By Terri Estes

The horseshoe crab is one of the most fascinating creatures on the planet. They may look like a creepy helmet with legs and a very long, sharp tail, crawling along the ocean floor, but there is so, so much more to them.

Though they are called crabs, horseshoe crabs are not actually crabs at all. They are arthropods. The three major classes of arthropods are insects, arachnids and crustaceans. The horseshoe crab belongs to its own class called Merostomata (which means legs attached to mouth) and is more closely related to spiders and scorpions than to crabs and lobsters.

Horseshoe crabs are considered living fossils, meaning they have existed, nearly unchanged, for more than 450 million years. This is probably because they are very adaptable, and their body structure is extremely effective for survival. They actually predate dinosaurs, flying insects and man on this planet.

A female horseshoe will lay tens of thousands of eggs on sandy shores along the Eastern coast of the Atlantic during the springtime. This occurs from Maine, all the way to Mexico. This is super important to our ecology because those eggs and larva are a main source of food for migratory birds and marine animals like turtles and many species of fish.

Not many of those eggs survive, but the ones that are lucky enough to become juveniles move into deeper waters, where they can burrow into the sand for protection. A horseshoe crab does not reach full maturity for 10 years. They are really hardy and can live for over 20 years.

Horseshoe crab blood is extremely important in modern medicine. The blood contains a compound called Limulus amebocyte lysate (LAL). LAL coagulates or becomes clumpy in the presence of small amounts of bacterial toxins and is used to test the sterility of medical equipment as well as virtually all injectable drugs.

Although they may look scary, horseshoes are actually quite harmless. Their long pointy tail is not used to “sting.” It is actually used as a tool by the animal to right itself if it gets flipped over by a wave. During the springtime, when many horseshoes come ashore to mate and lay eggs, they may get turned over and have trouble righting themselves. If you happen to see one in trouble, gently pick it up by the sides of its hard shell and turn it over. Never pick a horseshoe crab up by its tail as this may injure them.

Photo by Don Riepe.

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