‘Living fossil’: the blue-blood arthropod that has changed little in 445m years

The mangrove horseshoe crab – one of the species in the horseshoe family Illustration: Petra Korlević/The Guardian

They pre-date dinosaurs, eat food using their legs and contain blue blood that we use to keep us healthy. Horseshoe crabs are the supreme, if offbeat, survivors of the invertebrate world.

Often called “living fossils” for visually obvious reasons – horseshoe crabs resemble a sort of prehistoric helmet (their Japanese name translates as “helmet crab”) – these animals have barely changed appearance along a lineage that stretches back 445m years.