Moonlight, warm waters, and high tides are the perfect conditions to bring in the one of the most mysterious marine animals to Nantucket shores. In late May through June, horseshoe crabs will start their journey from the the ocean floor to shoreline to lay their eggs.

Female Horseshoe Crab
Photo: Libby Buck
Mating Male & Female Horseshoe Crab
Photo: NCF Science & Stewardship Dept.

The females are noticeably larger in diameter as compared to the males.  The females will climb up on to the shoreline dragging a male behind her. They are able to do this with special modified claws. Once the female decides on a location, she will then bury herself into the sand and lay up to 4,000 eggs in each nest. After laying she will then continue to drag the male over the eggs so he can fertilize them. The female may have several nests and can lay up to 100,000 eggs in one season! There is a high competition to mate with the females, sometimes the females will have many males forming a cluster around her.

A typical horseshoe crab cluster: One female with four males
Photo: Libby Buck

The next morning many beachgoers will see long winding tracks on the shorelines. Sometimes at the end of the tracks they will still see the horseshoe crabs seemingly stuck in the sand. These crabs are not in any danger and it is best to leave them alone. They are just waiting for the next high tide and will return to the ocean.

Horseshoe crabs tracks leading to buried horseshoe crabs
Video: Libby Buck

If you encounter a horseshoe crab that has been flipped upside down (legs in the air) and out of the water they are in danger. They need you help!  Please flip them back over by just holding on sides of the shell and return them to the ocean. Please DO NOT pick them up by their telson (long pointy tail), this could easily break off and that is their only body part that helps right themselves when they get flipped over. Once they are back in the ocean, they will revive themselves and be on their way. This fantastic instruction video from U.S. Fish & Wildlife demonstrates the proper way to flip one over:

Three stranded horseshoe crabs that were returned safely to the water
Photo: Libby Buck

Since 2009, the NCF Science & Stewardship department has been contributing to the Atlantic States Marine Fisheries Commission’s (ASMFC) region-wide population surveys.  We monitor the horseshoe crabs in Madaket Harbor. By monitoring these localized populations, it will help the ASMFC determine regulations and management options. During the spawning season (May-June), NCF staff & volunteers will go out and collect data around every full moon and new moon. Luckily, we can say that the Nantucket population has an upward trend with an all-time high count of 932 horseshoe crabs in 2019!

A horseshoe crab survey by NCF Science & Stewardship Staff
Photo: Alex Etkind

From 2009-2012, the Science & Stewardship department tagged many horseshoe crabs to figure out if they return to the same spawning areas every year. From that information, it was determined that the Nantucket horseshoe crabs are not adventurous and will return to the same beach. You can help us and other researchers by reporting these tagged crabs if you come across one alive or dead on the beach. It’s very simple, just record the tag number and where you found it then report it to this website  Once the report is completed and sent, you will be sent back information on where the crab was tagged and a the coveted “horseshoe crab pin”!  So keep your eyes open this horseshoe crab season!

Tagged horseshoe crab and the horseshoe crab pin with certificate!
Night-time horseshoe crab survey
Photo: Kelly Omand

The Nantucket Conservation Foundation is a private, non-profit land trust that depends on contributions from our members to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us now!