Horseshoe Crab
Authors of Subpage
James Cummings and Chad Kinzly
Introduction
To many the Horseshoe Crab is simply an armored oddity occasionally seen skittering just below the surface of the water. Its long hardened tale and armored shell with rows of small spikes might give you pause and ensure you don’t get to close. The Horseshoe Crab is a carnivore, however they are quite harmless to humans, preferring to eat clams and mollusks rather than your juicy toes.
Officially known as Limulus polyphemus, the first word Limulus comes from a Latin word meaning oblique or askew. Polyphemus refers to the giant in Greek epic the Odyssey because of the misconception that the Horseshoe Crab has only one eye. The Horseshoe Crab was given its common name for his armored shell looking similar in shape to Horseshoe, however they have gone by many other names over the years horse foot, sauce pan, or helmet crab to name a few.
Despite its name the Horseshoe crab closer related to spiders, ticks, and scorpions than it is to crabs. These wonderful little aquatic arthropods are seen in the waters along the eastern shore of North America.
Scientific Classification - Limulus polyphemus
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: Xiphosura
Family: Limulidae
Genus: Limulus
Species: L. polyphemus
Niche and Habitat
Atlantic Horseshoe Crabs are found along the eastern shore of North America with concentrations along the United States and in the Gulf of Mexico. Though Horseshoe Crabs can be found throughout Asia they are a different species. They prefer soft sand and spend almost their entire life cycle in the water. The Horseshoe Crabs come on to the beach to lay their eggs, once hatched the spawn will live there first few years in the beach shallows. Once grown they will travel to 30-200 meter deep water where they will live until they return to the beach for spawning.
Horseshoe Crabs are carnivorous and primarily eat clams and muscles that they dig up out of the sand on the sea floor. There eggs are a source of food for most sea birds and once fully grown they are preyed upon by sea turtles and sharks.
Depending on there environment the shell of the Horseshoe Crab can host various species of sponges, barnacles and lichens.
Evolutionary History
Horseshoe were originally with the extinct eurypterids in Merostomata. Recently studies suggest a relationship between the eurypterids and the arachnids switching the Horseshoe Crab to the group Xiphosura. It is believed that Horseshoe Crabs evolved in the shallow seas of the Paleozoic Era (570–248 million years ago).
Horseshoe Crabs are considered living fossils since they have changed so little in such a long stretch of time. A similar species has been found as far back as 400 million years and fossils show that the Horseshoe Crab as we know it was living at least 148 million years ago in the Jurassic Period. This means that their Horseshoe Crab predates most dinosaurs and their close cousins predate even the oldest dinosaur fossils found.
What has made this species so successful that it has changed so little? Horseshoe Crabs are very tolerant they have the ability to survive in various different temperatures and levels of salinity. They have a very effective but simple set design with a hard protective shell and locomotion from its five sets of legs as well as its gills that function as a paddle helping them swim. All these factors have allowed them to require very little evolution.
Environmental Impacts
Horseshoe Crabs have very little impact on the environment. As creatures who have been on earth for hundreds of millions of years, they have become part of the ecosystem and have neither a positive or negative impact on the environment. However, there are concerns about their dropping population due to coastal development on breeding grounds and humans harvesting them for various reasons. Since the Horseshoe Crabs eggs provide food for many species as well as the adults providing a source of food for predators, continued drops in their numbers could have heavy consequences on the ecosystem.
Conservation
The Horseshoe Crab has been harvested for many reasons over the years from being utilized as animal feed and bait for fishing. Currently the primary concern to the population is in medical company’s harvesting the blood of the Horseshoe Crab. The blood is used to create Limulus amebocyte lysate which is used in medical procedures and studies to detect bacterial endotoxins. This has created a large industry on the fishing of Horseshoe Crabs to harvest their blood as it is the most valuable liquid on earth with an average value of $15,000 per quart. Once the blood has been harvested, most companies return the Horseshoe Crabs to the ocean but there is still much debate about the survival rates of these exsanguinated creatures.
To combat these concerns the Atlantic States Marine Fisheries Commission (ASMFC) has put quotas on the number of Horseshoe Crabs that can be harvested and restricting the harvest of females. The ASMFC sets these quotas based on abundances of both Horseshoe Crabs and certain species of shorebirds.
Friends or Foes?
Horseshoe Crabs eggs and larva are predated on by many species of shorebirds in the early part of their life cycle. Once grown to Adulthood they are primarily only predated on by sea turtles and sharks. The heavy armor of their shell allows them protection from most other seagoing predators.
Depending on the environment they are in there shell are host to many forms of lichen, sponges and barnacles that do not harm the Horseshoe Crab and provide camouflage.
The Horseshoe Crab is susceptible to some parasites including the tube worm and limulus leeches to name a few.
Videos for Horseshoe Crabs
Web Resources for Audience to Explore
https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Horseshoe-Crab - The Natural Wildlife Federations page for the Horseshoe Crab gives brief information on various animals and options for donation.
http://horseshoecrab.org/ - A website hosted by the The Ecological Research & Development Group (ERDG) with the purpose of providing information on the Horseshoe Crab.
https://en.wikipedia.org/wiki/Atlantic_horseshoe_crab#Names_and_classification - Wikipedia page fore the Atlantic Horseshoe Crab providing a plethora of information.
http://www.asmfc.org/species/horseshoe-crab - Atlantic State Marine Fisheries Commission website page covering the Atlantic Horseshoe Crabs presence and effect on recreation and fisheries.
http://bioweb.uwlax.edu/bio203/2011/volz_mack/index.htm - An informative website based entirely on the Atlantic Horseshoe Crab
https://science.howstuffworks.com/life/biology-fields/horseshoe-crabs-endangered-biomedical-bloodletting.htm - A web page specifically discussing the issue of the harvest of Horseshoe Crabs blood for medical research.
References
“Atlantic Horseshoe Crab.” Wikipedia, Wikimedia Foundation, 30 May 2019, en.wikipedia.org/wiki/Atlantic_horseshoe_crab#Names_and_classification.
“Atlantic States Marine Fisheries Commission.” Species - Atlantic States Marine Fisheries Commission, www.asmfc.org/species/horseshoe-crab.
Author. Limulus Polyphemus: Atlantic Horseshoe Crab~ Homepage, bioweb.uwlax.edu/bio203/2011/volz_mack/index.htm.
Davis, Matt. “Horseshoe Crabs Are Drained for Their Blue Blood. That Practice Will Soon Be over.” Big Think, Big Think, 19 Mar. 2019, bigthink.com/surprising-science/horseshoe-crab-blue-blood?rebelltitem=1#rebelltitem1.
Fick, Loraine. “Horseshoe Crabs Endangered by Biomedical Bloodletting.” HowStuffWorks Science, HowStuffWorks, 20 Mar. 2019, science.howstuffworks.com/life/biology-fields/horseshoe-crabs-endangered-biomedical-bloodletting.htm.
“Horseshoe Crab.” National Wildlife Federation, www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Horseshoe-Crab.
“The Horseshoe Crab.” The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research, horseshoecrab.org/.
US Department of Commerce, and National Oceanic and Atmospheric Administration. “Are Horseshoe Crabs Really Crabs?” NOAA's National Ocean Service, 14 Mar. 2019, oceanservice.noaa.gov/facts/horseshoe-crab.html.