Merostomata

(Horseshoe crabs)

Phylum Arthropoda

Class Chelicerata

Subclass Merostomata

Number of families 1

Thumbnail description
Marine creatures distinguished by a large, hard exoskeleton that includes an arched, horseshoe-shaped shield in front (prosoma), a middle portion (opisthosoma), and a thin tail (telson); they are among the oldest living organisms

Evolution and systematics

The subclass Merostomata is one of three branches of the chelicerate line of arthropods; the other two branches include sea spiders and terrestrial spiders. Thus horseshoe crabs are more closely related to spiders and scorpions than to other crabs. Horseshoe crabs date to the Carboniferous period (350 million years ago [mya]). Ancestral relatives from the Cambrian period (550 mya) have been found. Horseshoe crabs are classified into a single order (Xiphosura) and family (Limulidae). Four species are recognized. Many scientists now categorize Merostomata as a class rather than a subclass.

Physical characteristics

The body of a horseshoe crab is covered by a smooth greenish to dark brown exoskeleton. The exoskeleton consists of three major parts: an arched, horseshoe-shaped shield in the front, the prosoma; a middle portion, the opisthosoma; and a thin tail, the telson. The prosoma bears two pairs of simple eyes on the top and a pair of compound eyes on ridges laterally along the outside. Under the exoskeleton, eight pairs of appendages are aligned along the lengthwise axis of the prosoma. The first seven pairs function in feeding. The eighth pair is fused and covers five pairs of book gills in the opisthosoma. The book gills maintain water flow for respiration, movement, and reproduction. Spines protrude from the outer edge of the opisthosoma; the number of spines varies by species. The long, thin telson extends from the back of the body.

Horseshoe crabs must shed their exoskeleton, or molt, to grow. Individuals molt 16 or 17 times during their lives. Six of these molts occur within the first year. As adults, females

are larger than males. In the smallest species, Carcinoscorpius rotundicauda, females reach 15 in (38 cm) in length and 5 in (12.5 cm) in width. In Tachypleus tridentatus, the largest species, females attain a length of 33.5 in (85 cm) and a width of 15.5 in (39.3 cm).

Distribution

Western Atlantic coast and regions of the Indian and Pacific oceans.

Habitat

Horseshoe crabs inhabit saline portions of estuaries or near-shore coastal areas. They often live in coves, bays, or wetlands protected from strong wave action. They remain in sandy or muddy sublittoral areas except when they move onto beaches for spawning.

Behavior

As larvae, horseshoe crabs swim vigorously for hours, but they adopt diurnal activity patterns as juveniles and adults. When resting, horseshoe crabs often bury themselves in shallow burrows. Crawling along the substrate is the primary means of locomotion, but horseshoe crabs sometimes swim upside-down by using the book gills for propulsion. As adults, horseshoe crabs migrate annually from deeper near-shore waters to beaches for spawning. Individuals that are flipped onto their backs use the telson to arch the body and roll over.

Feeding ecology and diet

Larval horseshoe crabs do not feed. Feeding begins after the first juvenile stage is attained. Horseshoe crabs do not have jaws, so they use their legs to grasp and crush prey. Horseshoe crabs scavenge on almost any food items they encounter in the sediment, such as mollusks and worms. They also scrape algae off rocks. Adults are eaten by opportunistic predators, including sharks, sea turtles, sea gulls, and terrestrial mammals. Most predation occurs on young horseshoe crabs, the larvae and eggs being eaten by fish. The eggs provide an important food source for many shorebirds during spring migration from South America to the Arctic.

Reproductive biology

Horseshoe crabs are long-lived and mature later than other invertebrates. Males mature between 9 and 11 years of age and females, between 10 and 12 years. Horseshoe crabs spawn during the spring and summer. Spawning occurs at high tide on low-energy beaches of estuaries, bays, and coves. One species (Carcinoscorpius rotundicanda) moves upstream into rivers to spawn.

During mating, the male grasps the edge of the female's opisthosoma. The female uses her legs and prosoma to dig a nest, into which she deposits a cluster of eggs. The eggs are fertilized by the male, and the pair moves 4–8 in (10–20 cm) farther in the sand and repeats the process. As the female digs the second nest, the excavated sand is pushed backward to cover the previous nest. Individual horseshoe crabs are capable of spawning more than once per season. The eggs hatch into trilobite larvae; after molting into juveniles, horseshoe crabs settle to the seafloor.

Conservation status

No species is listed by the IUCN. However, horseshoe crab populations have declined as the result of harvesting and habitat destruction.

Significance to humans

Horseshoe crabs have been harvested for food and bait. They also have been processed into fertilizer. Perhaps most important, horseshoe crabs have enabled numerous human health advances. Studies of the eyes of horseshoe crabs have led to therapies for human eye disorders. The blood of horseshoe

crabs forms a substance, Limulus Amebocyte Lysate (LAL), that is used to identify gram-negative bacteria in medical fluids and drugs and on surgical devices. Nontoxic and biodegradable chitin from horseshoe crabs is used in products such as contact lenses, surgical sutures, and skin lotion. The chitin forms a chemical that removes metals and toxins from water, and its fat-absorbing properties help remove fat and cholesterol from the human body.

Species accounts

List of Species

American horseshoe crab

Limulus polyphemus

order

Xiphosura

family

Limulidae

taxonomy

Limulus polyphemus Linnaeus, 1758.

other common names

English: Atlantic horseshoe crab, horseshoe crab, king crab.

physical characteristics

The American horseshoe crab is a large species. Males are smaller than females. Mean body length is approximately 14 in (35.7 cm) for males and 17 in (43.8 cm) for females. The edge of the carapace is domed. The prosomatic carapace forms a circular arch, whereas the opisthosomatic carapace is hexagonal and elongated toward the posterior of the body. This species has large compound eyes. The American horseshoe crab is greenish brown to blackish brown.

distribution

Atlantic coast of North America, from Long Island to the Yucatan Peninsula.

habitat

To a depth of more than 200 ft (60 m) in coastal areas; sandy beaches for spawning.

behavior

The American horseshoe crab lives in deeper offshore waters during the winter and migrates into shallow coastal waters as the spawning season approaches. Adults range a maximum of 22–25 mi (35–40 km). Juveniles move into marine waters offshore of the natal beach at the end of the first summer.

feeding ecology and diet

The diet consists largely of bivalve mollusks, such as clams, and polychaete worms. Gulls feed on adult horseshoe crabs that become stranded on beaches during spawning, and many types of migratory shorebirds consume the eggs.

reproductive biology

Mature individuals migrate to sandy beaches to spawn in the spring. As the tide rises, males approach the beaches in large groups. Females follow and couple with the males. Spawning occurs mostly at night and near the high-tide line. Females bury approximately 20,000 eggs in a series of clusters that are fertilized by males. The eggs hatch into trilobite larvae after 13–15 days.

conservation status

Not listed by the IUCN.

significance to humans

In the United States, the American horseshoe crab is harvested as bait for the conch and eel fisheries. From 1850 until the 1970s, the horseshoe crab was processed for fertilizer. The blood is used to make LAL for detecting gram-negative bacteria.

Japanese horseshoe crab

Tachypleus tridentatus

order

Xiphosura

family

Limulidae

taxonomy

Tachypleus tridentatus Leach, 1891.

other common names

English: Eastern horseshoe crab, three-spine horseshoe crab

physical characteristics

The Japanese horseshoe crab has a large body and relatively small compound eyes. Males average approximately 20 in (51 cm) in length, and females are approximately 23.6 in (60 cm) long. The prosomatic carapace is domed. The opisthosomatic carapace is hexagonal but not strongly elongated, and spines protrude from its margins. This horseshoe crab is greenish gray.

distribution

Discrete coastal areas in the Indian and Pacific oceans—Inland Sea and North Kyushu, Japan; south of the Yangtze River in China; southwest Vietnam; Philippines; Borneo; North Celebes; northern Sulawesi; northeast Java and southwest Sumatra.

habitat

Deep water in winter, shallow coves in spring; muddy or sandy substrate.

behavior

Larvae spend the first winter in the sand of the natal beach. Young then migrate to nearby mud flats. After becoming approximately 6 in (15 cm) long, individuals move into the ocean. They migrate from deeper offshore waters during the winter into shallow coastal waters in preparation for the summer spawning season.

feeding ecology and diet

The Japanese horseshoe crab eats invertebrates it encounters as it moves along the substrate. Predation on adults is minimal; fish and birds may consume eggs.

reproductive biology

The Japanese horseshoe crab spawns near the high-tide line on sandy and gravel beaches during evenings in July and August. Males and females form pairs before approaching the beach. The pair deposits and fertilizes approximately 20,000 eggs divided among as many as 10 nests. Eggs hatch after approximately five weeks, and trilobite larvae emerge.

conservation status

Not listed by the IUCN.

significance to humans

The blood is used to detect gram-negative bacteria. The Japanese horseshoe crab is sold for human consumption in several Asian countries, where it is considered a delicacy.

Resources

Books

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.

Other

Smithsonian Marine Station at Fort Pierce. "Limulus polyphemus (Horseshoe crab)." 25 July 2001 [12 Aug. 2003]. <http://www.sms.si.edu/IRLSpec/Limulu_polyph.htm>.

Katherine E. Mills, MS