Tracking Horseshoe Crab Migration in the Delaware Bay

In partnership with Cellular Tracking Technologies and generous donors Wendy and Rob Wilson who contributed via local partner non-profit organization Cape May Point Science Center, staff and volunteers with American Littoral Society deployed our PowerTags in late spring 2023 on horseshoe crabs in the Delaware Bay.

Shane Godshall of American Littoral Society teaches volunteers and donors about horseshoe crab ecology. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachnids such as spiders, ticks, and scorpions. Photo by Kelly Ball.

The crabs were tagged with CTT's battery-powered PowerTags (black tag atop the shell in this pic). Each tag was adhered to the crab's shell using epoxy. It is our practice to use sample biological materials to experiment with the efficacy of attachment methods for deployment. Although horseshoe crabs had not previously been tagged in this way, gluing onto turtle carapaces has been a common practice, one that other clients worked extensively with on similar aquatic projects. Over time, the epoxy and the tag will fall off.  Photo by Jess Formento.

The crabs were then monitored for an hour following tagging to ensure that the epoxy cured before their release back into the water. Photo by Jess Formento.

American Littoral Society's Advocacy for "Living Fossils"

 American Littoral Society (Society) was established in 1961 by NOAA researchers and scientific divers who were interested in the "littoral," or near-shore, zone and wanted to bridge the gap between science and the public. In time, this undertaking became one of the largest community science data collection projects in the world for fish aggregate studies. To date, the Society has championed efforts in advocacy, conservation, education, and habitat restoration.

In fact, it was in the past few decades that the Society particularly began focusing on habitat restoration. In 2008, they succeeded in instituting a moratorium in New Jersey to stop the harvesting of horseshoe crabs (Limulus polyphemus) as bait until a sufficient recovery of both the state's Red Knots (Calidris canutus) and horseshoe crab populations could be proven.

As Habitat Restoration Coordinator Quinn Whitesall McHerron recently explained, this particular moratorium would not have been possible if it weren't for the horseshoe crab's importance as a keystone species in the Delaware Bay. "In the early '90s, scientists started noticing that shorebird populations in New Jersey were plummeting, in direct correlation with decline in crab populations," said Quinn. A dramatic increase in human over-harvesting of the crabs for bait resulted in a dramatic imbalance to the Delaware Bay's ecological equilibrium in particular.


 Quinn trains volunteers from CTT on how to properly affix the PowerTags to each crab's shell. Photo by Kelly Ball.


The Delaware Bay is a hotspot of sorts for spawning horseshoe crabs, and this can be witnessed during May and June during the peak of their spawning season. More southern-based crab populations spawn throughout the year, but there is a small window in spring for the mid-Atlantic population. Their spawning coincides with the movement of ravenous migratory shorebirds along the Atlantic Flyway. When the temperature of the Delaware Bay's shallow, protected back bay water reaches around 59 degrees F, this triggers horseshoe crabs to storm the beaches to spawn. Explained Quinn, "They come from the edge of the continental shelf--they walk!" At the same time, migratory shorebirds such as Red Knots, Ruddy Turnstones (Arenaria interpres), and Semipalmated Sandpipers (Calidris pusilla) descend upon area beaches, desperate to refuel en route to their Arctic breeding grounds. Red Knots especially have a need to refuel, as their route brings them from their winter habitats located on the tip of South America.

During their stopover along the bay, the birds will spend about 3 weeks feasting on nonviable horseshoe crab eggs. "They're very fatty eggs, so a perfect food source, and very critical for local ecology," says Quinn. “It should also be known, the shorebirds don’t eat the clustered eggs that are buried beneath the sands. The surfaced, unviable eggs are what the shorebirds eat. Eggs become unearthed due to erosional forces from wave action and as other female crabs are trying to lay eggs." When weather conditions are conducive to spawning, such as they were in spring 2023, female crabs will lay about 90,000 eggs throughout May and June, with about 4,000 eggs in each clutch. However, if there is a cold dip early in the season, it can adversely impact the timing and, therefore, the food source essential to migratory birds that depend on these "refueling stations" to continue their journey. When this occurs, says Quinn, "there are essentially few [sic] eggs available, and birds will continue on with their migration, without gaining enough weight."

Why Is Big Pharma Interested in Horseshoe Crabs?

Outside of ecology, horseshoe crabs are incredibly important to humans. Why? According to Shane Godshall, Habitat Restoration Project Manager, "They have a component in their blood that coagulates when it comes into contact with bacteria. Biomed and pharma industries use this to check that vaccines and injectable drugs are safe for human use."

The process of harvesting the component from the crabs is, at the very least, a major stressor for the crabs. "In order to 'bleed' the crabs, they are hauled out of the bay and taken to a facility where a needle is pierced into the hinge of each crab," Shane explained. "Then they are supposed to remove 'x amount' of blood and released alive." The controversial practice is the exclusive right of a single company whose deals are confidential. In essence, nobody really knows how many crabs are taken and nobody really knows how the crabs are affected by the harvest. However, the company claims that this does not harm the crabs.

Questions surround the mortality rate for bled crabs. Additionally, there is supposition that, when bled, females cannot lay eggs for a number of years. Shane shared, "This harvesting company spends millions of dollars annually to block acceptance of the synthetic alternative in the U.S. Meanwhile, in the European Union and many countries, this alternative is widely accepted and used." The Society, therefore, advocates for the synthetic version used by American pharmaceutical company Eli Lilly.

Although the claim is that bleeding crabs does not harm them, it is unknown whether--and to what degree--bleeding impairs a female's ability to lay eggs. Photo by Kelly Ball.

Why Tag Horseshoe Crabs?

The Society team has been tagging horseshoe crabs in the Delaware Bay for about 10 years as part of the US Fish and Wildlife Service’s Cooperative Tagging Program. Since 2014, this tagging program, using traditional round plastic tags, has shown that Delaware Bay crabs are, in fact, remaining in Delaware Bay and also show evidence of some genetic differences of subpopulations and variations in species. (Chesapeake, Delaware, Barnegat and Bays all have separate subspecies.) "Generally, it’s a way to track how effective our management decisions are, and how beach restoration projects are working in the real world," explained Quinn.   

PowerTags lined up and awaiting their crab vessels! Photo by Kelly Ball.

By utilizing #PowerTags, along with the requisite SensorStation and Node grid, researchers aim to understand the movement and site selection of female horseshoe crabs and answer other important questions about their spawning season. The group seeks to understand more about how often females emerge on the beach during high tide, how their general movement prior to spawning might illuminate when they decide to move, and what factors inform those movement decisions. Said Quinn, "We're very interested in the correlation between egg density and when females come ashore. We want to know more about the movement of females in particular. For example, how long does it actually take them to arrive on the beach? Female crabs only come up about 20 times during May and June to lay eggs. We're curious as to exactly when she does come up–every high tide, twice a day? We know that males come up just about every single high tide. Do the females need a rest period, or do they come up multiple times in quick succession, then finish halfway through the spawning season?"

The peaks represent the times when the crabs are pinging, meaning that they came ashore. Also overlaid is the tide data to show where that lines up with high tide. The big wide peak a little before 5/24 00:00 UTC represents when CTT met with the American Littoral Society staff to begin tagging, when the PowerTags were still sitting in a box on the beach. All of the smaller and skinnier peaks shown after that are the crabs coming ashore!

There are other groups that study horseshoe crab movement. "U.S. Fish & Wildlife is the program leader. Organizations up and down the coast do this type of work, and a few universities do this as well, but by and large the Littoral Society is looking specifically at this region’s management," explained Quinn. Prior to this project, Society staff contacted a few university scientists to ensure the radio tags would not impair spawning or overall well-being. Added Shane, "The Littoral Society's program was started to gauge success with habitat restoration, which is where our funding and efforts are focused. Studying the patterns of where the crabs go helps to inform where to focus future habitat restoration efforts."

How You Can Help 

Educating the public about the horseshoe crabs' intrinsic value to the region is also key to restoring their populations. During our interview, several ALS staff offered thoughts on what misconceptions exist about this species, as well as tips for helping our chelicerate friends:

~Horseshoe crabs are NOT a risk to people. They aren’t going to hurt you. 

~Their tails do not contain a stinger.

~On a related note: don’t pick them up by their tails!

~If you see crabs on the beach laying on their back, turn them over.

~The epoxy used to adhere the radio tags onto crabs is not harmful to them. This method has been in use on turtle carapaces for many aquatic studies. Eventually, the epoxy and tag will fall off.

~Many commercial fishery employees disagree with the decline in crab population, explaining, “I see them everywhere.” The point is that the population isn’t near what it used to be historically.  In the past, the entire New Jersey coastline would be covered in horseshoe crabs, but they have largely been eliminated and the remaining populations contain very diminished numbers.

~From the advocacy side, numbers are NOT at the point where females can again be harvested.

Wendy and Rob Wilson allocated a generous donation to underwrite this project. Photo by Kelly Ball.


Many thanks to the following key staff at The American Littoral Society for helping to bring this story to life: Lindsay McNamara; Quinn Whitesall McHerron; Shane Godshall; Toni Rose Tablante; Lucia Ruggiero; and Don Riepe. 


This is what it's all about: Toni Rose of American Littoral Society holds a handful of unviable eggs. Access to these eggs during spring migration is a life-or-death proposition for migratory shorebird species like Red Knots. Photo by Kelly Ball.