cracked:

Those are actually horseshoe crabs and we humans are in fact harvesting their blood, which is blue, like a seasonal Mountain Dew flavor sold in Hell’s worst bodega.

Actually horseshoe crab blood is blue because it contains a lot of copper. They use that particular mineral to do the job that iron does in human (and other animal) blood, and instead of hemoglobin they have something called hemocyanin, which transports oxygen throughout their bodies. Their blood also deals with infection in a way that’s much different than ours, and clots aggressively around any foreign body to the point where it’s easily seen under a microscope – this is what makes their oozing goop absolutely vital to the pharmaceutical industry.

The gel-like substance that surrounds the slightest bacterial infection in horseshoe crabs was co-discovered by the tremendously named scientist Fred Bang. The chemical that causes it to occur, called coagulogen, allows us to very accurately test medications for contamination, can detect anomalies as small as one part per trillion, and is used on just about every single drug that comes out. And the way to get it is to capture a shitload of crabs, jab a needle in their assholes, and subject them to a process called “rack & bleed.”

Uhh, Modern Medicine … You OK?