The Horseshoe Crab
(Limulus polyphemus)

Classified Information

Kindgom Animalia
   Phylum Arthropoda
      Subphylum Chelicerata
         Class Merostomata
            Order Xiphosurida
               Family Limulidae
                  Genus Limulus
                     species polyphemuss

There are 4 species of horseshoe crab found on earth today. Three species exist along the coast of southeast Asia and the fourth is our familiar horseshoe crab, Limulus polyphemus, of the Atlantic coast. Once thought as a living fossil dating back over 300 million years, Limulus polyphemus, actually has no fossil record and only dates back about 20 million years.

Biologically Speaking

Range:
The Atlantic coast and the gulf of Mexico with the greatest abundance between VA and NJ with DE Bay being the center of distribution.

Anatomy:
The body of a horseshoe crab is divided into three distinct sections, a cephalothorax, abdomen, and telson. The cephalothorax and abdomen are attached by a simple hinge joint similiar to our knee or elbow. The abdomen and telson are attached via a ball and socket joint like our shoulder. The telson is used by the horseshoe crab to dig and to right itself when over-turned.

A horseshoe crab has 10 pairs of eyes but most of us are familiar with the 2 large compound eyes on the upper surface of the cephalothorax. The large eyes are very similar to the eyes that are common to many arthropods and are used primarily to detect movement.

Their are 7 pairs of appendages which are used for walking, manipulating food, and in male horseshoe crabs, mating.

Horseshoe crabs have six pairs of gill books located on the underneath side of the abdomen. The first pair is known as the operculum and is used to protect the remaining five pairs that act as the respiratory organs. Gill books aid juvenile horseshoe crabs with swimming.

Horseshoe crabs have "blue blood" due to the presence of a copper-containing molecule known as hemocyanin, which has a blue tint when exposed to air. This molecule is used to transport oxygen in the blood of the horseshoe crab compared to the iron-containing molecule hemoglobin (which is red) that we use to transport oxygen. The hemocyanin molecule is common to the blood of most artropods and mollusks.

Feeding:
Horseshoe crabs are omnivorous and feed on mollusks like razor clams and soft shelled clams, a variety of marine worms, and benthic algae. Horseshoe crabs could be classified as organisms that "eat on the run". Since they have no jaw, the middle segment of each leg is equipped with spines that crush food as it is passed along to the mouth located at the base of the legs. By performing this process while moving along the bottom the horseshoe crab is able to eat. Unlike some of us that have difficulty walking and chewing gum at the same time!

Growth:
Growth occurs through the process molting as in blue crabs and lobsters. However, during the final steps of molting the horseshoe crabs exits it's old exoskeleton by crawling "out the front" as opposed to crabs and lobsters which exit "out the back". Horseshoe crabs will molt several times during the first two to three years and about once a year thereafter. Female horseshoe crabs can reach lengths of 60 cm while males are smaller. The average lifesapn is about 19 years.

Reproduction:
Horseshoe crabs reach sexual maturity in nine to twelve years. Spawning in the Bay region typically begins in May and coincides with full moons and evening spring tides. Females release up to 80,000 eggs in a season. Eggs are laid in nests dug by the female on beaches and are fertilzed by males. In 2-4 weeks trilobite larvae emerge from nests to return to the water with the receding tides. These larvae can grow up to 4 cm in width during the first year. Eggs are important ecologically because they are a major food source for migrating birds coming from South America.

Ecology "Crash" Course

Although adult horseshoe crabs have relatively few natural enemies, juvenile Atlantic loggerhead sea turtles will feed upon horseshoe crabs during the summer as they migrate into the Bay. Horseshoe crabs are also an important link in the food web of migratory shore birds. There are at least 11 species of migratory shore birds that rely upon the horseshoe crab eggs as a fat-replenishing food source during their northern route to Canadien breeding grounds in mid-May and June. This link is vital to these species since Delaware Bay is the both the primary breeding ground for horseshoe crabs in the Mid-Atlantic and the second largest staging area for shorebirds in North America. This relationship was first recognized in the early 1980s after the resurgence of an overharvested horseshoe crab population attracted large numbers of migratory shorebirds.

Use and Management

Man has utilized the horseshoe crab for years as a source of food, fertilizer, animal feed, weapons, and tools. In recent decades this crab has been taking on a more predominant role in the field of biomedical technology. A compound called Limulus Amoebocyte Lysate (LAL) found in the hemolymph of the horseshoe crabs is invaluable in detecting the presence of endotoxins in pharmaceutical and medical products. These crabs have also played an important role in human eye, cancer, and vitamin deficiency research and the development of sutures, medical dressings, contacts, and skin creams.

The increase harvest of horseshoe crabs for use as bait in the catfish, eel, conch, and whelk industry has lead to the adoption of the Chesapeake Bay and Atlantic Coast Horseshoe Crab Fisheries Management Plan in 1994. The plan included the protection and monitoring of spawning populations and recognized spawning grounds. In 1995, in cooperation with Limuli Laboratories in New Jersey, Maryland DNR began a tagging program to track migration and spawning behavior.

More Information...

On the Web
www.dnr.state.md.us/fisheries/fishfaqs.html
www.aqua.org/animals/species/prhcrab.html
www.marinelab.sarasota.fl.us/WHORSESH.HTM

Dr. Bill Hall, University of Delaware
Marine Education Specialist
Sea Grant Marine Advisory Service
700 Pilottown Road
Lewes, DE 19958-1298