The Blue Blood of Horseshoe Crabs

Horseshoe crabs are ancient sea animals that have existed for more than 400 million years. Despite their name, they are not true crabs; they are more closely related to spiders. One of their most unusual features is their blood, which is light blue. Human blood is red because it uses iron to carry oxygen, but horseshoe crab blood uses copper instead.

In the 1950s, scientists discovered something even more remarkable about this blood. It contains special cells that react to certain harmful substances produced by bacteria. When these cells detect the substances, they form a thick clot that traps them. Researchers realized that this reaction could be used as a test. Since the 1970s, medical companies have used a product made from the crabs' blood to check that vaccines, injections, and medical devices are free of these dangerous substances.

To obtain the blood, workers collect crabs from beaches, remove part of their blood, and then return them to the sea. However, a significant number of crabs die after the process, and populations in some areas have declined. As a result, scientists have developed a synthetic version of the test that does not require any blood. Many companies are now switching to it, although others still prefer the traditional test.