Natural History
The horseshoe crab is a "living fossil" that possesses a body plan which has persisted for over 400 million years. The horseshoe crab's persistence has been attributed to its generalist nature. However, with few exceptions the distribution of horseshoe crabs has been tied to estuaries and associated spawning habitats through geologic time.
There are four species of horseshoe crab, and these four species are the only living representatives of the Merostomata and Xiphosura. Despite their common name, horseshoe crabs are not crabs and although they are in the same phylum as crustaceans, they are in reality most closely related to the trilobites which have been extinct for over 500 million years. The four species of horseshoe crab are named in the figure below which also highlights their respective distribution. All four species are similar in terms of ecology and morphology.
There are four species of horseshoe crab, and these four species are the only living representatives of the Merostomata and Xiphosura. Despite their common name, horseshoe crabs are not crabs and although they are in the same phylum as crustaceans, they are in reality most closely related to the trilobites which have been extinct for over 500 million years. The four species of horseshoe crab are named in the figure below which also highlights their respective distribution. All four species are similar in terms of ecology and morphology.
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References:
Anderson, Lyall and Shuster, Carl. The American Horseshoe Crab: A History of Skeletal Structure: Clues to Relationships among Species. Cambridge, MA: Harvard University Press, 2004.
Anderson, Lyall and Shuster, Carl. The American Horseshoe Crab: Throughout Geologic Time: Where Have They Lived? Cambridge, MA: Harvard University Press, 2004.
Ecological Research & Development Group (ERDG): The Horseshoe Crab. c2002-2009. Dover, DE. http://horseshoecrab.org/
Pechenik, JA. Biology of the Invertebrates, sixth edition. New York, NY: McGraw Hill Higher Education, 2010.
Shuster CN, Barlow RB, Brockman JH, editors. The American Horseshoe Crab. Cambridge, MA: Harvard University Press, 2004.
Anderson, Lyall and Shuster, Carl. The American Horseshoe Crab: Throughout Geologic Time: Where Have They Lived? Cambridge, MA: Harvard University Press, 2004.
Ecological Research & Development Group (ERDG): The Horseshoe Crab. c2002-2009. Dover, DE. http://horseshoecrab.org/
Pechenik, JA. Biology of the Invertebrates, sixth edition. New York, NY: McGraw Hill Higher Education, 2010.
Shuster CN, Barlow RB, Brockman JH, editors. The American Horseshoe Crab. Cambridge, MA: Harvard University Press, 2004.