What has an armored body; five pairs of legs; a long, pointed tail; is related to spiders; and has changed very little over the last 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.
The world's largest spawning population of horseshoe crabs is found in Delaware Bay. Horseshoe crab spawning begins in late April and goes through mid-August, although peak spawning in the mid-Atlantic is from May 1 through the first week of June.
During high tide, horseshoe crabs migrate from deep water to beaches to spawn. The female digs a nest in the sand and deposits 4,000–30,000 eggs that the male will fertilize with sperm. A single crab may lay 100,000 eggs or more during a season.
At low tide, adult crabs go back into the water but may return again 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 higher.
This fierce-looking yet harmless creature is critical to the survival of migrating shorebirds. Each spring, shorebirds migrate from wintering grounds in South America to breeding grounds in the Arctic. These remarkable 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 such as 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 the birds must leave Delaware Bay with enough energy reserves to not only make the trip to the Arctic but survive without food until well after they have laid their eggs. If they have not accumulated enough fat reserves at Delaware Bay, they may not be able to breed.
At the same time that migrating shorebirds arrive to rest and feed along Delaware Bay, horseshoe crab activity is high. While the crab buries these eggs deeper than shorebirds can reach, waves and other horseshoe crabs expose large numbers of eggs. These prematurely surfaced eggs will not survive, but provide food for many animals, including the shorebirds.
Horseshoe crabs have a commercial role. In the past, horseshoe crabs were harvested for fertilizers and even as food for chickens and livestock. Today, horseshoe crabs are used as bait for eels, whelks and catfish.
Horseshoe crabs have also become an integral part of the medical industry. A byproduct of horseshoe crab blood, Limulus Amoebocyte Lysate, is used to test the sterility of injectable medicines, dentistry instruments, needles and even heart valves. The biomedical community collects up to 30 percent of an individual horseshoe crab's blood, then the crab is returned to the water.
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 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.
Protection of sandy beaches is essential for spawning horseshoe crabs and migrating shorebirds. Shoreline erosion control structures used to protect property, like bulkheads and riprap, can block access to spawning beaches, eliminate sandy beach habitat or entrap and strand spawning crabs during times of high wave energy.
This spectacular yearly event also brings people to the beaches. Remember to keep a safe distance. If these birds are approached too closely on the beach, they take ﬂight and move to other areas. This uses energy that they need to build up their fat reserves for the long journey ahead. It's best to view them with binoculars or spotting scopes.
Keep dogs on a leash and cats indoors. And, limit motor vehicle use on the beach to designated areas only.
The best time to observe shorebirds feeding on horseshoe crab eggs at Delaware Bay is the last half of May. For information and viewing locations contact:
- Delaware Department of Natural Resources & Environmental Control: 302-739-9902, www.dnrec.delaware.gov/Pages/Portal.aspx
- Cape May Bird Observatory: 609-884-2736, www.birdcapemay.org/
- Bombay Hook National Wildlife Refuge: 302-653-6872,www.fws.gov/northeast/bombayhook/index.html
- Prime Hook National Wildlife Refuge: 302-684-8419, www.fws.gov/northeast/primehook/
- Cape May National Wildlife Refuge: 609-463-0994, www.fws.gov/northeast/capemay/
- Delaware Bay Estuary Project: 302-653-9152 www.fws.gov/delawarebay/
Horseshoe Crab Survey
The Delaware National Estuarine Research Reserve is looking for volunteers to survey horseshoe crabs in May and June. Contact the Horseshoe Crab Spawning Survey Coordinators at DNERRhsc@gmail.com, or call 302-739-NERR for details.