Is a horseshoe crab really a crab?

No.A horseshoe crab is not a true crab. The U.S. Fish & Wildlife Service describes it as a member of an ancient group of arthropods, closely related to spiders and scorpions rather than to crabs.

What's actually true

Horseshoe crabs aren't crabs. Horseshoe crabs walk on the seafloor with a hard shell, so the name seems fair. The U.S. Fish & Wildlife Service says they are not true crabs, but members of an ancient group of arthropods. Their closer relatives are spiders and scorpions.

Takeaway: Horseshoe crabs are ancient arthropods, closer to spiders and scorpions than to true crabs.

Why people believe it

It looks the part: a hard shell, lots of legs, and a life on sandy beaches next to real crabs. Early naturalists named animals by appearance, and the name stuck before anyone sorted out the family tree. Both crabs and horseshoe crabs are arthropods, so they are distant cousins, which makes the mix-up feel close enough to never get corrected.

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