The horseshoe crab, Limulus polyphemus, is a common resident of Florida's Gulf coast. An animal that hails from prehistoric times, they are not actually crabs at all but are instead more closely related to arachnids such as spiders and ticks. Horseshoe crabs can be found from the Gulf of Maine down the Atlantic coast of Florida and across the Gulf of Mexico to the Yucatan Peninsula.
Juveniles spend their time on intertidal flats while adults move deeper to subtidal flats. They feed on clams and worms buried in the sediment, using their legs to "chew" their food by grinding it against their undersides. Males are smaller than females, measuring 7-9 inches across compared to a female's 9-12 (Life Stages, 2008). Horseshoe crabs can be seen at any time of day but are most active at night, particularly during spawning season.
The season begins in May and runs through July, with the most activity occurring on nights with a full or new moon. Females dig nests between the tide lines and lay around 4,000 eggs in the sand (Species Profile, 2006). Spawning coincides with migratory shorebird arrival since the horseshoe crab's eggs are an important part of the birds' diet. Those eggs that escape predation or injury will hatch in about 14 days. Once hatched, the juveniles return to the sea on the tide and settle in to their new home.
Shells found on the shore aren't necessarily from crabs that have died. To grow bigger, horseshoe crabs must shed their exoskeleton in a process called molting. If the shell is split in the front along the seam between top and bottom edges, it's a molt. A horseshoe crab must go through 16 molts (17 if it's a female) to reach adulthood (Botton, et al. 2003) Since they molt only 1 to 2 times a year after the first few years, that means they aren't adults until they're 8-10 years old! The length of their lifespan is uncertain but they can live at least 20 years (Life Stages, 2008).
Horseshoe crabs have been severely impacted by human activity. Native Americans found that they made very good sources of fertilizer and taught early settlers to use them on their crops. By the late 1800's, 4 million crabs were harvested annually for use as fertilizer (Species Profile, 2006). The practice continued to grow until the 1960's when chemical fertilizers became more widely used. By that time populations had declined sharply. Although the pressure from the fertilizer industry lessened, horseshoe crabs continued to be threatened by their popularity as bait in eel and conch fisheries Recognizing a need for action, Delaware, Maryland and New Jersey began regulating the horseshoe crab fishery and the population decline slowed (Species Profile, 2006).
Preservation of horseshoe crab populations is important not just from an ecological viewpoint but a practical one as well. Our knowledge base on eyes and vision stems from research on the horseshoe crab. Their blood is used to detect life-threatening bacteria strains and toxins and for many applications is the most accurate test available. It is also used to detect spinal meningitis and some forms of cancer and may be beneficial in treating them as well (Medical Uses, 2008). These benefits make the horseshoe crab an important resource that should be carefully monitored.
The Florida Fish and Wildlife Commission is asking the public to assist with a spawning survey. If you see crabs on our local beaches, go to http://research.MyFWC.com/horseshoe_crab to fill out a report which will help biologists determine nesting sites around the state. While you're exploring the shore, remember that a horseshoe crab is at its most vulnerable on its back and will die if unable to right itself so if you find one upside down, turn it over. Every little bit will help to preserve this interesting animal for the future.
Distribution, abundance, and survivorship of young-of-the-year in a commercially exploited population of horseshoe crabs Limulus polyphemus. Botton, Mark L. Loveland, Robert E. Tiwari, Athena. Marine Ecology Progress Series Vol. 265: 175-184, 2003
http://www.horseshoecrab.org/ accessed April 27, 2008
http://www.ocean.udel.edu/horseshoecrab/History/lifestages.html accessed April 27, 2008
Species Profile: Horseshoe Crab Management Plan Seeks to Conserve Resource for Multiple Uses. Atlantic States Marine Fisheries Council, July 2006