Ancient Creature Lurks Along Coast

What has an armored body, 5 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 Yucantan 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.

Horseshoe crab spawning begins in late April and goes through mid August, peaking in late May to June. During high tide, horseshoe crabs migrate from deep water to beaches to spawn. The female digs a nest in the sand and deposits up to 30,000 eggs that the male will fertilize. At low tide adult crabs go back into water. Horseshoe crab spawning increases on nights with a full or new moon when gravity is stronger and high tides are higher.

At the same time, migrating shorebirds arrive to rest and feed along the shores. Horseshoe crab spawning activity is so high 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 fertilizer and 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.

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.

The population of horseshoe crabs in Chesapeake Bay and along the Maryland and Virginia portions of the coast is unclear. Both states have also taken steps that help to protect horseshoe crabs, especially while spawning. Trawling is the primary method used to harvest horseshoe crabs. In Maryland trawling, scraping and dredging is prohibited between April 1 and June 30 within Chesapeake Bay, coastal bays and 1 mile of Atlantic Ocean. Maryland restricts hand collection from beaches. 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 are developing sound management practices to insure that these ancient animals continue in their ecological and economical roles in along the Atlantic Coast, Delaware Bay and Chesapeake Bay.

Back to CBFO home page

 

Shorebirds feasting on horseshoe crab eggs. Photo by Sheila Eyler, USFWS
Shorebirds feasting on horseshoe crab eggs.
Photo by Sheila Eyler, USFWS

Crab eggs are about the size of a BB. Photo by Greg Breese, USFWS
Crab eggs are about the size of a BB.
Photo by Greg Breese, USFWS

Horsehoe crabs spawning on a Delaware beach. Photo by Greg Breese. USFWS

Horsehoe crabs spawning on a Delaware beach.
Photo by Greg Breese. USFWS