The Science of Sustainability

Rendezvous With Horseshoe Crabs

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Horseshoe crabs are related to spiders and scorpions.

Horseshoe crabs are more closely related to spiders and scorpions than to crabs.

pdf Rendezvous with a Horseshoe Crab Educator Guide ( pdf ) A resource for using QUEST Philadelphia video in the classroom; created by PBS/NPR partner station WHYY.

DELAWARE BAY, DELAWARE – Each spring the shores of the Delaware Bay in Delaware and New Jersey host a spectacular mating event: the annual horseshoe crab spawn.

Despite their name, horseshoe crabs are not actually a crab, but more closely related to spiders and scorpions. Their bodies are covered in a stone-like shell, the size of a soccer ball cut in half. They have evolved little in the hundreds of millions of years that they have lived in the ocean, and their mating ritual could be just as ancient.

Guided by sea temperatures and lunar cycles, these prehistoric-looking animals scrape their way from the sea floor to the beach to meet their mates.

"When the high tide hits on those really good nights and you get a full moon, you'll see just thousands and thousands of crabs," said Jim Hewes, a science teacher at Smyrna Middle School, in Delaware.

Jim Hewes, science teacher and horseshoe crab enthusiast.

Jim Hewes, science teacher and horseshoe crab enthusiast.

It would be an understatement to call Hewes a horseshoe crab enthusiast.

Each year he sacrifices sleep to head to the beach for midnight horseshoe crab counts as a volunteer with the Delaware Department of Natural Resources and Environmental Control. This year, Hewes led a small crew of volunteers on Kitts Hummock Beach, near the town of Dover.

They walk in regular paces for about a mile stretch of beach, stopping periodically to count the number of crabs within a square meter chunk of shoreline.

“We're counting the horseshoe crabs because for a long time we've been seeing a decline in the population,” Hewes explained.

Horseshoe crabs crowd onto the beach at night.

Horseshoe crabs crowd onto the beach at night.

Conch and eel love the taste of a horseshoe crab. For years, fishermen could grab as many horseshoes as they could handle, chop them in half, and drop them into a cage to trap conch and eel.

But the harvest had unintended consequences. Shore birds – who travel with bewildering accuracy from their wintering grounds in South America to show up exactly during the horseshoe crab spawn – rely on the eggs the crabs lay during their mating frenzy.

The birds “come here completely emaciated and really need to work hard to put on a lot of weight," said Kevin Kalasz, a wildlife biologist with the Delaware Division of Fish and Wildlife, "and horseshoe crab eggs are the best food source for them.”

The birds need to bulk up so they can make the final leg of their migration up to the Arctic, where they lay their own eggs.

About 20 years ago, shore bird numbers, particularly of one species called the red knot, dropped dramatically.

“Red knots have declined approximately 80 to 90 percent in the last 20 years," said Kalasz. "They're currently considered a candidate species under the Endangered Species Act."

Kalasz said that the presumption was that horseshoe crab harvests limited the number of eggs available for red knots to eat. State governments responded by putting limits on the number of horseshoe crabs harvesters could take, but it's still too early to tell whether the harvest policies have impacted the bird populations, he said.

Hewes is optimistic that horseshoe crabs are beginning to rebound. As he stands knee-deep in a pile of horseshoes, he counts the animals and his colleague records the sample.

“So far I'm seeing a lot of good numbers,” he said.

For Hewes, the surveys are not just part of helping to protect the horseshoe crabs and the birds that depend on them. It's also an effort to preserve something that makes Delaware special.

“It's one of the littlest states around, and we've got the biggest numbers of horseshoe crabs,” he said. “It doesn't happen anywhere else like this in the world.”

Watch the Science on the SPOT story on horseshoe crabs:

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Category: Biodiversity, Biology, Environment, Video

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Kerry Grens

About the Author ()

Kerry Grens has been a health and science reporter at WHYY since 2007, filing stories for radio, tv and web. Her radio career began through a AAAS fellowship at KUNC in Colorado, and Kerry has since worked as a reporter at New Hampshire Public Radio and The Scientist magazine. Her stories have appeared on NPR, Marketplace, Voice of America and in Nature magazine, and she is a regular contributor to Reuters Health. Kerry's reporting has earned numerous awards from state chapters of the Society of Professional Journalists and the AP Broadcasters Association. Kerry has a bachelors degree in biology from Loyola University Chicago and a master's degree in biological sciences from Stanford University.