Are horseshoe crabs really crabs?
No, horseshoe crabs are in a class by themselves; they are more closely related to spiders, scorpions, and ticks. They differ from true crabs in that they have no antennae and no mandibles (mouth parts for grinding food). Like spiders, they have a pair of chelicerae (small appendages for moving food into the mouth).

Are horseshoe crabs really ancient?
Yes and no, that idea comes from the fact that 350 million years ago, ancestors of horseshoe crabs were abundant. The anatomy of the species we have today is not much changed from those older forms. The life span of an individual horseshoe crab is also remarkable—it can live for up to 20 years.

Can a horseshoe crab hurt me?
Horseshoe crabs do not bite or sting. Despite the ferocious look of the tail, it is not used as a weapon. Instead, Horseshoe crabs use their tails for righting themselves if they are flipped over by a wave. They do have spines along the edge of their carapace, so if you must handle them, be careful.

What do horseshoe crabs eat?
They eat almost anything. Horseshoe crabs are mainly predators. They feed on small clams, crustaceans, and worms; however, they will also eat other animals and even algae. Because they have no mandibles or teeth, they crush hard food between their legs before passing it to their mouth. Like birds, horseshoe crabs also have gizzards for grinding food before it reaches their stomachs.

What is so special about horseshoe crab blood?
Only horseshoe crabs have a blood-clotting agent known as LAL, which clots in the presence of certain bacterial toxins. These toxins are difficult to detect by other means. The FDA requires the use of LAL to test all injectable and intravenous drugs produced in the U.S. The good news is that up to one-third of a horseshoe crab’s blood can be removed without killing the animal.

Why do I see so many dead horseshoe crabs on the beach?
Although horseshoe crabs may become stranded and die on a beach, most of the “dead” ones are actually molts. Like all arthropods (including crustaceans and insects), horseshoe crabs have a hard exoskeleton (shell) on the outside of their body. In order to grow, the crab must shed its old exoskeleton and form a new, bigger one. Unlike true crabs, which back out of their old exoskeletons, horseshoe crabs push forward, leaving their molts behind them.

Where can I find more information about horseshoe crabs?
Visit the Crustaceans and Other Marine Arthropods Section for more information.