Horseshoe crabs have bright blue blood. They are like aliens. (Does this one not look like a dead alien?) Nature, you are weird.

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Robert Krulwich explains why the crabs’ blood is so beautifully blue:

Their blood kind of sloshes around in their bodies carrying oxygen to various organs, as our blood does.

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Our blood is red because we use hemoglobin to move oxygen around. Hemoglobin has iron in it, which gives off a reddish hue. (Think of rust.) Horseshoe crabs use a copper-based molecule called hemocyanin to distribute oxygen. In nature, copper turns things blue or blue-green. So that’s why their blood is blue; it’s copper-based.

Besides being blue, horseshoe crab blood has another amazing property: It clots when it comes into contact with foreign bacteria. In other words, this stuff’s ideal for detecting impurities. And humans use it to do just that, in products like pharmaceutical drugs.

But humans are amazing, too, and they have found a way to steal some of this blood (it’s worth more than $10,000 a quart!) without killing the crabs. Okay, without killing most of them. They hook up the crabs to vampire machines, take what they want, and return the crabs to the bay. Most survive the process, though some have to be taken to Miracle Max afterwards.

At least one human, biologist Carl Shuster, worried a little bit about the 3 to 15 percent of crabs that don’t survive their ordeal on The Machine. He talked the government into creating a horseshoe crab preserve, where crabs can live free of fear of the red-blooded, white-faced monsters that grab them from their homes and suck out their blood.