The Horseshoe Crab

Taxonomy

Kindom Anamalia

Phylum Arthropoda

Subphylum Chelicerata

Class Merostomata

Order Xiphosura

Family Limulidae

Abstract

The Horseshoe Crab (Xiphosura) is one of the most studied arthropods. For being an invertebrate it has a complex set of internal systems that were known to have developed millions of years ago. The complex systems of the horseshoe crab that have intrigued scientist for decades include the circulatory system, reproductive methods. The circulatory system was reconstructed using scanning electron microscopy, light microscopy and transmission electron microscopy. The reconstruction depicted the complexity of the circulatory system and also suggested high levels of interactions with an advanced nervous system. Another study investigated the blood of the Horseshoe Crab and determined the mechanism in which Limulus causes blood coagulation and initiates the immune response. Researchers found that reproduction varied slightly by species and varies slightly within species depending of population size and local climate. Population size had an impact on whether females were monandrous or polyandrous. Large climate variability causes seasonal breeding and lower climate variability causes year round breeding.

Distribution and Introduction to the Horseshoe

The Horseshoe Crab is often referred to as a living fossil due to its prehistoric appearance. Horseshoe Crab fossils are thought to be as old as 450 million years old which predates dinosaurs by more than 200 million years. Current horseshoe crabs have a strong resemblance to their ancestor. Scientist believed that the Horseshoes lack of evolution was due to having a small gene pool however when investigated researchers found they have a large pool. The lack of evolutionary change is due to being well adapted and not needing to evolve. orsHorThe Horseshoe Crab has been studied extensively due to its unmatched success and its continued importance to the marine food web. Horseshoe Crabs are found in the Pacific, Atlantic and Indian Oceans. They are mostly found within 30 degrees latitude from the equator and are not adapt to cold waters. The Horseshoe Crab is most populous near the coast congregating around soft sand or muddy waters. They are not considered to be pelagic arthropods and are rarely found far off coast. Although the Horseshoe Crab has been successful for millions of years recently we have seen declining populations in the North Atlantic coast of North America as well as the North Pacific cost of Asia. Scientist are not certain about the cause of the declining species but it is speculated the decline is due to changes in water conditions. This could prove to be detrimental to local and global marine food webs.

Circulator System

The Horseshoe crab has a complex circulator system that has been well studied to in order to determine how it works. The researchers used various methods for making a 3D reconstruction of the circulatory system. The heart was examined using scanning electron microscopy. Light microscopy and transmission electron microscopy was used to reconstruct some of the other major structures. The 3D reconstruction allowed researchers to more accurately see its complexity and how it works together. The morphology of the circulatory system varies slightly by species however, all species contain complex artery-like vessels. In this study researches inadvertently gained some insight into how the nervous system works with and regulates the circulatory system (Göpel, 2015). An aspect of the circulatory system that is of particular importance to the scientific community is their blue blood. Like other crustaceans their blood is blue due to the use of copper rather than iron in their blood. The blood contains Limulus amebocyte lysate, which is highly valued in the medical community because it accurately tests equipment and drugs for sterility. Researchers found that bacteria contain Lipopolysaccharide (LPS, endotoxin) which causes Limulus to initiate coagulation in the blood. The coagulation of the blood initiates an immune response in the Horseshoe Crab for protection against infection. When Limulus is applied to unsterile equipment the blood will also coagulate in response to bacteria. (Conard, 2006)

Reproductive System

The Horseshoe Crab is a marine organism and spends the majority of its time in the water around the coasts. However it does not mate in water. Reproduction in the Crabs varies slightly by species. Scientist were interested to see how the mating process worked and if females would seek out to mate with more than one male. Horseshoe Crabs mate during warmer climate. Depending on where the population is found they can either reproduce seasonally or year-round. Horseshoe Crabs like to mate during times of high tide and typically within a few days of the full moon, so researchers believe that a full moon is a cue for Horseshoe Crabs to copulate. Populations that only reproduce during certain periods typically come together in large groups on the beach to mate the congregations are known as mating nests. Depending on the size of the beach, the number of Horseshoe Crabs forming a nest can vary considerably from only a few dozen to a couple thousand. Once the Crabs gather on the beach the males walk adjacent to the shoreline looking for potential females. Once a female is found the male makes a move on the female and attaches himself to the females back using specialized anterior claws. The couple moves away from the shoreline to find a more suitable place to mate. The male then fertilizes the female’s eggs and the female lays the eggs in the sand. Some males are unable to find females, however they have some success reproducing with females that have already copulated. Once the eggs are laid they will take a few weeks to hatch. The small Horseshoe Crabs closely resemble their adult versions (Liew, 2015). They make the journey to towards the water however only a small fraction make it to the water the majority of the eggs are consumed by predators primarily birds and some do not make it to the water. Researchers found whether a female was monandrous or polyandrous was partially dependent on the number of males looking for a females in the nest. If there is a lot of male to male companion there were more females who were polyandrous. Researchers found that if a female was monadrous during a breeding seasons she would continue to be monandrous in subsequent breeding periods. Additionally if a second male attempted to mate with the female the female would cease copulation altogether.

Conclusion

The Horseshoe Crab has been around more than 450 million years and is one of the oldest living species. The continued success can be attributed to its complex internal systems as well as its hard exoskeleton which makes it less susceptible to predation. The long existence of the Horseshoe has made it a vital part of most marine food webs and needs to be protected from exploitation. The reconstruction of the circulator system allows us to see how more complex invertebrates deliver gases, nutrients and other chemicals. Blue blood is fascinating since the vast majority of organisms are known to have red blood, however the chemical Limulus is of particular importance. Reproduction in the Horseshoe Crab is similar across species, differences can be attributed to different needs in different populations. The Horseshow crab has proven time and time again to be a remarkable organism.

Work Cited

Conrad, M. L., Pardy, R., Wainwright, N., Child, A., & Armstrong, P. B. (2006). Response of the blood clotting system of the American horseshoe crab, Limulus polyphemus, to a novel form of lipopolysaccharide from a green alga. Comparative Biochemistry & Physiology Part A: Molecular & Integrative Physiology, 144(4), 423-428. doi:10.1016/j.cbpa.2006.03.013

Göpel, T., & Wirkner, C. S. (2015). An “ancient” complexity? Evolutionary morphology of the circulatory system in Xiphosura. Zoology, 118(4), 221-238. doi:10.1016/j.zool.2014.12.004

Johnson, S. L., & Brockmann, H. J. (2012). Alternative reproductive tactics in female horseshoe crabs. Behavioral Ecology, 23(5), 999-1008.

Liew, P. L., Ng, W. L., & Tan, S. G. (2015). Levels and patterns of genetic variation in an Asian horseshoe crab species, Tachypleus gigas Müller, from the Malay Peninsula. Marine Biology Research, 11(8), 879-886. doi:10.1080/17451000.2015.1024135