Squeezing Blood From a Horseshoe Crab, For Medicine

A still from the PBS Nature documentary Crash ( PBS ) 

The Atlantic:

The thing about the blood that everyone notices first: It's blue, baby blue.
The marvelous thing about horseshoe crab blood, though, isn't the color. It's a chemical found only in the amoebocytes of its blood cells that can detect mere traces of bacterial presence and trap them in inescapable clots.
To take advantage of this biological idiosyncrasy, pharmaceutical companies burst the cells that contain the chemical, called coagulogen. Then, they can use the coagulogen to detect contamination in any solution that might come into contact with blood. If there are dangerous bacterial endotoxins in the liquid—even at a concentration of one part per trillion—the horseshoe crab blood extract will go to work, turning the solution into what scientist Fred Bang, who co-discovered the substance, called a "gel."
"This gel immobilized the bacteria but did not kill them," Bang wrote in the 1956 paper announcing the substance. "The gel or clot was stable and tough and remained so for several weeks at room temperature."
If there is no bacterial contamination, then the coagulation does not occur, and the solution can be considered free of bacteria. It's a simple, nearly instantaneous test that goes by the name of the LAL, or Limulus amebocyte lysate, test (after the species name of the crab, Limulus polyphemus).
The LAL test replaced the rather horrifying prospect of possibly contaminated substances being tested on "large colonies of rabbits." Pharma companies didn't like the rabbit process, either, because it was slow and expensive.
So, now, the horseshoe blood test is a big business. "Every drug certified by the FDA must be tested using LAL," PBS's Nature documentary noted, "as do surgical implants such as pacemakers and prosthetic devices."

Wow, I sure as hell didn't know that.  I also didn't know that horseshoe crabs were ground up back in the day for fertilizer.  But that blue blood...eww.