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Animal profile
Limulus polyphemus
The Atlantic horseshoe crab is not a crab at all but a chelicerate, more closely related to spiders and scorpions than to crustaceans. Adults spawn in large numbers on beaches around the spring tides of late spring. Its eggs are a critical food source for migrating shorebirds.
Horseshoe crab blood is blue, copper-based, and clots on contact with bacterial endotoxin. That reaction became the standard test for contamination in vaccines and implants, so hundreds of thousands of crabs are bled each year and returned to the sea.
Horseshoe crab blood is blue, because it carries copper-based haemocyanin rather than iron-based haemoglobin. Its amoebocytes clot on contact with bacterial endotoxin, a reaction turned into the standard safety test for vaccines and implanted devices.