It’s horseshoe crab breeding season

Thu, 05/28/2020 - 5:45pm

The following was submitted to The Block Island Times from the Block Island Conservancy:

Right now, tiny horseshoe crabs are emerging from sandy nests by the thousands around the Great Salt Pond. While exploring Andy’s Way over the weekend, we found young horseshoe crabs smaller than a dime burrowing underneath the sand, as well as several breeding pairs of adult horseshoe crabs. This remarkable annual breeding event is always a treat to observe.

Adult horseshoe crabs began appearing in our waters within the last month or so. With males leading the way, the adults crawl up from deep ocean waters to shallow tidal flats all along the East and Gulf Coasts in the late spring and early summer. When the females arrive, they release pheromones into the water to attract males. The male horseshoe crabs, which are about two-thirds the size of the larger females, grasp onto the backend of the female, who then tows them to shore using a special appendage. Once on the beach, the female digs a nest and deposits her eggs. The male will then pass over the eggs and fertilize them. The breeding pair will repeat this process multiple times, producing 80,000 to 100,000 fertilized eggs in a single evening, usually during new or full moon high tide cycles.

After about three to six weeks, larval horseshoe crabs hatch from the nests. The larvae look like adult horseshoe crabs, except without a tail. Most of the eggs and larvae will not make it to adulthood; young horseshoe crabs are a popular and important food source for numerous birds and fish. As the larvae develop, they move to deeper waters that offer more protection. During their first year of life, horseshoe crabs molt, or shed their exoskeleton, several times. For the next 10 years, the young horseshoe crabs will grow and live in these waters until they reach adulthood and sexual maturity.

Though called horseshoe crabs, these creatures are actually not crabs, but rather arthropods that are more closely related to spiders and scorpions than crustaceans. Horseshoe crabs have six pairs of legs under their carapace, which is the large protective shell that contains most of its body. The front two legs have a pincer-like shape and are used to grasp their prey, mainly worms and clams, and move them to their mouths, which are located at the center of their body. They have four eyes on top of the shell and five light-receptor organs beneath the shell.

Behind the legs are five overlaid flaps that are the animal’s gills. Each flap contains 100 paper-like leaves that absorb oxygen from the water and are called “book gills” because they lay on top of each other like the pages of a book.

Extending off the back of their carapace is the telson, a tail that helps them maneuver and right themselves if they flip over. The telson is not a stinger and should never be used to handle the animal as it can break off, which would likely kill the animal. If you want to pick up a horseshoe crab, gently grasp the edges of the shell and lift it up.

An even better way to get an up-close look at horseshoe crabs is by examining their molts, the shed exoskeleton that’s a perfect replica of the animal. You can frequently find the molts washed up in the wrack line at Andy’s Way. Sometimes we get calls from people concerned that there has been a huge horseshoe crab die-off, but thankfully they’re usually just seeing these discarded shells. To molt, the horseshoe crab buries itself beneath the surface. The shell splits around the front rim and the crab walks out and expands by 25 percent. A new soft shell will harden within the next 12 hours.

Horseshoe crabs are another of Block Island’s “living fossils.” Their appearance has not changed in 300 million years, making them older than the dinosaurs.

Today, horseshoe crabs are under threat from climate change, habitat loss and over-harvesting. Thankfully Block Island’s populations seem to be holding steady, but please take care while observing these incredible creatures.

To read more of the Block Island Conservancy’s stories, sign up by emailing Executive Director Clair Stover-Comings at, or by going to their website at and fill out their sign-up form.