From horseshoe crabs. Before you complain about how this hurts the crab population, the crabs that have their blood drained are, for the most part returned alive. Look, before anybody gets concerned about the 30% or so that die, the crab lifecycle expects that 90% of the larvae hatched will not make it to mating. Followed up with the fact that mated crabs die soon after egg laying.
I grew up on the water and horseshoe crabs are interesting creatures. They’ve been around since before the dinosaur, essentially unchanged for hundreds of millions of years. I’m not surprised that horseshoe crabs have excellent disease resistance. It goes along with the very hard shell and the shape that allows the crab to crawl along the bottom.
A remarkable creature.