What has an armored body, five pairs of legs, a long, pointed tail, is related to spiders and has changed very little over the past 360 million years? The horseshoe crab!
Named for its distinctive horseshoe-shaped body, the horseshoe crab (Limulus polyphemus) is found along the Atlantic Coast from Maine to the Yucatan peninsula. This fierce-looking yet harmless creature is valuable both ecologically and economically. Horseshoe crab eggs are an important food for shorebirds migrating along the Atlantic Coast. Horseshoe crabs are also used as bait in eel, whelk and catfish fisheries. Their importance doesn't end there. Horseshoe crabs are also used extensively for medical research.
Each spring, shorebirds migrate from wintering grounds in South America to breeding grounds in the Arctic. The birds must stop along the way to feed, refueling themselves so they can complete their journey north. Delaware Bay is the prime stopover site and the birds' stop coincides with horseshoe crab spawning.
Spawning begins in late April and goes through mid-August, although peak spawning occurs late May to late June. Horseshoe crab spawning increases on those nights with a full or new moon when gravity is stronger and high tides are higher.
During high tide, horseshoe crabs migrate from deep water to beaches to spawn. The female digs a nest in the sand and deposits between 4,000 and 30,000 eggs that the male will fertilize with sperm. At low tide, adult crabs go back into water but may return again at the next high tide.
Horseshoe crab spawning activity is so intense at the time when migrating shorebirds arrive along Delaware Bay that many nests are disturbed and the eggs surface. Although the beaks of most shorebirds are too short to penetrate horseshoe crab nests, the birds can easily feed on those eggs that have surfaced prematurely.
In the past, horseshoe crabs were harvested for fertilizers and even as food for chickens and livestock. Currently, horseshoe crabs are used as bait for eels, whelks and catfish. As traditional fisheries, like oysters, have declined, watermen have turned to these other resources, creating a demand and market for horseshoe crabs as bait.
Trawling is the primary method used to harvest horseshoe crabs. A ban on trawling in Virginia has decreased that state's harvest of horseshoe crabs. But Maryland ships large numbers of horseshoe crabs to Virginia for its whelk fishery.
Horseshoe crabs have become an integral part of the medical industry. Horseshoe crab blood clots when exposed to endotoxins, poisons released by certain bacteria. A byproduct of horseshoe crab blood, Limulus Amoebocyte Lysate (LAL), is used to test the sterility of injectable medicines, dentistry instruments, needles and even heart valves.
Horseshoe crabs are caught, checked for health and bled using a stainless steel tube inserted into the animal's circulatory system. About 20 percent of a horseshoe crab's blood is collected in six to seven minutes. The animal is then returned to the water as required by the Food and Drug Administration.
About 200,000 horseshoe crabs are bled each year. Mortality of bled crabs averages about 10 percent. LAL is used internationally, and is more accurate, simpler and less expensive than similar tests.
Ninety percent of the horseshoe crab stock is located between Virginia and New Jersey. Since 1987, the commercial harvest of horseshoe crabs along the Atlantic coast has increased. From 1990 to 1994, the National Marine Fisheries Service reported the harvest of horseshoe crabs increasing from 685,648 pounds to 1,386,367 for the states of Delaware, Maryland and New Jersey combined. At the same time, a horseshoe crab spawning survey, conducted in Delaware Bay since 1990, indicates that the number of spawning adults has declined.
Problems exist with using spawning surveys as an indicator of abundance because they do not account for annual variations in peak spawning and abundance, and the number of beaches surveyed each time also varies. Current procedures for extrapolated data assume that horseshoe crabs are randomly distributed along beaches, while data from Chesapeake Bay indicate that they are not.
The stock in Chesapeake Bay and along the Maryland and Virginia portions of the coast is unclear. Both states have taken steps to find out more about this ancient creature. In 1994, Maryland began a horseshoe spawning survey to identify and document spawning areas. Reports of spawning are received on a Horseshoe Crab Hotline (410) 974-2241 and volunteers perform the survey.
So far, spawning areas have been located from the Chester River (north of the Chesapeake Bay Bridge) south to the coastal bays. Maryland also tags horseshoe crabs and anyone finding a tagged crab is asked to call the hotline and report the tag numbers. Ongoing research is looking at how environmental factors affect when and where the crabs spawn. In Virginia, commercial fisherman are required to report horseshoe crabs harvested, though most are caught largely as bycatch in other fisheries.
Both states have also taken steps that help to protect horseshoe crabs, especially while spawning. Most of the measures affect trawling.
In Maryland trawling, scraping and dredging is prohibited between April 1 and June 30 within Chesapeake Bay, coastal bays and within one mile of Atlantic Ocean. Maryland also restricts hand collection on beaches to Mondays and Thursdays during this time.
In Virginia, trawling is banned completely within state waters and within 3 miles of coast. Commercial harvest of horseshoe crabs requires a license in Virginia and recreational harvest, mainly for bait, is limited to 5 horseshoe crabs per person per day.
Together with other protection and surveying efforts in Delaware, New Jersey and other Atlantic Coast states, scientists will develop sound management practices that will assure that these ancient animals continue in their ecological and economical roles.
Kathryn Reshetiloff is with the U.S. Fish & Wildlife Service's Chesapeake Bay Field Office in Annapolis. Information for this article was generously provided by Maryland Department of Natural Resources and Virginia Marine Resources Commission.