https://phys.org/news/2018-07-invaluable-medical-industry-horseshoe-crab.html

More closely related to scorpions and spiders than to crabs, the horseshoe crab is commonly referred to as a "living fossil," having remained unchanged for millions of years. They are an important part of the ecosystem, found along the East Coat of America and Gulf of Mexico. Migratory shorebirds arrive to feed on horseshoe crab eggs during the spawning season, before continuing to northern breeding grounds.

The unique properties of horseshoe crab blood makes it invaluable for the biomedical industry. The blood is also increasingly used for air and water quality testing.