They look like props from the movie Jurassic Park - but these ancient organisms were around long before the dinosaurs. Some of their closest evolutionary relatives have been extinct for hundreds of millions of years. But while countless other species have come and gone, the horseshoe crab has survived and is of great importance.
At the end of May and during June, hundreds of thousands of these prehistoric creatures emerge from the waters of Delaware Bay to lay and fertilize their eggs in the wet sand. On some beaches they will be met by scientists and volunteers who will carefully count their numbers across a series of sampling plots.
The census was designed by USGS scientists in cooperation with researchers from the U.S. Fish and Wildlife Service, several state management agencies, a biomedical company, and several universities to determine the size and status of the Delaware Bay horseshoe crab population. This information is crucial because today these ancient organisms lie at the center of a far-reaching network of ecological, medical and economic relationships. The census is a project of the Atlantic States Marine Fisheries Commission, a multistate and federal partnership.
The horseshoe crabs of Delaware Bay provide a critical food source for about a million migratory shorebirds that pass through the region every spring. After spending the winter in South America, species such as the red knot, ruddy turnstone, semipalmated sandpiper and sanderling feast on protein-rich horseshoe crab eggs before moving on to their Arctic breeding grounds. For most of these birds, Delaware Bay is the most important stopover on a migratory journey of up to 10,000 miles.
Though horseshoe crabs are found along the Atlantic coast from Yucatan to Maine, the Delaware Bay population is the world's largest. USGS biologist and statistician Dr. David Smith, of the Leetown Science Center, says the bay's long stretches of protected, sandy beaches provide ideal habitat for spawning. "What has evolved is a dependence of migratory shorebirds on the superabundance of horseshoe crab eggs in Delaware Bay," Dr. Smith says.
But in recent years declines in horseshoe crab numbers have been reported from a number of locations. The evidence is spotty, and until recently no systematic horseshoe crab surveys have been conducted. But biologists say there are numerous reasons for concern: commercial harvesting of horseshoe crabs is increasing, and much of the sandy beach habitat needed for spawning has been lost.
Evidence also suggests that red knots and other migratory shorebirds are in serious decline, though the causes remain unknown. Biologists want to know if these trends are related to reductions in horseshoe crab eggs available to the migrating birds. "It's clear that we need to know more about horseshoe crab status and trends," says Dr. Smith.
A Vital Link
Despite their name, horseshoe crabs are more closely related to spiders and scorpions than to true crabs. In the early spring, as shorebirds wintering in South America begin preparing themselves for northward migration and breeding, horseshoe crabs along the Atlantic coast of North America emerge from their muddy beds on the continental shelf and begin their own journey towards shallow waters.
Birds and horseshoe crabs by the millions converge on the bay beaches of Delaware, Maryland and New Jersey. Horseshoe crabs gather in massive numbers just offshore, often waiting for the highest tides -- associated with the full or new moon -- to crawl up on the beach and lay their eggs. During these mass spawning events crab densities may exceed 100,000 per mile of beach, with each female laying thousands of eggs.
Egg-laying continues to a lesser extent at other evening high tides throughout May and early June. "Some crabs are spawning any time the tide is up," says Gregory Breese, a biologist with the FWS Delaware Bay Estuary Project. Shorebirds, said Breese, mainly eat the eggs when horseshoe activity is highest. Horseshoe crabs bury clusters of pale green eggs in the sand, but during mass spawnings vast numbers of eggs get churned up to the surface where they are eaten by the birds.
Breese says some shorebirds may journey non-stop from as far away as southern Argentina before arriving at Delaware Bay. With their energy stores exhausted, the birds gorge themselves on the rich, abundant food source. In less than two weeks they may more than double their body weight, allowing them to continue their journey to the far north.
Shorebirds aren't the only organisms for which horseshoe crabs and their eggs are a vital food resource. Many small fish feed on the countless eggs that are washed out into the bay, and later on the hatched larvae. Juveniles of larger ocean fish, which use the protected waters of Delaware Bay as a giant nursery, also consume eggs and larvae. Juvenile and adult horseshoe crabs are a dietary staple for the threatened loggerhead turtle.
Horseshoe crabs have long been important to humans as well. Their relatively simple biology has made them a popular organism for basic biomedical research. Important early advances in understanding human vision and neurobiology were made through studies of the horseshoe crab's primitive eyes and long optic nerve.
Today a multi-million dollar industry exists around the collection of horseshoe crab blood. A clotting agent called Limulus Amoebocyte Lysate (LAL) is extracted from the blood, and used to test for the presence of gram-negative bacteria. Human blood and all commercially produced intravenous drugs are tested for bacterial contamination using LAL.
Blood extraction from horseshoe crabs is usually non-lethal, and thus the pharmaceutical industry is not thought to be a major factor affecting crab populations. Of much greater concern to researchers and managers is the increased harvesting of horseshoe crabs for use as bait in American eel and whelk fisheries.
University of Delaware biologist Dr. Bill Hall says the high density of horseshoe crabs in the Delaware Bay region makes harvesting them for bait cost-effective. "This is one of the few places where it's economical to fish for them," he says. Concern over the possible effects of this harvest prompted Dr. Hall to join with Benjie Swan of Limuli Laboratories and Dr. Carl Shuster of the College of William and Mary to organize a volunteer-based survey of horseshoe crabs spawning on Delaware Bay beaches in 1990.
"There is much concern about the harvest," says Dr. Smith. "There's enough data to suggest that the number of crabs spawning in the bay has dropped recently, enough that shorebirds may be threatened." But though the volunteer surveys provided some indication of a decline, the project was not initially designed to provide a valid statistical sample of the entire Delaware Bay population.
Commercial harvesting isn't the only factor that may be putting the squeeze on horseshoe crabs. Delaware Bay's miles of sandy beaches are steadily shrinking. "There's some pretty high rates of shoreline retreat and beach loss due to erosion and apparent sea level rise," Breese says. Development and shoreline hardening can also eliminate horseshoe crab habitat.
USGS and other biologists note that if the horseshoe crab population were to become seriously depleted, recovery might require decades. Though prolific in terms of egg production, horseshoe crabs are long-lived animals which do not breed until they reach nine or 10 years of age.
A Cooperative Response
With concerns over horseshoe crabs mounting, ecologist Dr. Joe Margraf and colleagues at the USGS Maryland Cooperative Fish and Wildlife Research Unit began a study in 1996 to determine how horseshoe crab use of spawning beaches varies through time and space. Their goal was to develop a rigorous sampling method for use in population monitoring. The researchers found that, to be most representative and consistent, surveys should be concentrated in the shallowest waters and should be carried out on the higher of the two daily high tides, immediately following a new or full moon.
In 1998, the Atlantic States Marine Fisheries Commission implemented a management plan for the species along the Atlantic Coast. Adopted last October, the plan calls for the collection of information necessary to manage the horseshoe crab fishery and to insure protection for the diverse animals that rely on horseshoe crabs and their eggs for food. The plan also specifically mandates that the bay states of New Jersey, Delaware and Maryland develop and implement a statistically sound methods for monitoring the status of horseshoe crab populations.
This interstate management plan led to the current cooperative research program involving USGS scientists, resource managers from the three states, and outside researchers. Under Dr. Smith's guidance a new, statistically sound sampling protocol is being implemented, which will help researchers determine the true size of the population and keep track of any downward trends. "Starting this year, we'll be getting a much better handle on variability in horseshoe crab numbers," says Breese.
"Our current project builds on the volunteer-based survey and incorporates the recommendations of Dr. Margraf and his coworkers," Dr. Smith says. "We're focusing now on larger spatial and temporal scales -- how best to select beaches, and how often beaches should be sampled to monitor the bay-wide population."
Another component of the cooperative effort -- funded in part through the USGS State Partnership program -- is to assess genetic variability in horseshoe crabs across their geographic range. Investigators are confident that unique DNA "markers" can be identified that will allow them to determine the distribution and movement patterns of different horseshoe crab populations. Fishery managers can use this information to monitor fishing pressures on specific populations.
Currently, says Dr. Smith, researchers do not know whether horseshoe crabs taken by trawlers farther out on the continental shelf belong to the Delaware Bay breeding population. "It's not clear 'whose' crabs they're taking out there," he says. "Are they from Delaware Bay or somewhere else?" The answer -- which the genetic studies should provide -- will affect how horseshoe crabs are managed in the Delaware Bay region.
Peter Himchak, of the New Jersey Division of Fish, Game and Wildlife, says the new horseshoe crab census is an ambitious undertaking, which will require extensive volunteer support. Eight beaches on both sides of the bay have been chosen for sampling. Teams of volunteers and staff of natural resource agencies will count adult horseshoe crabs along established transects during evening high tides. Biologists will also conduct sample counts of horseshoe crab eggs. Dr. Bob Loveland of Rutgers University and Dr. Mark Botton of Fordham University will lead the effort to measure spawning success by counting live eggs deposited in the sand by female horseshoe crabs at the same beaches where the crab surveys take place.
Researchers say the expanded census program is building on the success of previous volunteer surveys. Dr. Hall continues to coordinate the survey effort. "The census attracts a lot of dedicated volunteers, people who are not biologists," says Dr. Smith. "It's great to see this kind of interest."
Additional volunteers are needed to assist biologists in counting horseshoe crabs during night-time high tides. Interested persons should contact Ms. Benjie Swan at (609) 465-6552 or Dr. Bill Hall at (302) 645-4253, [email protected]. More information about the project can be found online at http://ael.er.usgs.gov/groups/stats/Limulus/
The horseshoe crab research is one of several projects supported this year under the USGS State Partnership Program. Cooperators include the New Jersey Division of Fish, Game and Wildlife; the Delaware Division of Fish and Wildlife; and the Maryland Department of Natural Resources. The State Partnership Program is a competitive funding initiative that seeks to foster collaboration among scientists and resource managers from States, Tribes, the USGS, and other Department of Interior agencies.
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