These living fossils have been around for 400 million years and are related to arachnids. They are sensitive to light. The horseshoe crabs blood is blue due to copper. They are an endangered species because of medical research. I’m fascinated by these creatures that were considered “junk” when I was a kid. Their tails are sensitive.. the crab has a tendency to flop.. its best to gently turn over from the side to get them back on their feet. There is a large migration at Slaughter beach in Delaware every May. I missed this year but plan on going down in 2019 to check off the yolo list. Volunteers are tagging them to try and help. They outlived the dinosaurs.