Merostomata- Horseshoe Crabs the History

Merostomata- Horseshoe Crabs

The history of the Horseshoe Crab is one of confusion, the traditional association of the coruscations has given them their common name, yet with more close analysis the association is clearly only a loose one.

At one time they were considered true crustaceans, thus the name "crab." However, these dark brown Arthropods that we usually have in our popular touch tank are only distantly related to such crustaceans as the true crabs, shrimps and lobsters. By examining their underside, we can clearly see many similarities between the horseshoe crabs, members of the class Merostomata, and spiders, scorpions and ticks, which belong to the class Arachnida. (Sturtevant 2004)

The formal name for the American species is Limulus polyphemus. Additionally, historical confusion about the species is its age. It is often called a living fossil yet it has no fossil record whatsoever.

While we sometimes see the horseshoe crab referred to as a "living fossil," it is more appropriate and factual to eschew such an oxymoron, since Limulus polyphemus has no fossil record whatever. The genus Limulus ranges back only about 20 million years, and, therefore, is not so old as was thought at one time. (Sturtevant 2004)

Though much older relative of today's horseshoe crabs are known to have been present on the American continent about 520 million years ago.

A judging from fossils found in the Burgess Shales of British Columbia, Canada. While these relatives and their descendants enjoyed a phenomenal evolutionary success, surviving environmental changes that led to the rise and fall of the dinosaurs, they did not undergo great diversification. Actually a modest four species grouped into three genera inhabit the earth today. (Sturtevant 2004)

The relatively small set of species associated with the horseshoe crab name are relegated in relatively small numbers to only a few regions of the world, waterways.

Our Limulus is only found along the entire Atlantic coast from Maine south and along the Gulf of Mexico as far as the Yucatan. The other two genera are found along the coast of southeast Asia and adjacent islands such as Japan. (Sturtevant 2004)

Another notable fact about the horseshoe crab is that like humans it has no natural enemies and therefore lives in relative peace, at the will of the retention of its specialized habitat. The habitat of the Merostomata class is largely associated with its life cycle.

Spawning adults prefer sandy beach areas within bays and coves that are protected from wave energy. Beach habitat also must include porous, well-oxygenated sediments to provide a suitable environment for egg survival and development. Differences between coarse- and fine-grained sand, as well as how rapidly the sand drains, affect nest-site selection and nesting synchrony. The preferred sites are usually located next to large intertidal sand flat areas, which provide protection from wave energy and an abundance of food for juveniles.

The eggs, juveniles and adults also thrive in highly salient water.

Horseshoe crabs use different habitats at different life stages. For example, protected beaches provide essential habitat for horseshoe crab spawning efforts, while nearshore shallow waters are essential nursery habitat." (Natural History Web site: (http://www.horseshoecrab.org/nh/habitat.html)

Specific habitat requirements for adult horseshoe crabs are not known. Although the animals have been found at depths greater than 200 meters, some studies suggest that adults prefer depths of less than 30 meters. During the spawning season, adults typically inhabit bay areas adjacent to spawning beaches and feed on bivalves. In the fall, adults may remain in bay areas or migrate into the Atlantic Ocean to overwinter on the continental shelf. (Natural History Web site: (http://www.horseshoecrab.org/nh/habitat.html)

There are many species that depend on the horseshoe crab for food and habitat and the interactions between all such species determine the health and wealth of the ecosystem, associated with the Horseshoe crab. Shorebirds, of many kinds are dependant upon the Horseshoe Crab larvae for food.

At least 11 species of migratory birds use horseshoe crab eggs as their primary food supply during their 2 to 3-week stopover. The eggs replenish their fat supply during their trip from South American wintering areas to Arctic breeding grounds. Most of the eggs consumed are from nests that have been disrupted by waves and storms, so the feeding of the shorebirds does not have an adverse affect on the breeding success of the horseshoe. (Natural History Web site: (http://www.horseshoecrab.org/nh/eco.html)

Additionally, many other interactions occur between species in the shared ecosystem including Finfish and Sea turtles, who also rely on the larvae for essential nutrients.