Bay Journal

Migratory birds shore up appetites on horseshoe crab eggs

  • By Kathy Reshetiloff on May 18, 2017
Horseshoe crabs emerge from the waters of Delaware Bay to mate and lay their eggs. (Gregory Breese / U.S. Fish and Wildlife Service) Rufa red knots must leave Delaware Bay with enough energy reserves to make the trip to the Arctic and survive without food until well after they have laid their eggs. (Gregory Breese / USFWS) A single horseshoe crab may lay 100,000 eggs or more in a season.  (Gregory Breese / USFWS)

Timing is everything. Each spring, shorebirds migrate from wintering grounds in South America to breeding grounds in the Arctic. These birds have some of the longest migrations known.

Delaware Bay is the prime stopover site and the birds’ stop coincides with horseshoe crab spawning. Shorebirds like the red knot, ruddy turnstone and semipalmated sandpiper, as well as many others, rely on horseshoe crab eggs to replenish their energy reserves before heading to their Arctic nesting grounds.

The birds arrive in the Arctic before insects emerge. This means that they must leave Delaware Bay with enough energy reserves to make the trip to the Arctic and survive without food until well after they have laid their eggs. If they have not accumulated enough fat reserves at the bay, they may not be able to breed.

The world’s largest spawning population of horseshoe crabs occurs in Delaware Bay. 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. A single crab may lay 100,000 eggs or more during a season.

Horseshoe crab spawning begins in late April and runs through mid-August, although peak spawning in the mid-Atlantic takes place May 1 through the first week of June.

At low tide, adult crabs go back into the water but may return at the next high tide. Horseshoe crab spawning increases on nights with a full or new moon, when gravity is stronger and high tides are even higher.

At the same time that migrating shorebirds arrive to rest and feed along Delaware Bay, horseshoe crab activity is high. While the crab buries its eggs deeper than shorebirds can reach, waves and other horseshoe crabs expose large numbers of eggs. These surface eggs will not survive, but they provide food for many animals. The shorebirds can easily feed on eggs that have surfaced prematurely.

Coastal states have taken measures to ensure that horseshoe crabs remain an integral part of this coastal ecosystem. The National Marine Fisheries Service established the Dr. Carl N. Shuster, Jr. Horseshoe Crab Sanctuary in federal waters off the mouth of Delaware Bay.

The rectangular-shape sanctuary protects the large spawning population of horseshoe crabs in the Delaware Bay and maintains the superabundance of crab eggs available to migratory shorebirds. It is bounded by Delaware waters to the west and extends 30 miles to the east, to just south of Atlantic City, NJ, and just north of Ocean City, MD. No commercial harvesting of horseshoe crabs is allowed within sanctuary waters. In addition, the mid-Atlantic states restrict the harvesting of horseshoe crabs during specific times of the year.

The protection of sandy beaches is essential for spawning horseshoe crabs and migrating shorebirds. Shoreline erosion control structures used to protect property, like bulkheads and rip rap, can block access to spawning beaches, eliminate sandy beach habitat or trap and strand spawning crabs during times of high wave energy.

This spectacular yearly event also brings people to the beaches. The best time to observe shorebirds eating horseshoe crab eggs at Delaware Bay is the last half of May. Remember that if you approach these birds too closely on the beach, they will take flight and move to other areas. This uses energy that they need to build up their fat reserves for the long journey ahead.

When using the beach, keep a safe distance so the birds do not stop feeding and take flight. View them from a distance with binoculars or spotting scopes. Keep dogs on a leash and cats indoors. Limit motor vehicle use on the beach to designated areas only.

To learn about shorebirds and viewing locations contact:

Rufa Red Knot (Calidris canutus rufa)

  • Is one of six subspecies of redknot; the rufa red knot winters at the tip of South America in northern Brazil, throughout the Caribbean and along the U.S. coasts from Texas to North Carolina.
  • Breeds in the Canadian Arctic.
  • Travels close to 20,000 miles round trip per year.
  • Makes non-stop flights of 2,000 miles or more.
  • Doubles its weight in two weeks. More than 90 percent of its diet in this time period is horseshoe crab eggs.
  • Its population has fallen by about 75 percent since the 1980s, largely the result of the overharvesting of horseshoe crabs, changes to shoreline habitats and coastal development. It is designated as threatened under the Endangered Species Act, meaning it is at risk of becoming endangered throughout all or a significant portion of its range.

Horseshoe crab (Limulus polyphemus)

  • Named for its distinctive horseshoe shape, armored body.
  • Has five pairs of legs and a long, pointed tail.
  • Is actually related to spiders and has changed very little over the past 360 million years.
  • Is found along the Atlantic Coast from Maine to the Yucatan Peninsula.
  • Has several pairs of eyes. Two on the head are sensitive to polarized light and can magnify sunlight 10 times. Another pair of simple eyes on the head can sense ultraviolet light from the moon. Multiple eye spots are also found on the underside of the head and tail
  • A byproduct of horseshoe crab blood, Limulus Amoebocyte Lysate (LAL), is used to test the sterility of many medical devices.

About Kathy Reshetiloff

Kathryn Reshetiloff is with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office in Annapolis.

Read more articles by Kathy Reshetiloff

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