Pictured Above: “Horseshoe Crab Family Gathering” by Willy Conley, submitted to the 2023 Maryland DNR Photo Contest.
Thousands of horseshoe crabs (Limulus polyphemus) have started landing on Maryland’s shores this spring for what is believed to be the world’s oldest wildlife migration. Dating back an estimated 350 million years, the annual spawning occurs from May through July, peaking with high tides on or around the full and new moons in June.
One spawning female horseshoe crab will deposit an average of 20,000 eggs into the sand, with a mate grasping onto her shell and fertilizing the eggs. In addition to perpetuating the ancient species, the eggs are a seasonal buffet for migratory shorebirds preparing to return to their summer nesting grounds in northern Canada.
Horseshoe crab larvae are also an important food source for juvenile Atlantic loggerhead turtles, striped bass, American eel, and flounder. For human health, the horseshoe crab’s copper-based blood is also used for testing for bacterial products used in medicine. Specially permitted fishing operations collect the animals, draw their blood in a biomedical facility, and release them back into the water.
Despite their armor and tail, horseshoe crabs are gentle creatures that do not bite or sting. The tail is not a weapon; it is used to plow the crab through the sand and muck, act as a rudder in the water, and to right the crab when it accidentally tips over. Humans can help horseshoe crabs stuck upside down in rock jetties by gently flipping or moving the animal, using both hands, but never picking it up by its tail.
Maryland Department of Natural Resources biologists and the Maryland Coastal Bays Program monitor the returning horseshoe crab population for ecological and scientific research purposes. The department encourages the public to report any spawning activity and sightings of horseshoe crabs to DNR’s Horseshoe Crab Volunteer Angler Survey.
The best opportunities to view horseshoe crabs are evenings around the full and new moons in June (June 15 and June 29 in 2026).
Prime Viewing Locations
If you are in the Alexandria, VA area and want to observe this 350-million-year-old migration, you can visit local spawning beaches around the Bay:
Flag Ponds Nature Park (Calvert County, MD: A well-known hot spot for horseshoe crab spawning in Southern Maryland.
Maryland’s Coastal Bays & Ocean City: Sunset Park near the Ocean City Inlet is a premier site to witness the gathering.
How You Can Help & Get Involved
While the Chesapeake Bay Foundation (CBF) primarily focuses on advocacy, oyster restoration, and living shorelines, there are several ways you can actively participate in tracking and protecting horseshoe crabs:
Maryland DNR Surveys: The Maryland Department of Natural Resources (DNR) monitors shoreline usage. You can report sightings directly to the Maryland DNR Horseshoe Crab Survey to help identify spawning habitats.
Citizen Science: In nearby Worcester County, the Maryland Coastal Bays Program coordinates annual volunteer spawning and stranding surveys.
Resources:
Maryland Department of Natural Resources
Maryland Coastal Bays Program
Chesapeake Bay Foundation
Quick Facts
Species type: Native
Size: 2 feet in length
Habitat: Juveniles are found in shallow, protected waters with a sandy bottom; adults live in deeper waters
Range: Found year-round in the middle and lower Chesapeake Bay, as far north as the Chester River. Most visible in spring and summer, when they spawn on sandy beaches.
Diet: Feeds mostly on worms and mollusks such as razor clams and soft shell clams
Lifespan: Can live more than 20 years
Conservation status: Stable
Appearance: Horseshoe crabs grow to two feet in length. They have a hard, rounded, brownish-green exoskeleton, a spike-like tail and five pairs of jointed legs. Their widely spaced eyes look like bumps on the top of their shell. Their gills have folds of membranes that look like the leaves of a book. Juveniles are sand-colored.
Feeding: The horseshoe crab eats mostly worms and mollusks such as razor clams and soft shell clams. It spends most of its time rooting through bottom sediment looking for food. Because they lack jaws, these crabs use the spiny bases of their legs to crush and grind their food, then push it into their mouths.
Predators: Several types of shorebirds eat horseshoe crab eggs. Various fish, invertebrates and sea turtles feed on eggs and larvae. Humans catch adult horseshoe crabs to use as bait and for medical research.
Reproduction and life cycle: Spawning takes place in spring and summer (peaking in May-June), usually during evening high tides when the moon is full or new. Large numbers of adults crawl up onto sandy, protected beaches to mate and lay eggs. Females lay clusters of about 4,000 greenish eggs in the sand around the high-tide mark. They return to the beach to lay more eggs during high tides throughout the season. Eggs take about one month to develop and hatch. Young swim to shallow, sandy, protected nursery areas, where they remain for up to two years.
Did you know?
- Horseshoe crabs are not true crabs, and are more closely related to terrestrial spiders than blue crabs.
- Often called “living fossils,” horseshoe crabs have existed for more than 300 million years.
- Although their spikes and spines may look dangerous, horseshoe crabs are harmless.
- Just like other arthropods, horseshoe crabs must molt to grow. They leave their old shells behind and grow a new, larger shell.
- Horseshoe crabs swim upside down and can survive out of the water for an extended period of time if their gills are kept moist.
Sources and additional information
Life in the Chesapeake Bay by Alice Jane Lippson and Robert L. Lippson
Maryland Fish Facts: Horseshoe Crab – Maryland Department of Natural Resources
Animal Diversity Web: Limulus polyphemus – University of Michigan Museum of Zoology
Limulus polyphemus – MarineBio
The Horseshoe Crab – Ecological Research & Development Group