How does the blood of a 450-million-year-old arthropod help prevent lethal infections in humans? And could we exhaust the supply?  Zachary Crockett wades in. SOURCE:Dina Fine Maron, senior wildlife crime investigative reporter at National Geographic. RESOURCES:"When the Horseshoe Crabs Are Gone, We’ll Be in Trouble," by Deborah Cramer (The New York Times, 2023)."Horseshoe Crab Blood Is Vital to Modern Medicine. A New Lab-Made Alternative Could Save the Species," by Kristoffer Whitney, Jolie Crunelle, and The Conversation (Fortune, 2023)."Horseshoe Crab Blood Saves Lives. Can We Protect These Animals From Ourselves?" by Dina Fine Maron (National Geographic, 2022)."For Atlantic Horseshoe Crabs, Love Is a Battlefield," by Dina Fine Maron (National Geographic, 2022)."Horseshoe Crab Blood: The Miracle Vaccine Ingredient That's Saved Millions of Lives," by Katie Pavid (Natural History Museum, 2020)."The Role of Horseshoe Crabs in the Biomedical Industry and Recent Trends Impacting Species Sustainability," by Jordan Krisfalusi-Gannon, Anthony L. Dellinger, et al. (Frontiers in Marine Science, 2018)."Video: Horseshoe Crabs Mate in Annual Beach 'Orgy,'" by Heather Duner MacAdam (National Geographic, 2014)."Crash: A Tale of Two Species," S26.E7 of Nature (2011). EXTRAS:"Baby Blue Blood Drive," by Radiolab (2020).