Horseshoe crabs (Limulus polyphemus) can be found on many Florida beaches. You may have had the opportunity to come across these prehistoric creatures spawning during a high tide, or perhaps, found one of their molts on the shore.

Horseshoe crabs along the Gulf Coast of Florida are of special conservation interest because they remain unprotected from harvesting protections and face many threats.  

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The Role of Horseshoe Crabs

Horseshoe crabs and their eggs are extremely important food sources for migratory shorebirds, including red knots (Calidris canutus), a declining shorebird that has one of the longest migration paths of any bird. 

“Red knots migrate thousands of miles between their breeding grounds in the arctic tundra and their wintering grounds in South America twice a year,” said SCCF Shorebird Intern Riona Lahey. “These journeys cost considerable amounts of energy, making food availability extremely important for the survival of these birds.”

When horseshoe crabs in the northeast U.S. were overharvested in the 1900s, red knot populations rapidly declined. Scientists quickly learned that horseshoe crabs are vital to the survival of the red knot, leading to the protection of both species. 

The Atlantic States Marine Fisheries Commission (ASMFC) has protections in place to ensure the harvest of horseshoe crabs along the east coast of Florida remains at sustainable levels. However, Gulf Coast horseshoe crabs are not protected by the ASMFC, so they are of special conservation interest.

Humans also benefit from horseshoe crab blood, which contains limulus amoebocyte lysate (LAL), which is used to test for bacterial contamination in medical products. LAL is important for maintaining high safety standards, preventing infections, and ensuring the effectiveness of pharmaceuticals, vaccines, and implants. Biomedical collection permits are required to collect blood from horseshoe crabs, and the animals must be released afterward. Recent efforts have been underway to use synthetic alternatives.

Development, shoreline armoring, the aquarium trade, beach driving, sea level rise, and erosion also threaten horseshoe crab populations in Florida. 

About Horseshoe Crabs

Despite their name, Horseshoe crabs are not crabs or crustaceans. They are arthropods that belong to their own family, Limulidae, and are closely related to spiders and other arachnids.

The oldest fossil records show that horseshoe crabs have existed for at least 445 million years. Limulus polyphemus is one of four living horseshoe crab species — the other three are found in Asia. This species has remained nearly unchanged since before dinosaurs roamed the Earth, so they’re often referred to as “living fossils.”

Before horseshoe crabs reach maturity at 9-10 years old, they molt their shells up to sixteen or seventeen times! You can tell if a shell has molted if there is a slit running along the front, which indicates the horseshoe crab has exited.

Horseshoe Crab Spawning and Community Science

Although horseshoe crabs in Florida can nest all year, peak spawning occurs in the spring and fall at high tides during full and new moons. They can nest in pairs or groups with multiple males. The female lays thousands of eggs in a hole in the sand, then the male(s) externally fertilize them. 

SCCF’s shorebird team has been documenting horseshoe crabs spawning throughout the summer. In preparation for fall nesting, the team is looking forward to attending a public training with the Florida Horseshoe Crab Watch (FHSCW) on Aug. 30.

Are you interested in participating in horseshoe crab tagging? Sign up for the training >>

FHSCW is a partnership between the FWC Fish and Wildlife Research Institute and the University of Florida with the goal of learning more about Florida’s horseshoe crab populations through community science. Trained volunteers assist biologists in surveying, tagging, and re-sighting crabs. 

“This work is important for understanding our unique Gulf Coast populations and protecting them from the many threats they face,” Lahey said.

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