Why is it called a “horseshoe crab”?
The name horseshoe crab comes from the shape of its hard shell (called the carapace).
The shell is wide and rounded with a slight indentation, making it look like a horseshoe.
Because of this shape, English speakers started calling it a horseshoe crab.
The animal also has a long spike-like tail called a telson. It does not use this to sting. Instead, it uses it to flip itself over if it ends up on its back.

Why isn’t it a real crab?
Even though it looks similar to a crab, it is not actually a true crab.
True crabs belong to the group:

Crustaceans (Crustacea)
crabs
lobsters
shrimp
Horseshoe crabs belong to a different group entirely.

What is it related to then?
Horseshoe crabs belong to a group called chelicerates. This group also includes:
spiders
scorpions
mites
ticks
This means they are more closely related to spiders and scorpions than to crabs.

A “living fossil”
Horseshoe crabs have existed for over 400 million years and have changed very little over time. Because of this, scientists often call them “living fossils.”
They were already around long before dinosaurs appeared.

Another fascinating fact
Their blue blood is extremely important in medicine. Scientists use it to test medicines and vaccines for harmful bacteria.