Citizen Science

ReTurn the Favor: Horseshoe Crab Survey

Horseshoe crabs spawning in Mispillion Harbor, Delaware. Photo by Gregory Breese/USFWS.

Looking for a citizen science project that takes you on a beach vacation?

Want to help a creature that was around long before the dinosaurs and still lives with us today?

The Horseshoe Crab Spawning Survey is the project for you!

Each year throughout May and June people gather at 12 beaches in New Jersey and 13 in Delaware to count spawning crabs, because Delaware Bay is home to the largest population of horseshoe crabs in the world!

Many organizations participate in the survey, but only two in New Jersey, the Nature Conservancy and the Wetlands Institute regularly train new volunteers from the public.

The Nature Conservancy’s New Jersey chapter holds survey walks on two beaches: Sunray Beach in Villas and Higbee Beach in Cape May.

This year, I went to Sunray Beach to learn more and help out with the survey.

Why is it important?

Horseshoe crabs are survivors. Not true crabs, but more akin to spiders or scorpions, they’ve been around for 445 million years. That’s three ice ages and a slew of other environmental changes that they overcame.

However, their numbers began to decline in the 1990’s and have not yet recovered.

Used as bait for eel pots and whelk, horseshoe crabs are very valuable, up to 5 dollars for a large female. And they are very easy to harvest – they come to beaches in large groups, move fairly slowly, and don’t bite or pinch.

As a conservation measure, the state of New Jersey passed a law in 2008 making it illegal to harvest horseshoe crabs, and Delaware, Virginia, and Maryland all have strict harvest quotas, but the population has been slow to recover.

“It’s astonishing how quickly harvest can devastate the population and how long it can take for the population to recover after bait fishing has been closed,” says Preserve Program Manager Adrianna Zito-Livingston.

Spawning crabs on Sunray Beach. Photo by Lisa Feldkamp/TNC.
Spawning crabs on Sunray Beach. The female is digging into the sand and there are two males on top.  Photo by Lisa Feldkamp/TNC.

The key driver of horseshoe crab conservation has been their importance to shorebirds.

Shorebirds, like the imperiled red knot, stop by Delaware Bay on their way from Argentina to the Arctic.

During this stop they need to nearly double their weight in order to survive the rest of the journey and they do that by eating the eggs of the spawning horseshoe crabs.

The crabs lay so many eggs, thousands each night for multiple nights, that they can provide a feast for the birds and still produce plenty of baby crabs.

“Their eggs are like cheesecake, full of lipids so that shorebirds can put on fat fast – and they only have a few weeks to put on weight before flying north.” That’s how Zito-Livingston describes the dynamic.

Perhaps most importantly for people, horseshoe crabs are used in the biomedical industry.

Horseshoe crabs are true blue bloods – their blood is copper-based, which gives it a blue sheen.

In the presence of gram-negative bacteria, the blood gels. That prevents the bacteria from entering their blood stream.

People realized that they could use horseshoe crab blood as an easy test for the presence of bacteria.

“Any time you use vaccine needles or surgical implants, they were tested with horseshoe crab blood,” says Zito-Livingston.

Don’t worry! They don’t have to kill horseshoe crabs to get their blood. Technicians drain around 30% of the crabs’ blood and then release them. Most survive this process.

That’s a lot of good reasons to be sure that horseshoe crabs stay around and the spawning survey is an important part of that effort.

Numbers from the spawning survey not only help scientists to understand how crab populations are doing, they are used to set policies that protect horseshoe crabs and the shorebirds that depend on them. For instance there New Jersey has had a moratorium on harvesting crabs for five years and there are restrictions on harvest in other states.

How do you get involved?

The first thing to do is to sign-up to volunteer at Sunray Beach or at Higbee Beach.

These are popular walks and there is limited space, but there are still openings, so do it now!

The times and dates might seem strange, but they are that way because the crabs time their spawning based on the tides and those are affected by the moons.

Horseshoe crabs have to bury their eggs on shore, deep enough that waves and predators don’t disturb them but close enough to the surface that they stay warm.

The ideal time period to get eggs up on the shore where they won’t be disrupted is between the full moon and the new moon from May to June.

“From years of study we know that the best time to see the crabs is at high tide during the full moon in the third week of May,” Zito-Livingston explained.

It’s also important to take counts at night because it is less disruptive to visiting shorebirds. For a month of the survey period, beaches are closed to the public to protect shorebirds and so every precaution is taken by survey organizers and volunteers to ensure that the survey has a low impact on birds and crabs alike.

Using a quadrat to survey crabs at Sunray Beach. You can also see how high the waves get. Photo by Lisa Feldkamp.
Using a quadrat to survey crabs at Sunray Beach. You can see how high the waves get. Photo by Lisa Feldkamp/TNC.

Once you have signed up for a date, plan your trip to New Jersey. I recommend staying in Cape May for charm and convenience getting to the beaches.

Make sure you bring along a flashlight and some rain boots, because you will be walking in the surf at night. Don’t be afraid to get wet!

Once you get to the beach, volunteers gather for an anatomy lesson.

First you will learn the wrong way to hold a horseshoe crab – never pick one up by the tail!

Crabs use their tails to turn back over if they get flipped. A tail can easily break off if you pick them up that way – so don’t do it, use the front of the shell.

Knowing how many of the crabs are males and how many are females is important to understanding the health of the population.

The easiest way to tell males and females apart is that females are much bigger. They must also be older (8-10 years) than males (5-7 years) to reach sexual maturity.

If you look at the underside of a horseshoe crab, you get another clue. Only males have specially shaped “boxing-glove” like pincers that help them latch onto the backs of females.

When they are spawning, the females work on digging into the sand to deposit eggs while the males (sometimes several) pile on top to fertilize the eggs.

After the anatomy lesson, the steps are easy and ensure that you count a sample of crabs on the beach:

  1. Volunteers split into two groups.
  2. Each group has a volunteer pacer. This volunteer learns how many paces it takes them to walk 10 meters and makes sure that their group goes 10 meters each time before dropping a quadrat in the “crab line” (the area of beach along which crabs are spawning).
  3. Each group gets a one-meter-squared quadrat (see the photo above) to drop every 20 meters. The number of male and female crabs that are in it when it first falls are counted.
  4. The group agrees on the numbers for males & females and reports them to the recorder (a veteran volunteer or organizer who records all numbers for both groups).
  5. This process is continued (with the groups staggered) until both groups have covered about a 0.5 km strand of beach.

The start and end time of the survey are recorded – the whole process usually takes around 2 hours.

Everything is done very carefully to ensure that the data gathered represent a random sample of spawning crabs that can be compared to data from other beaches or from years past to get an accurate sense of how horseshoe crab populations are changing.

Other metrics like wave height and weather are recorded to give scientists a sense of the factors that might have influenced the count.

For instance, the count I participated in took place after a storm and choppy waves were making it hard for crabs to get a foothold on the beach. Though some crabs appeared to be spawning successfully – there were many more crabs washing back and forth in the waves than were firmly on the beach.

If you would like to participate, but are worried that you wouldn’t be able to get an accurate count, remember that veteran volunteers and organizers are there to help every step of the way.

I volunteered at the same time as a great group of young people from the New Jersey Academy of Aquatic Sciences and they did an amazing job of following the protocols despite the waves.

Even children under 18 can participate as long as an adult is present to help them.

Hurry up – sign-up for the Horseshoe Crab Spawning Survey and give these amazing creatures that do so much for us a fighting chance to be around for another 445 million years.


Lisa Feldkamp

Lisa loves all things citizen science and enjoys learning about everything that goes on four legs, two wings or fins - she even finds six and eight-legged critters fascinating at a safe distance. She has a PhD in Classical Literature and Languages from the University of Wisconsin - Madison and enjoys reading Greek and Roman literature or talking about mythology in her spare time. More from Lisa

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  1. Please share the sign up link to register for the survey. The hyper links in the articles don’t work.