Q. What gives blood its colour
A. The colour of blood is determined by its respiratory pigments, which carry oxygen and their metallic content. The pigments, of which there are several different types, bind oxygen where it is found in high concentrations, like the lungs, and release it to areas where it is needed, like tissues.
Q. What causes blue blood
A. The pigment that carries oxygen around a horseshoe crabs body, hemolymph, contains copper. On contact with air, this gives the blood a blue appearance. In humans, and most vertebrates, the oxygen carrying pigment is haemoglobin, which contains iron giving the blood its deep red colour.
Q. Does the horseshoe crab have any other special features
A. Yes, it isn't a crab at all. When it was first discovered and named it was thought to be a crustacean based on its appearance. However, it's an Arthropod, more closely related to spiders and scorpions.
In addition, it's believed that the horseshoe crab has no natural enemies, other than humans.
A by-product of the horseshoe crabs' blood is also an important compound in medicine used to test for a fatal bacteria in all injectible dugs and medical devices, like artificial hips.
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by Lisa Cardy