HORSESHOE CRABS: Merostomata

PHYSICAL CHARACTERISTICS

Adults horseshoe crabs range in length from 3.5 to 33.5 inches (89 to 850 millimeters). The females are larger than the males. Horseshoe crabs have a large, arched forebody covered by a horseshoe-shaped carapace (KARE-a-pays), or upper shell, followed by a smooth abdomen with spines on the sides, and a thin tail. There are two pairs of simple eyes, or eyes with one lens, on top of the carapace and a pair of compound eyes, or eyes with multiple lenses, on ridges toward the sides. The mouthparts are made up of a pair of pincherlike mouthparts and a pair of clawed leglike appendages. There are four pairs of clawed walking legs. The walking legs have seven segments, the last two form pinchers on the first four pairs of legs. The bases of the fourth pair of legs are fitted with special structures called flabella. The flabella are used to clean the book gills. The last pair of legs ends in four leaflike structures. These legs are used to push through, and sweep away mud, silt, and sand as the horseshoe crab burrows through the sea bottom in search of food.

The solid midsection, or abdomen, has six pairs of flaplike limbs. The first pair is joined together and protects the reproductive opening, through which the crab lays its eggs. The other five pairs form the gills, the organs through which the crab breathes underwater. They are called book gills because they resemble the pages of a book. Movable spines stick out on each side of the midsection. Different species, or types, of horseshoe crabs have different numbers of spines. A long thin tail extends from the end of the midsection and is used for steering through the water. Individual crabs that are trapped on their backs on the beach use the long tail to flip themselves over.

GEOGRAPHIC RANGE

Horseshoe crabs inhabit coastal regions of eastern North America and the Indo-Pacific. Of the four species of horseshoe crabs in the world, just one is found along the eastern and Gulf coasts of the United States.

HABITAT

Horseshoe crabs live just offshore along the coast or in salty estuaries (EHS-chew-air-eez), the wide parts at the lower ends of rivers, where the river meets the sea. They prefer coves and bays, which are small inlets of the sea, or wetland, meaning land that is covered with shallow water or that has very wet soil. The crabs like clean, sandy bottoms or muddy bottoms protected from strong wave action. Adults move onshore to mate on beaches at night.

GIANT TERRORS OF ANCIENT SEAS

Despite their name, horseshoe crabs are not related to true crabs. Instead, they are related to sea spiders and true spiders and their relatives. The Merostomata are among the oldest living animals, with fossils (FAH-suhls), the remains of ancient animals, dating back 350 million years. Horseshoe crabs are the only living members of the subclass, which includes the now extinct giant water scorpions, or giant sea scorpions. They are called "living fossils" because they have changed very little over millions of years. Some 290 million years ago they were plentiful on the bottoms of ancient seas, as well as in brackish, or salty, waters and freshwaters. The sea scorpions, also called eurypterids, were similar to horseshoe crabs but thinner and longer and with a smaller shell and an abdomen divided into many segments. They were the largest arthropods ever to have roamed the earth. One species measured 9 feet (2.7 meters) in length. Giant sea scorpions probably ate fish and other giant sea scorpions.

DIET

The young, or larvae (LAR-vee), do not feed until after they molt, or shed their outer skeleton, called an exoskeleton, for the first time. Adults and older larvae both eat almost any food items they find on the bottom and also attack clams and worms, or scrape algae (AL-jee), tiny plantlike organisms, off rocks. Horseshoe crabs lack jaws and instead use their legs to grasp and crush their food. Food is ground with spines at the bases of the crab's legs. In fact, the name Merostomata comes from the Greek words meros, meaning leg, and stoma, or mouth—referring to the role the legs play in chewing food.

BEHAVIOR AND REPRODUCTION

Every spring and summer adults migrate (MY-grayt), or move, from deeper waters onto nearby beaches to mate and lay eggs, or spawn. Spawning occurs at high tide on the beaches of estuaries, bays, and coves. One species swims upstream into rivers to spawn.

A male grasps the edges of the female's shell with his clawed legs, while the female digs a hollow in the sand with her legs and the front section of her body. As the female lays the eggs into this hollow, the male fertilizes (FUR-teh-lye-sez) them. The pair moves to a nearby site and begins the process all over again, using the sand dug from the new site to cover the previously laid eggs. Horseshoe crabs may spawn more than once a year, laying up to several thousand eggs after each mating.

The eggs hatch into larvae. The larvae swim constantly but soon settle to the seafloor after their first molt. They rest by burying themselves in shallow burrows in the mud and silt. Although they usually crawl on the bottom, horseshoe crabs also swim upside down using their book gills to propel themselves through the water.

Horseshoe crabs molt six times in their first year and sixteen to seventeen times during their entire lives. Males reach adulthood between nine and eleven years of age. Females become adults when they are ten to twelve years old.

HORSESHOE CRABS AND PEOPLE

Horseshoe crabs are used for food and as bait to catch various ocean-dwelling animals. Their bodies were once ground up and used as fertilizer. Certain scientific studies of their eyes have led to a better understanding of how the human eye works, resulting in improved treatments for human eye disorders. Their blood contains a substance that is used to identify bacterial contamination, or dirtying, of drugs, surgical equipment, and vaccines, or medicines that are used to protect the human body against disease. Other substances in their blood show promise in fighting viruses and cancer. Their exoskeletons are used in the manufacture of skin lotion, contact lenses, and surgical sutures, or stitches. Chitin (KYE-tehn), an important chemical component of the horseshoe crab's exoskeleton, is used to remove harmful substances from water. It also is taken as a dietary supplement.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists just one species of horseshoe crab as Near Threatened, meaning that they are at risk of being threatened with extinction in the future. All horseshoe crab populations are thought to be declining worldwide owing to harvesting, or gathering by humans, and habitat destruction.

HORSESHOE CRAB (Limulus polyphemus): SPECIES ACCOUNT

Physical characteristics: The horseshoe crab, also known as American horseshoe crab, is greenish brown to blackish brown. Males are smaller than females. The average body length of males is 14 inches (356 millimeters) and of females is 17 inches (431 millimeters).

Geographic range: The horseshoe crab inhabits the Atlantic and Gulf coasts of North America, from Long Island to the Yucatán peninsula.

Habitat: These crabs live off the coast to depths of more than 200 feet (61 meters).

Diet: The horseshoe crab eats clams and marine worms.

Behavior and reproduction: Adults live in deeper, offshore waters during the winter, ranging as far out as 25 miles (40 kilometers). In spring they migrate into shallow waters and then onto sandy beaches to spawn. Males approach the beach as the tide rises, followed by the females. Spawning occurs mostly at night near the high-tide line. Females bury as many as twenty thousand eggs in several clutches, or groups, which are fertilized by males. Eggs hatch after thirteen to fifteen days.

Horseshoe crabs and people: In the United States the horseshoe crab is harvested as bait for the conch and eel fisheries. From 1850 until the 1970s the horseshoe crab was processed for fertilizer. Components of its blood and exoskeleton are used for a wide variety of medical purposes.

Conservation status: This species is listed by the World Conservation Union (IUCN) as Near Threatened. It is threatened by too much harvesting and habitat destruction. ∎

FOR MORE INFORMATION

Books:

Day, Nancy. The Horseshoe Crab. New York: Dillon Press, 1992.

Tanacredi, John T., ed. Limulus in the Limelight: A Species 350 Million Years in the Making and in Peril? New York: Kluwer Academic/Plenum, 2001.

Periodicals:

Rudloe, A., and J. Rudloe. "The Changeless Horseshoe Crab." National Geographic Magazine (April 1981) 159, no. 4: 562–572.

Web sites:

"Chitin Research." Sea Grant. http://www.ocean.udel.edu/horseshoecrab/Research/chitin.html (accessed on August 19, 2004).

"Eurypterida: All about Sea Scorpions." Palaeos: The Trace of Life on Earth. http://www.palaeos.com/Invertebrates/Arthropods/Eurypterida/ (accessed on August 19, 2004).

"Eurypterida." Eurypterids.net. http://eurypterids.net/index.html (accessed on August 19, 2004).

"The Horseshoe Crab." Ecological Research and Development Group. http://www.horseshoecrab.org/ (accessed on August 19, 2004).

"The Horseshoe Crab: Putting Science to Work to Help "Man's Best Friend." NOAA Research. http://www.oar.noaa.gov/spotlite/archive/spot_delaware.html (accessed on August 19, 2004).