Merostomata

(invertebrate zoology) A class of primitive arthropods of the subphylum Chelicerata distinguished by their aquatic mode of life and the possession of abdominal appendages which bear respiratory organs; only three living species are known.

(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

Resources

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.

Smithsonian Marine Station at Fort Pierce. "Limulus polyphemus (Horseshoe crab)." 25 July 2001 [12 Aug. 2003].

[Article by: Katherine E. Mills, MS]

A class of the phylum Arthropoda, sub-phylum Chelicerata. Merostomes are aquatic chelicerates, characterized by abdominal appendages bearing respiratory organs. Most merostomes are extinct; only the horseshoe crabs, comprising four species and three genera (Carcinoscorpio and Tachypleus of eastern Asia and Limulus polyphemus of eastern North America) survive.

The body of a merostome consists of a prosoma, or head, which lacks antennae, has a pair of compound eyes and a pair of simple median eyes, and bears the chelicerae (pincers) and five pairs of uniramous walking legs with gnathobases for mastication. The opisthosoma, or trunk, consists of 12 or fewer segments which may be freely articulating or partly or entirely fused into a solid shield; the opisthosoma bears the respiratory appendages. The telson (tail) is a solid, usually spikelike, structure. See also Chelicerata; Living fossils.

Merostomata
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Chelicerata Class: Merostomata
Orders
Order Eurypterida Order Xiphosura

Merostomata is a class of marine Chelicerata which includes horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. Some favor the abandonment of the term class Merostomata, in favour of splitting the class into two classes, Xiphosura and Eurypterida. This is because Merostomata is thought to be paraphyletic, with the Eurypterida more closely related to the Arachnida, forming the group Metastomata (Weygoldt & Paulus 1979) or Cryptopneustida (Boudreaux, 1979). This change has not been incorporated into most textbooks, which typically use the traditional "Merostomata" without reference to the Eurypterida.

The Merostomata are the only chelicerates to possess compound eyes; the arachnids have lost the ancestral arthropods' fully developed latero-anterior compound eyes.

References

• Boudreaux H. B., 1979. Arthropod phylogeny with special reference to insects. John Willey & sons, New York, Chichester, Brisbane, Toronto. 320 pp.

• Weygoldt, P. & Paulus, H.F. 1979. Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. Zeitschrift für zoologische Systematik und Evolutionsforschung 17: 85–116, 177–200.

External links

• http://www.earthlife.net/chelicerata/merostomata.html

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