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Formally known as the Limulus polyphemus in latin/greek (limulus is latin for "odd" and polyphemus is greek for "large eyed/ cyclops, one eyed"), the horseshoe crab is perhaps, one of the oldest living arthopods and are sometimes referred to as "living fossils" due to the fact that they have evolved very little from their predecessors the trilobite. The following report outlines the general habitat of the Limulus polyphemus as well as exploring five different structural, behavioural and physiological adaptations this animal has evolved in order to survive in the modern world.


General Information

Scientific Classification


Level
Classification
Kingdom
Animalia
Phylum
Arthopoda
Class
Merostomata
Order
Xiphosura
Family
Limulidae
Genus
Limulus
Species
Polyphemus




Horseshoe Crab
Horseshoe Crab

Habitat

For most if not all animals, the habitat or the natural surroundings of which an animal lives in is very important to an animal’s survival and well-being. The horseshoe crab Limulus polyphemus is no different, requiring a very delicate and unique environment of parts of the North American coastline in order to lay eggs, develop and become adults. Beginning with the Spawning areas of the Limulus polyphemus, there will also be an examination of other important features of the habitat essential for Horseshoe crab survival. The horseshoe crab can therefore be found in a wide range of places, including the ocean and estuaries and bays.



crab habitat
crab habitat
The spawning areas of the horseshoe crab in North America are mainly located in sandy beaches scattered along the delta of the Delaware River (see fig. 02). Once found in large parts of the North American coastline, recent human destruction of the local habitat (including cities such as Atlantic City) have begun to reduce the coastline areas available for the habitation of the horseshoe crab and the Ecological Research and Development Group (ERDG) was founded in 1995 as a means of retaining and ensuring a brighter future of the four remaining species of horseshoe crab.


Similarly, throughout the early years of development, the Limulus polyphemus will tend to remain close to sand flats or muddy marshes. Water in the areas are often said to be at a very delicate level of salinity for the development of the crabs. It will often be not until these animals are ready to reproduce will they move out into the ocean. Once they become adults, they become more robust and are able to survive more extreme conditions including large variations in the salinity and temperature, of which such changes are extremely common in the oceans.


The habitat of the horseshoe crab naturally links it to other flora and fauna and therefore, its appetite. On most occasions, the Horseshoe crab will eat bits of dead fish or annelid worms when in muddy swamp areas as they are most abundant there. However, once at sea, the Limulus polyphemus will dine on a much wider variety of food, mostly molluscs such as small squids, some octopus and other marine invertebrates. Unfortunately, the unique habitat of the crab also gives it a fair share of predators. Although adult horseshoe crabs have only one main predator, human kind, the juveniles and young hatchlings with their weak and fragile shells provide an excellent opportunity for seagulls and other predators (see fig. 3).

Climatically, Delaware Bay is a comfortable place with mild summers and colder winters. This particularly suits the Horseshoe crab which requires warmer summer months for the eggs to develop. In the colder months, the larger adult craps will retreat to the deeper levels of the ocean while juveniles may stay in cooler waters.

Crab Habitat 2
Crab Habitat 2


Adaptations

Structural Adaptation One


Perhaps one of the most intriguing structural adaptations of the horseshoe crab Limulus polyphemus is that it has a unique set of compound eyes. As can be seen in the image from the “general information” section (fig one), it is possible to see the wide distribution of the eyes throughout the body. While the two frontal eyes are compound, the other “eyes” (median, tail, side) are actually merely sensors which can detect varying degrees of light. For the horseshoe crab, this is a most helpful adaptation because it will assist the animal in gaining a sense of direction when travelling in murky waters. In addition, although they will not be beneficial in tracking predators (it has none anyway), it will allow the Limulus polyphemus to track down prey.

There is a possible explanation to this adaptation having occurred during years of evolution. It maybe that as the horseshoe crab began increasingly spending time in murky and often muddy marshes, it would be more beneficial to have eyes on all sides of the body in order to make searching for food easier. Another factor which could have influenced the development of this structural adaptation is that increasing numbers of Limulus polyphemus and other animals meant that having more eyes may assist in finding more food at sea, giving a distinct advantage.


Structural Adaptation Two

Another structural adaptation of the horseshoe crab is the telson, also known as the tail. The tail is a very useful feature for the Limulus polyphemus because it not only helps to keep the animal steady, but if strong currents overturn the crab, it helps to flip the horseshoe crab over to its correct position. In addition to this, the telson maybe used for more mobility when swimming under the ocean. Moreover, on land, the tail is also proven to be an extremely important rudder in order to maintain the balance of the Limulus polyphemus. Unfortunately, there is a popular misconception that the tail of the Horseshoe crab not only looked formidable but was thought to have been passed down from ancient sea scorpions, since testing, scientists have now agreed that trilobites are a closer ancestor.

As the current most likely ancestor of the Horseshoe crab is the trilobite, the environmental pressure that could have given rise to this may link closely. It is possible that the trilobite, when flipped over under strong currents was unable to flip itself over and therefore if landed on a beach would eventually die. Therefore as time progressed, a tail may have begun to develop in order to assist in maintaining balance while swimming in heavy currents as well as flipping back if the Limulus polyphemus has been ‘capsized’.


Behavioural Adaptation One

At the beginning of summer, both male and female adult crabs will migrate from the deeper oceans to Delaware Bay (covered in “habitat” section). This interesting behaviour allows the male Limulus polyphemus’ to move slowly across the seabed, searching for the scent of a female counterpart. Once found, the male will climb onto the female’s back until spawning occurs and the eggs are laid.

This behavioural adaptation may have evolved as a result of compensating for the Limulus polyphemus’ poor eyesight. In addition, as molluscs and other creatures live far from the shores, it will be important for the animal to migrate there to feed. However, it is also important for the animal to lay its eggs in the beach (see below) and therefore, there was and there still is a pressure for the animal to migrate.


Behavioural Adaptation Two
One of the most important behaviours of the Limulus polyphemus is the laying of eggs on land rather than at sea. Note that it is not on dry land but still accessible by water during higher tides. The reason behind this behaviour is that it allows the eggs to develop much quicker due to the higher temperature variations. In addition, by laying eggs on land and covered by sand, it also protects the eggs from scavengers and other prey.

A good guess for the environmental pressure that could have led the Limulus polyphemus to lay eggs on land rather than out at sea maybe that its ancestors, which were mostly unable to traverse on land, laid eggs which took very long to develop and may have been eaten by the time they could hatch. Over time, the horseshoe crab would develop until it could walk on land and ensure the survival of its young. Another environmental pressure may have been the climate. As the plates moved northward, the climate would begin to drop and therefore, in order for the eggs to survive, they may have to be moved to warmer land as a pose to the sea.


Physiological Adaptation

The immune system of the Limulus polyphemus is both simplistic and extremely effective. This system is most closely related to the circulatory system as the granulocytes in the blood assist in the clotting. When the Horseshoe crab is injured or bacteria are able to enter the blood stream in any way, the granulocytes will immediately seal of the injured area. Meanwhile, Limulus Amebocyte Lysate (LAL) will react with the foreign particle and trap it in a gel-like material.

Perhaps one of the environmental pressures attirbuted to this is the fact that sea water is filled with microbes and as a means of protecting itself, this effective immune system was developed. Another possibility maybe that it’s ancient ancestor had an uneffective immune system and therefore it began evolving into this modern immune system as a means of “survival of the fittest”.


References/Bibliography


Beach Net!. (n.d.) Horseshoe Crabs – The Ancient Mariners. Retrieved 4th June 2008 from: http://www.beach-net.com/horseshoe/Bayhorsecrab.html

ERDG. (2006). The Horseshoe Crab. Retrieved 5th June 2008 from: http://www.horseshoecrab.org/

Wikipedia. (2008). Horseshoe Crab. Retrieved 4th June 2008 from: http://en.wikipedia.org/wiki/Horseshoe_crab

National Geographic Society. (2008). Horseshoe Remain Mystery to Scientists. Retrieved 6th June from: http://news.nationalgeographic.com/news/2002/06/0621_020621_wirehorseshoecrab_2.html