Touching life 400 million years ago

Spending a Night NYC Horseshoe Crab Monitoring

Here are things you may not know about Atlantic horseshoe crabs:

  • They are one of the world’s oldest species. Scientists have discovered fossils of horseshoe crab ancestors that lived over 445 million years ago (dinosaurs appeared 200 million years ago).
  • Horseshoe crab blood is blue. Their blood is copper-based, not iron-based like ours.
  • One mamma horseshoe crab will lay up to 120,000 eggs during a single spawning season.
  • Horseshoe crabs, and their eggs, glow under UV light/blacklight. The younger the crab, the brighter they glow.
  • Certain endangered migrating shore birds depend on horseshoe crab eggs to survive, including the Red Knot

High tide hit Big Egg Marsh in Queens at 10:52 p.m. on Tuesday June 20th. The incoming tidewater lapped against my knees… a good 3 or 4 inches above the tops of my rainboots… as I carefully and slowly walked along the shoreline trying not to step on the hundreds of horseshoe crabs who had migrated in this night to mate and spawn their eggs.

The only light coming from my headlamp and with the wind pushing the incoming tide, it was a bit disorienting to determine where to put my next step.

The bodies of female adult horseshoe crabs are the size of dinner plates. If you include their tails, they could easily be two feet long. Males are smaller, faster. Tails quickly moving back and forth as they skitter through the water. They use their tails for stability, not for protection, a fact for which I was grateful as dozens of them swarmed around my feet.

The mosquitoes were intense and unforgiving. Any new skin exposed to the air without a layer of bug spray would quickly be covered in mosquitoes looking for a late-nite dinner. In the time it took me to push my long sleeves back from my wrist up to my elbows and spray a good dose of bug repellent, I counted no less than a dozen mosquitoes landing on my arm for a feast.

I was part of a small team of volunteers with the NYC Audubon Society, there to monitor the horseshoe crabs by counting them, checking for (and recording) pre-tagged crabs, and then tagging a few of our own after catching them, measuring them and checking for a few specific characteristics before releasing them back into the bay.

If you’ve never caught a live horseshoe crab it is like picking up a piece of ancient earth. Shells hard and dark, sometimes covered in barnacles, they’re faster than you think. They can weigh up to 11 pounds. All of which makes catching them in the water in the complete dark, under a new moon, using only your headlamp to track them, a challenge.

Picking them up means placing your hands on either side of the center edges of their body shells and quickly flipping them up and over so their 10 pairs of legs are flailing about in the air. From here, upside down, you can see their claws, their gills, and their jawless mouths, which, if you touch it, feels a bit like you’re running your finger along extra sturdy toothbrush bristles.

Their tail – while not a weapon – does whip up and down when you pick up a crab, as if it is doing violent sit ups. It’s an effort to thrust the tail up and down in hopes you drop them back into the water so they can escape. It is a bit unnerving if you have never done it. The tail may not be a weapon but it still pinches when it whaps you in the face.

One thing you should never do is catch them or hold them by their tail. It may look sturdy, but it is very fragile and easily damaged and if injured means it cannot properly navigate the waters. The tail also has photoreceptors that allow it to sense light, helping it understand where the shore is.

Our group had arrived at the site in the dark around 9:30. We took some initial shoreline measurements and set up our equipment. Dottie – our leader from the Audubon Society – took us through the process of what was to come and gave us tons of information about the crabs. Horseshoe crabs are not really crabs at all. They’re actually closely related to scorpions. A fact that also made me grateful they do not use their tails as weapons or for defense.

The crabs only come to shore in May and June during new and full moons – leaving the safety of the deep water to make their way up the Atlantic coast to breeding grounds where they mate and spawn eggs. Lots of eggs. They arrive by the hundreds of thousands along the shallow waters along the beaches.

Male clutching onto the back of a female as she makes her way to the shore to lay her eggs

Mating is unique. Males migrate to the inshore waters first and wait for the females to arrive. The females are much larger than the males. In the shallow waters, as the females begin to make their way towards the beach, the males battle to attach their front legs to a female’s shell.

There could be a dozen males vying for one female. When the female moves onto the very edge of the water, just at the very edge of the beach, she digs a small hole (a nest) where she deposits strands and strands of eggs, sometimes thousands at a time. At the same time the male releases his sperm over the eggs to fertilize them. It is not unusual for another male to come charging in and also release his sperm over the eggs too.

You can see the tiny eggs glowing in the blacklight

Females lay as many as 4,000 eggs in one nest and will create 5 to 7 nests in one visit to the beach. Each female returns to the shore many times during spawning season.

The eggs are the size of pearl couscous, almost indistinguishable from the sand except they glow under blacklight/UV light. We saw several nests of eggs during our time on the beach.

They will hatch in 2-4 weeks, during a high tide where the eggs are covered by the water. They use the waves to carry them out to sea.

We came across a nest of early eggs who were hatching. If you were to scoop some up in your hands you would see they are shaped exactly like their parents. Tiny, miniature versions of their earlier generations.  

It’s hard to believe something so tiny will hatch and make its way to the enormous ocean to one day become a full-grown adult.

NYC Audubon has dozens of teams spread out across several sites in Queens and Brooklyn tonight and for 12 – 15 nights in total (depending on migration pattern and moon phase). These beaches have the highest densities of horseshoe crabs on the East Coast.

Our monitoring of the population is part of a statewide project to survey spawning populations. The NYC Audubon is partnering with Cornell Cooperative Extension’s Marine program and Stony Brook University as well as the NY State Department of Environmental Conservation. The information is used to create conservation and management plans.

11 species of migrating birds, including several endangered and threatened ones, rely on the eggs to replenish their fat supplies as they migrate to northern breeding grounds. The horseshoe crabs and birds have developed an interesting symbiosis. Female horseshoe crabs will often dig up existing nests where all or some of the eggs did not hatch. This makes the eggs available to the birds – making it easy for the birds to get nourishment that way instead of needing to dig up fresh nests.

The crabs are threatened in many ways, all of which are human:

  • Biomedical use – Because horseshoe crab blood is copper-based and contains a special clotting component, an extract of it is used to test pharmaceuticals and medical devices for bacteria (to insure there is none). Vaccines and implants are primary areas where horseshoe crab blood is used for such testing. Pharmaceutical companies typically harvest the crabs by hand during spawning season. They transport them to labs where they are cleaned and then 1/3 of their blood is removed. After a bit of care to be sure the crab is healing, they return them about a week later to the ocean. This interferes with spawning. It is not uncommon for the crabs to die during the process.
  • Climate change is changing the temperature of the ocean, the patterns of ocean circulations, changing the temperature, height and depth of the water along the coastlines. This affects the migration and breeding of the crabs. Additionally, as humans build sea walls and other interventions to fight sea level rise, it often negatively affects their breeding grounds
  • Pollution of the water and beaches
  • Bait fisheries often harvest horseshoe crabs and cut them up to use as bait for eel. More breeding areas have been set aside as protected areas where such harvesting is not permitted, but the activity still exists.

During our count tonight we came across several crabs that had been tagged in previous years. The goal with these was to catch them to measure and inspect them, get a photograph of the tag, and then record it so it can be tracked throughout its life. One horseshoe crab we found was over 10 years old. Each of us in the group found at least one tagged specimen.

Tagging a female

After completing the counting, it was time for each of us to pick up a sample crab for measuring and tagging. I picked up a female. She was about 2 feet long and very dark, meaning old (the shells of the older crabs are darker than the younger ones). She had barnacles on her back section.

I tagged her with a tracking number and released her wondering where her journey would take her.

Making my long journey back from the beach in Queens to Manhattan NYC on the A train Subway at 1:30 in the morning, wet clothes and smelling like sea water, I was feeling exhausted, excited and amazed.

It was humbling to be able to hold and examine one of the Earth’s oldest living species – unchanged for 400 million years.

I could not help but wonder….. Horseshoe crabs have survived the prior 5 mass extinctions of Earth’s history. Even outliving the dinosaurs. Would they survive this current man-made one?

I am eager to learn more about the population pattern (increasing/decreasing) and I am already looking forward to helping out again next year!

Thank you for joining my journey this week,


If you’d like to learn more about NYC Audubon, get involved, or support their programs, please check out: Who We Are | NYC Audubon

To learn more about the New York Horseshoe Crab Monitoring Network, check out: New York Horseshoe Crab Monitoring Network (

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