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Horseshoe Crab Featured Creature Archive from Tampa Bay Estuary Program


Horseshoe crabs are extraordinary creatures. They've made their home in our seas for over 350 million years. Remarkably, their body form hasn't changed much from then to now, which is why they are often called "living fossils."

Horseshoe crabs are not really crabs at all. They are more closely related to spiders and scorpions than to true crabs. Unlike true crabs, horseshoe crabs lack antennae and jaws, and have seven pairs of legs instead of five. Their tough outer shell, or carapace, acts like armor and protects their legs and organs. Although the horseshoe crab has a long spike-like tail that appears threatening, the animal is harmless. Their tail is not used as a weapon, but rather to right the animal when it finds itself belly-up.

Horseshoe crabs face few natural predators, among them loggerhead sea turtles and tiger sharks. They are able to withstand environmental changes such as extreme temperature and salinity variations, and if they have to, they can live without food for up to one year! These factors may be reasons why the horseshoe crab has been able to survive for millions of years.

Summertime in Florida is peak spawning season for these ancient mariners. All along our beaches, horseshoe crabs are busy laying billions of eggs. The delicate, BB-sized green eggs serve as an important food source for Florida's migratory birds and marine life. This is why few eggs and offspring survive to maturity, and why it's important that horseshoe crabs lay thousands of eggs at a time.

Most nesting activity takes place during high tides, three days before and after a new or full moon. If you stroll down the beach during this time, you may notice the horseshoe crab's interesting mating behavior. Males travel along the shoreline and attempt to snag females with their hook-like front claws. If successful, the male grips onto the female's back as she tows him up onto the beach. The female stops every few feet to dig a hole and lay her eggs. As the male is pulled over the nest, he fertilizes the eggs.

Between two and five weeks, the eggs expand and the embryos morph into larvae. The larvae remain on the beach for several weeks until they molt into juveniles that resemble small adults. Young and adult horseshoe crabs spend their time scurrying along intertidal flats searching for clams, mussels and worms to feed on.

Horseshoe crabs are vital to the ecology of our marine environment. As mentioned, horseshoe crab eggs are a major food source for many animals. They also provide shelter for mussels, barnacles, sponges, and other invertebrates that cling to the underside of their carapace.

Horseshoe crabs are directly important to humans. Their blood is widely used in the biomedical industry to test for bacterial contamination in human blood and commercial drugs. The material that makes up their exoskeleton, chitin, is used to make contact lenses, skin cream and hair spray.

Currently horseshoe crabs are being overharvested in some states, and all Atlantic Coast states are being required to identify horseshoe crab spawning beaches. The Florida Wildlife Research Institute is collecting that information in Florida and needs your help! If you see horseshoe crabs mating or laying eggs, please report the information to horseshoe@fwc.state.fl.us. More information about the horseshoe crab research project may be found at http://research.myfwc.com ∞.

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