Animal · United States, Mexico
Atlantic horseshoe crab
Limulus polyphemus — the name this animal has in the language scientists everywhere share.
The Atlantic horseshoe crab is a helmet-shaped sea animal with nine eyes and pale blue blood, and it is not a crab at all but a relative of spiders. Its kind is about 445 million years old.
Picture credit: Kaldari (CC0) · Hans Hillewaert (CC BY-SA 4.0) · Wikimedia Commons
Where does the Atlantic horseshoe crab live?
Home: Shallow sandy bays along the American Atlantic coast.
In the wild it lives in these countries: the United States, Mexico.
Conservation status from the IUCN: vulnerable.
The short profile
Fully grown: up to 60 cm long, tail included. It weighs about 2 kilograms. On the menu: meat.
The story behind it
Under that brown shell an animal walks on ten legs and grinds worms and clams with bristles on its own legs, because it has no jaws. The long spike at the back is not a weapon: a horseshoe crab uses it as a lever to turn itself right side up when a wave rolls it over. On spring nights around full moon, thousands come ashore in Delaware Bay to lay eggs in the sand, and migrating shorebirds arrive at exactly that time to feed on them. The pale blue blood carries copper instead of iron, and it clots the moment it touches bacteria. Medicine uses that reaction to test whether vaccines and drips are clean, so the species is caught, bled and released in large numbers, and its population is falling.
Find it on the globe
Atlantic horseshoe crab is there as a tiny animated figure that breathes and looks around. From far away the planet is only a blue ball; detail arrives as you zoom in, and this one appears once you are close enough. Open it on the globe.
It sits at its real coordinates, not near them — the same numbers listed in the facts above. Switch off the layer for this kind of place and it disappears again; the buttons for that are down the right-hand edge.
Questions children ask
Is a horseshoe crab really a crab?
No. Despite the name it belongs to a group of its own, closer to spiders and scorpions than to crabs. The shape of its legs and mouthparts gives that away.
Why is horseshoe crab blood blue?
Because it carries oxygen with copper instead of iron, and copper turns pale blue when oxygen attaches to it. Human blood uses iron, which is why ours is red.
Can a horseshoe crab sting with its tail?
No. The spike looks dangerous but has no venom and is not used against anything. The animal presses it into the sand as a lever to flip itself back over.
Where these numbers come from
Every figure on this page was checked against public reference sources before publication. Where sources genuinely disagree, the text says so.
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