The American horseshoe crab is a common sight on Florida’s beaches. Horseshoe crabs are “living fossils” meaning they have existed nearly unchanged for at least 445 million years, well before even dinosaurs existed.

Horseshoe crabs are not actually crabs at all, they are much more closely related to spiders and other arachnids than they are to crabs or lobsters!

There are four species of horseshoe crabs still around today. Only one species, Limulus polyphemus, is found in North America along the Atlantic and Gulf coasts from Maine to Mexico. The other three species are found in Southeast Asia.

Despite existing for hundreds of millions of years, horseshoe crabs are nearly identical to their ancient relatives. This is because their body structure is extremely effective for survival, think, “if it ain’t broke, don’t fix it!”

How they look like

Horseshoe crabs have a tank-like structure consisting of a front shell called the prosoma, a back shell called the opisthosoma, and a spike-like tail called a telson.  Some people think horseshoe crabs are dangerous animals because they have sharp tails, but they are totally harmless. Really, horseshoe crabs are just clumsy and they use their tail to flip themselves back over if they get overturned by a wave.*

Though the horseshoe crab’s shell is hard, it is very sensitive to the world around it.  The crabs are especially sensitive to light. They have 10 eyes, a pair of compound eyes on the prosoma, and “photoreceptors” in other areas, primarily along the tail.

*Never pick up a horseshoe crab by its tail, as it can harm the animal.  Instead, gently pick it up by both sides of the prosoma using both hands.

Horseshoe crabs are known to gather in large nesting aggregations, or groups, on beaches particularly in the mid-Atlantic states such as Delaware, New Jersey and Maryland in the spring and summer, where their populations are largest. Horseshoe crabs can nest year-round in Florida, with peak spawning occurring in the spring and fall.

When mating, the smaller male crab hooks himself to the top of the larger female’s shell by using his specialized front claws, and together they crawl to the beach. The male fertilizes the eggs as the female lays them in a nest in the sand.

Some males (called satellite males) do not attach to females but still have success in fertilizing the female’s eggs by hanging around the attached pair. Most nesting activity takes place during high tides around the time of a new or full moon.

Horseshoe crab larvae emerge from their nests several weeks after the eggs are laid. Juvenile horseshoe crabs look a lot like adults except that their tails are smaller. The young and adult horseshoe crabs spend most of their time on the sandy bottoms of inter-tidal flats or zones above the low tide mark and feed on various invertebrates.

Why are horseshoe crabs important?

Horseshoe crabs are an important part of the ecology of coastal communities. Their eggs are the major food source for shorebirds migrating north, including the federally-threatened red knot. These shorebirds have evolved to time their migrations to coincide with peak horseshoe crab spawning activity, especially in the Delaware and Chesapeake Bay areas. They use these horseshoe crab beaches as a gas station, to fuel up and continue their journey.

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Many fish species as well as birds feed on horseshoe crab eggs in Florida. Adult horseshoes serve as prey for sea turtles, alligators, horse conchs, and sharks.

Horseshoe crabs are also extremely important to the biomedical industry because their unique, copper-based blue blood contains a substance called “Limulus Amebocyte Lysate”, or “LAL”.

This compound coagulates or clumps up in the presence of small amounts of bacterial toxins and is used to test for sterility of medical equipment and virtually all injectable drugs. That way, when you get a vaccine you know it hasn’t been contaminated by any bacteria.  Anyone who has had an injection, vaccination, or surgery has benefited from horseshoe crabs! Additionally, research on the amazing and complex compound eyes of horseshoe crabs has led to a better understanding of human vision.

Horseshoe crabs are also used in several fisheries.  The marine life fishery collects live horseshoe crabs for resale as pets in aquariums, research subjects, or as educational specimens, and  both the American eel and whelk fisheries use horseshoe crabs as bait along many parts of the Atlantic coast.

Threats to horseshoe crabs and research efforts

Horseshoe crab numbers are declining throughout much of their range. In 1998, The Atlantic States Marine Fisheries Commission developed a Horseshoe Crab Fishery Management Plan that requires all Atlantic coastal states to identify horseshoe crab nesting beaches. Currently, with the help of the public, biologists at the Fish and Wildlife Research Institute are documenting nesting sites of horseshoe crabs throughout the state. If you see horseshoe crabs mating and would like to report a sighting, please visit the Report Sightings page for more information.