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Horseshoe Crabs

By Kristen Swanberg, Senior Director of Education

When you hit the beach this summer be on the lookout for horseshoe crabs (Limulus polyphemus). Amazingly, they have been on the planet for over 360 million years. They are one of the oldest creatures living on earth.

HorseshoeCrabThese creatures look like crabs when you turn them over, but in fact they are more closely related to spiders and scorpions. They have a large brown protective shell, a spiny tail and five pairs of claws.  They have nine eyes: two compound eyes, five additional eyes on the top of their shell, and two eyes located near their mouth.  Their gills are called book gills because they look like pages in a book.  These gills have two uses — first to absorb oxygen from the water and second to help them swim.  Horseshoe crabs swim awkwardly, they turn upside-down in the water and use their book gills as paddles to propel them.

Horseshoe crabs live in salt water along bays and shallow coastal waters.  They spend most of their time searching and digging for food like clams and worms. They are like bulldozers, pushing their bodies along the sand.  To break up their food they grind it between closely set bristles on their legs, then they pass the meat to their toothless mouths.  They have to be walking in order to chew their food.

Despite their appearance, horseshoe crabs are harmless creatures.  They use their tails to help them flip over and not to stab or sting swimmers.

In the circle of life, many migrating birds depend upon horseshoe crabs as they travel north from South America.  Shore birds like the red knot time their migration to arrive when horseshoe crabs come ashore to lay their eggs.  Many of their green, pea-sized eggs become food for hungry migrating birds, so it’s a good thing each female lays about 90,000 eggs in one season!

Horseshoe crabs are commonly found in Rhode Island waters, but they are threatened by pollution, destruction of habitat, harvesting for bait, and increased use for medical research.  These ancient creatures are important to our ecosystem, so the next time you see a horseshoe crab take a close look, but be sure to leave it where you found it, and share what you’ve learned with others.

Fun Facts:
– Only four species of horseshoe crabs live on earth.
– Females are larger than males.
– It takes eight to ten years for horseshoe crabs to reach adulthood.
– They can then live up to 30 years.
– Their blood turns blue when exposed to oxygen; this is because they have copper in their blood instead of iron like humans.

There is so much to learn about horseshoe crabs.  If you are fascinated and want to know more check out these resources:

– Discover Nature at the Seashore by Lawlor and Archor

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