These prehistoric creatures aren’t crabs at all, but arthropods related to scorpions and spiders. They’ve lived on earth for 450 million years (which means they existed 200 million years before dinosaurs), have blue blood and 10 eyes. What they don’t have is an immune system. A clotting agent in the blood called coagulogen protects them from harmful bacteria and is used to test contamination in the human world, preventing risk of infection and septic shock. In Thailand, horseshoe crab eggs are a delicacy and usually incorporated into salad. Eggs have the appearance of flat lentils and vary in shades of green, yellow and orange. They’re firm and crunchy with a subtle saltiness, but are prized for their texture. You’ll find horseshoe crab roe on menus at seafood restaurants, especially those attached to fish markets, but it’s best to avoid preparing them yourself – parts of it are poisonous.