The biomedical industry depends on blood from horseshoe crabs for drug and environmental safety testing — but this commercial demand, together with capture for bait, climate change and habitat destruction, is threatening populations of these ‘living fossils.’ This in turn will detrimentally affect the surrounding ecosystem, such as migratory shorebirds who rely on horseshoe crab eggs for food. Sustainable alternatives to horseshoe crab blood tests should be developed in conjunction with better harvesting and conservation strategies.