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When people mention “blue blood”, it’s often a figure of speech referring to royalty or aristocracy, but when you’re talking about some of the oldest creatures on the planet, it’s no longer a figure of speech.
 
Horseshoe crabs (Limulus polyphemus) are certainly older than any aristocratic family, having been around for approximately 450 million years, and in a bizarre twist of nature, their blood is literally blue. While it may sound like something out of science fiction, their blood functions much the same as ours, circulating around their bodies to carry oxygen and nutrients to their organs. But while our blood uses iron-rich haemoglobin to transport oxygen, causing the red colour, horseshoe crabs use a copper-based molecule called hemocyanin instead. In nature, the copper turns things blue or blue-green, hence the otherworldly colour of their blood.
 
Besides being blue, horseshoe crab blood has another amazing property. When a horseshoe crab is injured, its blood senses any harmful bacteria and releases a mass of specialised granules that instantly clot to seal out the bacteria and prevent infection.
 
The crabs have had hundreds of millions of years to perfect this effective response, but humans have taken a shortcut. Because of its unrivalled ability to clot when it comes into contact with foreign bacteria, their blood is ideal for detecting impurities in products like pharmaceutical drugs. Indeed it provides the most effective means we have for detecting bacterial contamination in myriad medical products that must be completely sterile, and there is currently no known alternative that can replace it at the same level of performance.
 
Thankfully science has developed a way to extract some of this blood without killing the crabs, and at nearly £6,000 a pint, their blood makes them valuable enough to gain them a measure of protection from casual use as commercial fishing bait.