Citizen Scientists Help Track Delaware Bay Horseshoe Crab Population


As horseshoe crabs, with their helmet-like shells and long tails, glided gracefully across the water, volunteers groped up and down the beach in rain boots or bare feet. They called out males and females, many of whom were preoccupied with their annual mating ritual.

“Oh, that’s neat. I love it. It’s fun,” said first-time volunteer Amy Seaman of Annapolis, Maryland, who runs a “Save the Horseshoe Crabs” Facebook page. “I saw them all mating in the water all together like that. I’ve never seen so many at once.”

Volunteer Amy Seaman takes photos and videos for her Save the Horsehoe Crab Facebook page. (Emma Lee/WHY)

Seaman and the rest of the night crew were part of a volunteer program the Delaware Center for the Inland Bays has run every spring since 2008, attracting around 100 volunteers a year. Their objective: to count, catch and tag the horseshoes, in order to follow their population level.

“The horseshoe crabs are hard not to like. They’re very charismatic. They’re really interesting,” said Nivette Pérez-Pérez, community science manager for the Delaware Center for the Inland Bays.

More importantly, horseshoe crabs play a crucial role in the watershed ecosystem. Their eggs serve as a food source for dozens of species, including the endangered Red Knot – which depends on the eggs to replenish during their annual stopover from South America to the Arctic, where they end up by landing and mating.

Horseshoe crab eggs are an important food source for many species of birds and fish in Indian River Inlet. (Emma Lee/WHY)

A living fossil under threat

Conservationists like Pérez-Pérez have reason to worry: Red knot populations fell 75% between the 1980s and 2000s, according to the US Fish & Wildlife Service. That’s largely because of a drop in the supply of horseshoe crab eggs, conservationists say.

In the 1990s, populations of horseshoe crabs reached devastating low levels as fishermen used them as bait to catch eel, whelk and conch.

Since then, efforts have been made to limit the number of horseshoe crabs allowed to be harvested. The Atlantic States Marine Fisheries Commission, which coordinates the conservation and management of fish species on the Atlantic coast, sets quotas for the amount of horseshoe crabs that can be harvested as bait. This effort has helped stabilize the horseshoe crab and red knot populations to some extent, but their numbers are still far from historic levels.

Horseshoe crabs are also harvested for biomedical research. Their bright blue blood contains a compound used by medical researchers to test the sterility of injectable drugs, biomedical devices and vaccines. Their blood also contributed to the development of the COVID-19 vaccine.

Volunteers Dan Duffey and Karen Longo dive into the murky water of Indian River Inlet to count the number of male and female horseshoe crabs in a square meter during a population survey at James Farm Ecological Preserve. (Emma Lee/WHY)

Although horseshoe crabs are released after being bled by lab technicians, it is believed that 15-30% of those used for biomedical purposes eventually die. It is estimated that approximately 500,000 crabs are harvested along the Atlantic coast for biomedical purposes. The Atlantic States Marine Fisheries Commission does not issue quotas for this purpose.

Horseshoe crab fishing regulations vary from state to state: New Jersey has a moratorium on horseshoe crab fishing, but has a biomedical use exemption. Delaware does not have a moratorium on bait harvesting, but prohibits harvesting for biomedical purposes.

Horseshoe crabs are also threatened by development, pollution and climate change. All of these threats combined underscore the importance of survey work, said Dennis Bartow, senior naturalist with the Delaware Center for the Inland Bays.

“I’d like to see them continue through the sixth mass extinction, which could include us,” he said. “And with climate change, our environment is changing, our sea level is going up, it’s going to submerge our beaches, it’s going to flood our marshes, and their habitat is going to disappear. So we have to be aware of their population.

Horseshoe crabs spawn along the shore of the Indian River entrance to James Farm Ecological Preserve in Ocean View, Delaware. (Emma Lee/WHY)

Bartow and other conservationists advocated for the use of artificial horseshoe crab blood, which was developed. However, its use has not been considered in the United States

Bartow also thinks the use of artificial bait would benefit horseshoe crab populations. Other conservationists say artificial bait doesn’t work as well, but clam bait could be used.

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