Natural History
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A living fossil
One often hears Limulus referred to as a "living fossil" as if the actual animal alive today is millions of years old. Although this is not the case (A horseshoe crab can live for more than ten years) if one was able to peer back in time to the shallow seas 250 million years ago, long before dinosaurs, there would be the horseshoe crab, living much like it does today. There are few animals the size of Limulus alive today that were around that long ago.Although it is commonly known as a crab, Limulus is actually more closely related to spiders. Crabs and spiders are both classified as arthropods, which refers to their jointed legs. Arthropods are then divided into two major groups based on their mouthparts. There are madibulate arthropods which have "mandibles." Think of the "jaws" of an ant or a beetle. Crabs and insects are typical mandibulate arthropods. The other group are the chelicerate arthropods which have feeding appendages called chelicerae ("chell-iss-erra"). This group is represented by spiders, scorpions and the horseshoe crabs.
There are four species of horseshoe crabs found in the world today. Limulus is the only species found in the Atlantic Ocean. The remaining three are found along the Asian coast of the Pacific with overlapping ranges (see figures above). Some of the Asian species are actually used as food by some people who consume the eggs in the females but discard the males.