Why do horseshoe crabs have blue blood?
Quick Answer
Their blood is blue because it uses copper-based hemocyanin to transport oxygen instead of iron-based hemoglobin. The blood also contains cells used to detect bacterial toxins.
The Full Story
Horseshoe crab blood is blue because its oxygen-carrying pigment is hemocyanin, which contains copper and turns blue when oxygenated. This is different from human blood, which is red because it uses iron-based hemoglobin. Their blood has become medically important because it clots in the presence of bacterial endotoxins, a property used to test some medical products for contamination. The animals are ancient arthropods and have existed in roughly similar form for hundreds of millions of years. Their unusual blood chemistry has made them famous far beyond the shoreline habitats where they live.
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