I agree very much with the author that one reason why this is so mind-blowing is because: "I don't know about you, but the idea that every single person in America who has ever had an injection has been protected because we harvest the blood of a forgettable sea creature with a hidden chemical superpower makes me feel a little bit crazy."
Every FDA-certified drug must be tested by LAL. This means companies need a LOT of horseshoe crab blood. The crab's ecology allows them to acquire it, since they swim into very shallow water en masse to mate. Humans can collect them and drain their blood in the lab. Only about 30% of the blood of each crab is drained, however, in hopes that the crabs will survive, can be released into the wild, and reproduce so that the medical industry isn't the cause for their extinction, and loss of this very valuable resource (the blood can sell for $15,000 a quart).
Everything isn't all fine and dandy for the crabs, however. 10-30% of the bled crabs die, and the ones that survive and are released are lethargic and are less likely to move to shallow waters to mate.