The Horseshoe Crab: A Living Dinosaur

Ashley Breault | Marketing Intern | Ocean Studies and Communications Student at University of New England

horseshoe crab

Horseshoe crabs, Limulus polyphemus, are one of the many fascinating ocean animals you can find in the marsh of Odiorne Point State Park, right outside the doors of the Seacoast Science Center. Despite their name, horseshoe crabs are not crabs and are actually more related to spiders and scorpions. These cool critters can be found all along the Atlantic coast of North America, from Maine to Florida and even down to the Yucatan Peninsula. They live on the muddy, sandy bottoms of estuaries and bays, and have been found as deep as 30 meters, but prefer shallower waters of 5-6 meters. That makes the New Hampshire coast a perfect location to find these prehistoric looking creatures. Horseshoe crabs are known as “living fossils” because they evolved more than 230 million years ago. Their closest relative, trilobites, are extinct and can only been seen as fossils. This makes horseshoe crabs one of the oldest natural wonders!

Looking further at these cool creatures we can explore their life style and reproductive cycle.

In the late spring/early summer, adults migrate from deeper water to the shallow coasts to mate. Their behaviors are based on the moon and tidal cycles, making horseshoe crabs more active at night, specifically during full or new moons. This means horseshoe crabs are nocturnal.

horseshoe larval stage 1

Female horseshoe crabs dig small nests in the sand, lay their eggs, and then a few weeks later baby horseshoe crabs come crawling out during a high tide. These larval babies swim around the water column for a while before they start to molt, a process that is required for them to grow. They shed their outer exoskeleton, which allows them to grow bigger and stronger with every molt. They will then molt at least once a year until they die.

baby horseshoe crabs

After their first few molts, when they are about a year old, they settle to the ocean floor where they feed on worms, algae, and clams. They now look like real horseshoe crabs, with a hard outer protective covering, 6 pairs of legs, and 9 eyes.

horseshoe crab eggs

At about 10 years old, they reach maturity and are ready to start reproducing. A mature female can carry about 4000 eggs per clutch and will reproduce every year after reaching maturity until it dies, which could be anywhere between 20 and 40 years old.

These ocean animals are a lot more than just what they appear and are a lot different than what you might think. Horseshoe crabs have a lot to teach us and learning about their life cycle is just the beginning.