Class Merostomata (horseshoe crabs)

1. Separate sexes (gonorchoristic) exhibit pseudocopulation during mating.
2. External fertilization.
3. Juvenile stage is a small version of the adult.
4. Thick, horseshoe shaped carapace covers the large prosoma.
5. Bears a long post-anal, caudal spine.
6. Book gills located behind the sixth pair of abdominal appendages representing the respiratory structure
7. Spiny gnathobase at the base of the legs shreds food and pushes toward the ventrally located mouth.
8. Large, diffuse, lateral, compound eyes.

The merostomatans, commonly called horseshoe crabs, were once very diverse and abundant, but now are represented by only four species. Horseshoe crabs are relatively large marine arthropods which are classified as chelicerates due to the somewhat unique feeding structures (the chelicerae) and methods by which they eat. The merostomatans have features which are appropriate for marine environments and are not typical among the other chelicerates. Examples include external fertilization and the lack of a water conserving excretory system. The horseshoe crabs are bottom dwelling, nocturnal creatures, which are common in the shallow waters and sandy shores of the Atlantic coast. Merostomatans feed on worms, molluscs, and other small invertebrates in shallow ocean sediments.

• The horseshoe crab, is common to shallow waters of the Atlantic coast of North America from Canada to Mexico.
• Although technically merostomatans do not copulate, they engage in a pseudocopulatory act which stimulates the discharge of male and female sex cells so that external fertilization can take place. This differs markedly from the internal fertil ization that occurs from actual copulation in the terrestrialchelicerates.
• The horseshoe crab is one of the few surviving members of an ancient group of chelicerate arthropods first appearing in the Devonian period 400-350 million years ago.

1. Bonaventure, Joseph and Celia and Shirley Tesh. 1982. Physiology and Biology of horseshoe crabs. New York: A.R. Liss.
2. Carthy, J.D. 1965. The behavior of Arthropods. San Francisco: W.H. Freeman.
3. Clarkson, E.N.K. 1993. Invertebrate paleontology and evolution. New York: Chapman & Hall.