A species
Atlantic horseshoe crab
Limulus polyphemus
Not a crab but a spider cousin, with blue blood that helps test medicines.
You and the atlantic horseshoe crab last shared an ancestor about 580 million years ago (on 11 November, if life's history were one year).
This is no true crab, but a cousin of spiders. The Atlantic horseshoe crab may look like a crab in a helmet, yet it is closer to spiders and scorpions than to true crabs. Its smooth dome shell is shaped like an upturned horseshoe, olive-green on top and pale tan below, with about 10 jointed legs tucked underneath around its mouth.
Adults grow to about 0.6 metres long including the tail and about 0.3 metres wide, about as long as two school rulers end to end. Females are larger than males. Their blood looks blue because copper carries oxygen in it, while iron makes our blood red. The blood clots quickly around bacteria, and scientists use that clotting power to check that medicines and vaccines are clean.
They belong to a very old family. That family even survived the great extinction about 66 million years ago that wiped out the dinosaurs.
A tail that is not a weapon
The long tail spine is harmless. It works like a rudder for steering and a lever to flip back over when stranded upside down.
Ten eyes
It has 10 eyes in total, including two large eyes on top of the shell that help it find a mate.
Eggs that fuel a long flight
Its fatty eggs are a superfood for migrating red knots. The birds can double their weight during a two- to three-week stop on a round trip of about 18,000 miles between South America and the Arctic.