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Facts About Horseshoe Crabs

Ancestors of horseshoe crabs date back over 350 million years—long before the age of the dinosaurs.

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Four species of horseshoe crabs exist 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. Horseshoe crabs are not true crabs at all; they are more closely related to arachnids (a group that includes spiders and scorpions) than to crustaceans (a group that includes true crabs, lobsters, and shrimp). Horseshoe crabs are often considered “living fossils” because fossils of their ancestors date back over 350 million years—long before the age of the dinosaurs. Furthermore, horseshoe crab body forms have changed very little over all of those years.

The strange anatomy of the horseshoe crab is one of this animal’s most notable aspects. Unfortunately, the long, thin, spike-like tail of horseshoe crabs has given this species an unfavorable reputation. Many people view horseshoe crabs as dangerous animals because they have sharp tails. In reality, horseshoe crabs are harmless, and their tails are used primarily to flip themselves upright if they are accidentally overturned.

Horseshoe crabs are well known for their large nesting aggregations on beaches, particularly in Mid-Atlantic States such as Delaware, New Jersey, and Maryland. These nesting aggregations are commonly observed in Florida as well. During the nesting season, principally in spring and summer, male horseshoe crabs move parallel to the shoreline on sandy flats and intercept females as they pass by. A successful male attaches himself to a female 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. Males that do not find mates will often swarm mating couples and try to fertilize some of the females’ eggs. Most of this nesting activity takes place during high tides in the three days before and after a new or full moon.

Horseshoe crab larvae emerge from their nests several weeks after the eggs are laid. Juvenile horseshoe crabs resemble adults except that their tails are proportionally smaller. The young and adult horseshoe crabs spend most of their time on the sandy bottoms of intertidal flats and feed on various invertebrates.

Why are horseshoe crabs important?
Horseshoe crabs are an important part of the ecology of coastal communities. During the nesting season, especially in the Mid-Atlantic States, horseshoe crab eggs become the major food source for migrating birds. Over 50% of the diet of many shorebird species consists of horseshoe crab eggs. Many bird species in Florida have been observed feeding on horseshoe crab eggs. In addition, many fish species rely on horseshoe crab eggs for food.

Horseshoe crabs are currently exploited in three industries: the bait fishery, the marine live fishery, and the biomedical industry. Horseshoe crabs are used extensively as bait in the American eel and whelk fisheries along many parts of the Atlantic coast. The marine life fishery collects live horseshoe crabs for resale as aquarium, research, or educational specimens.  Horseshoe crabs are important to the biomedical industry because components of their blood coagulate in the presence of small amounts of bacterial toxins, thereby providing a method to test for bacterial contamination in commercial drugs and medical equipment. Research on the compound eyes of horseshoe crabs has led to a better understanding of the human visual system. The material that makes up their exoskeleton (called chitin) is used to make contact lenses, skin creams, and hairsprays, as well as sutures and dressings for injuries.

Threats to horseshoe crabs and research efforts
Horseshoe crab numbers are declining throughout much of the species range. Although scientists are unsure of the exact causes of this decline, it is probably due to a variety of factors, including the degradation of habitat and overfishing.

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 trying to document nesting sites of horseshoe crabs throughout the state. If you are interested in becoming more involved with the horseshoe crab survey, please visit the Survey for Horseshoe Crab Nesting Beaches in Florida for more information.