It’s the middle of the night, the moon is full, and you have waded shin-deep into the ocean, carrying a flashlight, and toting a plastic pole. The pole is connected with string to two additional poles. Standing 5 meters away from you, to form a square, are other volunteers, similarly equipped. Why? Because horseshoe crabs have returned for another spawning season, and it’s time to count them!
Every spring for the past 18 years, volunteers have conducted Horseshoe Crab Surveys at Duxbury Beach. Teams assemble during the new and full moon high tides of May and June to assess a long stretch of shoreline. Working systematically down the beach, they count horseshoe crabs, make note of their sex, and keep track of whether each crab is alone, in a pair, or in a cluster. Numbers vary, but last year the most populous survey revealed 63 females and 140 males!
It’s not always “beach weather” when the surveys take place. Nearly half of them are at night. Sometimes it’s windy. Sometimes it rains. Nevertheless, these surveys are probably our most popular Citizen Science opportunity, attracting participants from as far away as Boston and the North Shore.
Our volunteers are not alone out there. State-run surveys take place in 16 locations from Duxbury to West Wareham to the Cape and Islands, overseen by the Division of Marine Fisheries’ (DMF), often in conjunction with other citizen science groups. (Ours is a joint effort with MassBays and Duxbury Beach Reservation.) By documenting population trends, the surveys help DMF determine and fine-tune how to manage, conserve and protect local horseshoe crab populations.
Why do horseshoe crabs need protection? Overharvesting and habitat loss have resulted in a sharp decline in their population. They are also valuable as bait for the eel, conch and whelk fisheries. The pharmaceutical industry relies on a compound in their blood to check the safety of certain medical devices, drugs, and vaccines. From an ecological perspective, horseshoe crabs promote the overall health of the coastal zone – stirring up sediment on the ocean floor and oxygenating mudflats. They’re also an essential component of the food chain: their eggs are a key source of food for migrating shorebirds.
While a mating horseshoe crab should never be disturbed, they can be intriguing to observe from a distance. During spawning season, they gather in large numbers in bays and coves with beaches that are at least partially protected from surf. The males arrive first, and wait for the females. The females release pheromones to attract the males. Then, clasped together, they head to shore. Females dig nests in the sand and deposit pearlescent clusters of eggs – 4,000 at a time – which males then fertilize. The process can be repeated multiple times.
Which is good, because most of the eggs will be gobbled up before reaching the larval stage! Eggs that are not consumed by birds and fish will hatch about two weeks later, producing tiny, tailless horseshoe crabs that settle into the sandy bottoms of tidal flats for a year to mature, before departing for deeper waters. Over the next ten years, they continue to grow, molting numerous times before reaching adulthood, when the breeding cycle begins anew.
Horseshoe crabs are protected in Massachusetts during their spawning season, which extends from April 15 to June 7. A state regulation bars their harvest during this time, with the hope that protection through such a critical and vulnerable period will result in a population rebound. Numbers have generally increased since statewide counting began in 1998, but with enhanced protections in place since 2024, we’re hoping to see even better results going forward.
If you’d like to participate in Horseshoe Crab Counts this year, there are a few steps you can take right now. Sign up to volunteer with NSRWA. Be sure to select both Citizen Science and Horseshoe Crab Counts in your online application. And watch your inbox. Registration for this year’s required training (and counts) will begin in mid-April. We will share participant info with Duxbury Beach Reservation, our project partner.
In the meantime, if you enjoy Citizen Science, we would love to have your help with our annual herring counts, which began in late March and extend into June. Even if you miss the first week or two, we welcome you to the team. You’ll find a training session on our volunteer website, where you can also select your counting shifts.
If you enjoy the outdoors, be sure to check out NSRWA’s Explore South Shore. Every morning, we highlight one of the region’s best nature places on Facebook and Instagram. Not on social media? We’ve listed all the locations on our website as well!
by Kezia Bacon
April 2026
Kezia Bacon's articles appear courtesy of the North and South Rivers Watershed Association, a local non-profit organization devoted to protecting our waters. For membership information and a copy of their latest newsletter, contact NSRWA at (781) 659-8168 or visit www.nsrwa.org. You will also find thirty years of Kezia’s Nature columns there. For more information about the “That’s My Watershed!” Contest, visit https://www.nsrwa.org/2026-nsrwa-explore-south-shore-thats-my-watershed-contest-rules/
