Horseshoe Crabs and Red Knots Are In Peril
Across Delmarva, another bay - Delaware Bay – is a critical point for another “crab,” the horseshoe crab, a critical link to biodiversity and for migratory Red Knots and other shorebirds.
Horseshoe crabs lay millions of eggs on Delaware beaches, feeding shorebirds, fish, and other wildlife. Their large, hard shells serve as microhabitats for many other species, such as sponges, mud crabs, mussels, and snails. The world’s largest spawning population of horseshoe crabs is found in Delaware Bay.
Delaware Bay is also a critical stopover point for Red Knots on their annual migration from their wintering grounds at the southern tip of South America to their Arctic breeding area. This is one of the longest migrations in the avian world, more than 9,000 miles. Over half of the Western Atlantic flyway’s Red Knots, Ruddy Turnstones, and Semipalmated Sandpipers rely on the Delaware Bay each spring. It is the birds’ last chance to gain sufficient energy reserves for the breeding season.
Unique Ecological Relationship between Horseshoe Crabs and Red Knots
Horseshoe crabs are not really crabs; they are more related to spiders. They appeared on Earth over 400 million years ago, before dinosaurs. Today, horseshoe crabs are considered living fossils.
Males, often more than one at a time, trail behind the larger females and externally fertilize her 60,000 to 120,000 eggs, which she then buries near the high tide line. The small, green eggs are food for shore birds, fish, and other wildlife; they are critical for the migrating Red Knot.
After the surviving horseshoe crab eggs hatch, the juveniles return to the Bay. Once they mature in eight to 10 years, they come to shore again only to mate. Horseshoe crabs shed their outer shells 16 or 17 times before they become adults. Those shells, scattered on beaches throughout the summer and fall, are often mistaken for dead horseshoe crabs. They are the remains of the animals’ outgrown exoskeletons.
The Red Knot’s spring migration begins in mid-February from their wintering grounds in South America. By mid-April, they have reached southern Brazil, believed to be the last area in South America where they feed and gain weight. By the middle of May, large numbers of Red Knots arrive at Delaware Bay to refuel before completing their journey. Their arrival coincides with the spawning of horseshoe crabs on Delaware Bay’s beaches.
By the end of May or the first few days of June, the migrating birds have departed on the last leg of their journey, reaching the Arctic just as the snow is melting and abundant insect life is emerging. Once there, they have just enough time to establish nesting sites, perform courtship displays, incubate eggs, and replenish their energy reserves before their autumn migration begins again in mid-July.
Because the Arctic season is so short, there is almost no room for delay. If they have not accumulated enough fat reserves at the Delaware Bay stopover, or if the Arctic insects have not emerged until after the birds have laid their eggs, they will likely not be able to breed.
Threats
Delaware Bay’s horseshoe crabs are imperiled by overharvesting for bait and biomedical tests. Horseshoe crab blood contains copper instead of the iron that gives human blood its red color. That “blue blood” is the key ingredient in Limulus Amebocyte Lysate (LAL), used to test vaccines and medical equipment for bacterial contaminants that could be introduced during the manufacturing process. That blood, when harvested, currently goes for $60,000 a gallon.
Threats such as oil spills and other pollution, habitat loss from development or shoreline alterations to prevent erosion, along with other human disturbances, and sea level rise due to climate change are only a few of the threats to Red Knots. Competition with gulls for food and reduced food availability also affect the Red Knots’ complicated spring migration.
The population of Red Knots has dropped to the lowest level since counts began 40 years ago — fewer than 7,000, down from 90,000 in the early 1980s. The population drop this year is attributed to cold ocean temperatures which delayed the spawning of horseshoe crabs last year until early June. By then, the Red Knots and other shorebirds had already left for their breeding grounds. Because they were unable to refuel in the Delaware Bay, it is estimated that 40% of the migrating birds never arrived, lacking the energy reserves to reach the Artic.
Some Progress
Threats of declines among horseshoe crabs and their valuable eggs and the environmental threats to Red Knots and other shorebirds remain.
The Red Knots’ extinction has, so far, been prevented by regulations limiting the harvesting of horseshoe crabs for bait. The regulations moderated the steep decline of the horseshoe crab population; however, Red Knot numbers are slow to recover. This year’s setback puts the survival of the Red Knot at severe risk.
Awaiting FDA approval is Recombinant Factor C (rFC), a synthetic alternative to LAL, the “blue blood” needed for testing vaccines and medical equipment for contamination. The synthetic has been available since 2003 for vaccine and medical equipment testing but, so far, not to researchers.
Conservationists are calling for an immediate ban on harvesting horseshoe crabs for bait. Since 1996, Maryland’s commercial fishery has operated under a quota system for horseshoe crabs. In 2013, the harvest of female horseshoe crabs was prohibited, and quotas were set for males. New Jersey implemented a full moratorium on horseshoe crab bait harvest in 2008. Delaware allows the bait harvest of male crabs under a quota system.
Research to identify supplemental and artificial baits for the conch and eel fisheries is being conducted. Because horseshoe crabs do not breed before age nine or older, it may take some time before the population measurably increases.
The Atlantic States Marine Fisheries Commission, with representatives from each state on the Atlantic Coast, developed a horseshoe crab management plan, state-by-state harvest quotas, and created a 1,500 square mile Horseshoe Crab Sanctuary off the mouth of the Delaware Bay. Since 2004, a closed season during May helps to ensure that shorebirds have sufficient eggs to feed upon.
However, the Horseshoe Crab/Red Knot connection remains threatened whenever regulations are not enforced and the supply of the eggs that migrating birds depend upon is reduced.
Jessica Clark is a graduate of the University of Maryland School of Journalism. After a 30-year career as a Public Information Specialist and photojournalist for several federal government agencies in Washington, D.C., she retired to Georgetown, Delaware. She restored former Governor John Collins’ 1790s home on Collins Pond, volunteers for and promotes several nonprofits in local newspapers, teaches English as a Second Language in James H. Groves Adult High School, and is a Sussex County Master Gardener.
Common Sense for the Eastern Shore
© Common Sense for the Eastern Shore, 2025
Title image: Sunset at Eastern Neck National Wildlife Refuge, Kent Co. Photo: Jan Plotczyk
