Facts About Horseshoe Crabs and FAQ (2023)

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 fourspeciesof 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, horseshoecrabs are nearly identical to their ancient relatives. This isbecause their body structure is extremely effective for survival, think, “if it ain’t broke, don’t fix it!”

Horseshoe crabs have a tank-like structure consisting of a front shell called the prosoma, a back shell called the opisthosoma, and aspike-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 ifthey getoverturned by a wave.*

Though the horseshoe crab's shell is hard, it is verysensitive to the world around it. The crabs areespecially sensitive to light.They have 10eyes, a pair of compound eyes on the prosoma, and"photo receptors" in other areas, primarily along the tail.

*Never pick up a horseshoe crab by its tail, as it can harm the animal.Instead, gentlypick it up byboth 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 smallermale 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 theattached pair. Most nesting activity takes place during high tidesaround the time ofa new or full moon.

Horseshoe crablarvaeemerge from their nests several weeks after the eggs are laid.Juvenilehorseshoe 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 variousinvertebrates.

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, includingthe 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.

Many fish species as well as birdsfeed on horseshoe crab eggs in Florida.Adult horseshoes serve as prey forsea 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 compoundcoagulatesor clumps up in the presence of small amounts of bacterialtoxinsand 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 fromhorseshoe crabs! Additionally,research on the amazing and complexcompound eyesof horseshoe crabs has led to a better understanding of human vision.

Horseshoe crabs are also used in several fisheries. The marine lifefisherycollects 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 theReport Sightingspagefor more information.