Archive of Spotlight Feature Articles

The Horseshoe Crab
-- Putting Science to Work to Help "Man's Best Friend" --

University of Delaware
Sea Grant College Program

The largest population of horseshoe crabs in the world lives in Delaware Bay. This ancient animal plays a critical role in the bay's ecology and in human medicine. The crab's eggs fuel shorebirds migrating north to Arctic nesting grounds each spring. A compound in the crab's blood is used by the pharmaceutical industry to test intravenous drugs, as well as prosthetics, for bacteria. There is concern, however, that the Delaware Bay's horseshoe crab population is declining, and Delaware Sea Grant scientists are working to stem the tide.

Dog and cat lovers continually debate about which animal is man's best friend. But ask Bill Hall, marine education specialist for the University of Delaware Sea Grant College Program, his opinion, and he will tell you that the hands-down winner is the horseshoe crab. While few may know it, this prehistoric creature with the helmet-shaped body and spear-like tail has saved countless human lives.

"Horseshoe crabs are critical to the welfare of migrating shorebirds that stop along the Delaware Bay each spring to fuel up for the flight north to Arctic nesting grounds. Some of these birds double and even triple their weight by feasting on horseshoe crab eggs," Hall says.

"Yet the horseshoe crab is just as important to humans as it is to wildlife," he notes. "This animal's blood contains a unique clotting agent that the pharmaceutical industry uses to test intravenous drugs for bacteria. No IV drug reaches your hospital pharmacy without its horseshoe crab test. So if you or someone you love has ever been hospitalized, you owe a lot to the horseshoe crab."

Hall helps organize a regional census of the Delaware Bay's spawning horseshoe crab population. Now in its tenth year, the census is conducted on selected bay beaches each May and June by volunteers from Delaware and New Jersey. This year, the census will be expanded to additional beaches along both sides of the bay with support from the Atlantic States Marine Fisheries Commission.

"Delaware Bay is the world's population center for horseshoe crabs, but during the past few years, we've noted a significant downturn in the animal's population, from 1.2 million spawning females down to about 400,000," Hall says.

"Scientists believe the decline is due to overfishing of the crab for eel and conch bait and to the loss of the sandy beaches it needs for spawning," he notes. "The census is designed to help resource managers and scientists gain a better understanding of the horseshoe crab's status and what we can do to guard our 'golden goose.' "

Developing an Artificial Bait to Reduce Fishing Pressure On the Horseshoe Crab

In Sea Grant research at the University of Delaware, marine biologist Nancy Targett has been working to minimize fishing pressure on the horseshoe crab through biochemistry. She has made significant progress toward identifying the stimulant in female horseshoe crabs that makes them such an irresistible bait for eels and whelks. With this information, she wants to develop an artificial bait that will attract eels and whelks just as well as female horseshoe crabs do.

Targett and her graduate students are in the home stretch in chemically characterizing the attractant. The next step will be to incorporate the compound into a variety of artificial bait types and test their effectiveness. Several commercial fishermen in Delaware have contacted Targett, offering to test the baits when they are ready.

"The fishing industry is very supportive of this effort," Targett says. "With their help, our goal is to develop an artificial horseshoe crab bait that will work as well as the traditional one. The result should be a win-win situation for the fishermen as well as the horseshoe crab, resulting in more horseshoe crabs for spawning and sustainable uses in medicine."

Horseshoe Crab Facts Although called a "crab," the horseshoe crab is actually closely related to spiders and scorpions. It is also one of the Earth's oldest creatures, having appeared here 100 million years before the dinosaurs. Scientists have learned a lot about the human eye by studying the horseshoe crab's large compound eyes.The horseshoe crab also has numerous smaller eyes called photoreceptors on its top shell and along its tail. Sensitive to light, they help synchronize the crab's internal clock with daily cycles of light and darkness. The horseshoe crab's sword-like tail, called the telson, isn't poisonous as some people believe. When the crab has been overturned, it simply uses its tail as a lever to flip itself to an upright position.

Horseshoe Crab Model Connects with Kids

Outreach specialists at the University of Delaware Sea Grant College Program have spearheaded a number of efforts to educate the public about the horseshoe crab, from a public forum to set research and management priorities, to public service announcements in the program's long-running SeaTalk radio series. Listen to 60-second audio clip on horseshoe crab census.

The Horseshoe Crab Model, a life-sized paper model that can be cut out and taped together, also has been developed to help teach the public, especially youngsters, about the crab.

Developed with funding from the Delaware Department of Education, more than 25,000 copies of the model have been distributed to Delaware teachers and their students and a variety of other audiences in the U.S. and abroad.

After her fifth-grade science class tackled the model, Mrs. Sheila Anderson, a teacher at St. John the Beloved School in Wilmington, Delaware, wrote:

"Thought you might like to see the fruits of your labors (see photo). The horseshoe crabs were a great hit! We're studying classification of invertebrates and they fit right into science as well as 'across the curriculum' into our study of Indians in Delaware. We learned they used the shells to scoop water out of canoes and the tails to spear fish. Thanks for a great project!"

In addition to the model parts, the eight-page publication provides information about the horseshoe crab's ecological and economic importance along with a crossword puzzle to test readers' knowledge.

The Horseshoe Crab Model has been so popular that Delaware Sea Grant is now preparing a second printing of 30,000 copies. As a teacher aid, the second edition will include information on how the classroom activity meets national and state science education standards.

A four-page educational bulletin about the horseshoe crab also is available from Delaware Sea Grant. For more information, contact the University of Delaware Marine Communications Office at MarineCom@udel.edu.

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