A Very Ancient History
Not a true crab, the horseshoe crab is actually more closely related to spiders, scorpions, and ticks than to other crustaceans. This large marine arthropod gets its common name from its shell, or carapace, which is U-shaped. Brownish-gray in color, the carapace provides camouflage against the muddy or sandy seabed where the horseshoe crab lives.
They haven't changed much since the Devonian era, some 360 million years ago, well before dinosaurs. Fossil data suggests that there were never more than twenty species of horseshoe crabs. Today only four species, grouped into three genera, remain.
Two genera are found along the coast of southeast Asia and nearby countries such as Japan. The third is found along the entire Atlantic coast of the United States and along the Gulf of Mexico as far as the Yucatan. Believed to have once been far more widely distributed, Limulus is the only surviving species in its genus, which indicates that its lineage is very ancient.