
After this was discovered, a special test was developed called Limus amebocyte lysate (LAL) which collects blood from living Horseshoe Crabs that are released after donating a small but not dangerous amount of their blood to the medical detection of bacterial endotoxins.