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Horseshoe Crabs, Limulus polyphemus

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Description & Behavior

Limulus polyphemus (Linnaeus, 1758), commonly known as horseshoe "crabs" and Limulus albus (Bosc, 1802), Limulus americanus (Leach, 1819), Limulus cyclops (Fabricius, 1793), Limulus occidentalis (Lamarck, 1801), Limulus sowerbii (Leach, 1815), and Monoculus polyphemus (Linnaeus, 1758), were originally classified as a crab erroneously. They are actually a distant relative of crustaceans, and are more closely related to arachnids such as spiders, scorpions and ticks. Although they look prehistoric, and ancient relatives of Limulus polyphemus were present 520 million years ago as evidenced by fossils, this species has only been around for about 20 million years which is not enough time to consider this animal a "living fossil" as they are sometimes called.

Horseshoe crab fossil and tracks

Horseshoe crabs are fascinating creatures. They are one of the few animals that has no predators, other than humans who capture horseshoe crabs primarily for bait. A protein found in the blood of horseshoe crabs is used to detect impurities in intravenous medications; the animals are apparently not harmed during blood extraction. Horseshoe crab blood has also been used in cancer therapy research, leukemia diagnosis and to detect vitamin B12 deficiency.

Another interesting fact is that horseshoe crabs are quite literally "blue blood." Oxygen is carried in the blood of the horseshoe crab by a molecule that contains hemocyanin, which contains copper causing the blood to turn blue when exposed to air. Most red-blooded animals carry oxygen in iron-rich hemoglobin causing their blood to turn red when exposed to air.

The larger female horseshoe crab can reach up to 60 cm in length and can weigh up to 5 kg. The U- or horseshoe-shaped carapace (shell) is smooth and brown, although in some environments the carapace is covered with epiphytic plants and epizooic animals (organisms which live on the outer surface of other organisms and can grow so much that the carapace is no longer visible). This is usually observed toward the end of the horseshoe crab's lifespan of approximately 19 years. During its formative years, the horseshoe crab sheds its carapace periodically, or molts, to accommodate its growing body. The new skeleton is flexible so that it can accommodate the increased body size. The new carapace then hardens and its color forms during tanning of its protein component.

The body is divided into an anterior cephalothorax and a posterior abdomen. The spike-shaped tail, or telson, functions as a tool for digging in sand and a lever if the animal finds itself upside down. The tail might not always be effective however. In Delaware, a program known as "Just Flip 'Em" was established to encourage beachgoers to flip "stuck" horseshoe crabs stranded on the beach during mating season. The horseshoe crab is equipped with 4 pairs of jointed walking legs (pedipalps) each ending in a claw. The fifth pair is larger and allows the animal to lurch forward. The middle segment of each leg is covered with spines used to chew food before it is passed forward and into the mouth located at the base of the legs. Interestingly, locomotion and feeding are closely related, since the animal can chew only when it moves. More on feeding below.

Horseshoe crabs have 10 eyes located all over their bodies, most located on the back or sides of the animal. Some contain only photoreceptors such as the eyes located on their tails. The eyes found on the back each have about 1,000 photoreceptor clusters or ommatidia, each with a lens, cornea and photoreceptor cells. Horseshoe crabs have the largest rods and cones of any known animal that are about 100 times the size of humans'. In spite of the number of eyes, horseshoe crabs still have "poor" eyesight used only to sense light and locate mates.

Horseshoe crabs' respiration is conducted through 6 pairs of appendages attached to the underside of the abdomen called gill books. The first pair, called the operculum, protects the other five pairs, which are respiratory organs and houses the opening of the genital pores through which eggs and sperm are released from the body.

World Range & Habitat

There are 4 species of horseshoe crabs found in the world today. Limulus is the only species found in the Atlantic Ocean. The remaining three are found in SE Asia where the eggs of some species are used for food. In addition to Limulus polyphemus found off the US East Coast from Maine south to the Gulf of Mexico as far as the Yucatan peninsula, the remaining species are:

Tachypleus tridentatus: Malyasia, W Indonesia and China East Coast
Tachypleus gigas: Bay of Bengal, Indonesia to N Australia
Carcinoscorpius rotundicaudata: Thailand and Vietnam to SE Indonesia

Feeding Behavior (Ecology)

Horseshoe crabs are omnivorous scavengers, feeding upon small bivalves, mollusks, worms, dead fish and algae. They feed through the mouth located at the base of the legs, which are covered with thick bristles (gnathobases) that point inward used to grind the food as the animal walks.

Horseshoe crabs

Food is then pushed into the mouth by the chelicera, which is then passed into the esophagus where it is further ground and passed into the stomach and intestine. Waste is excreted through an anus located on the ventral side in front of the telson (tail).

Life History

Sexual maturity is not reached for nine to 12 years. Mating season for the horseshoe crab takes place during the spring and summer full moons. The male uses the first pair of claws to attach to the female. The female deposits between 2,000 and 30,000 eggs in each nest in the sand and the male then releases sperm to fertilize them. The eggs are 2-3 mm in diameter and are an attraction for migrating birds that time their arrival in the season to feed on Limulus eggs. When the moon is full again, the 1 cm long larvae hatch and crawl from where they are buried in the sand to return to the water. In about 1 year juvenile horseshoe crabs will reach about 4 cm in width.

Conservation Status & Comments

Since the 1990s, beach communities along the Atlantic coast have sponsored horseshoe crab surveys to ensure that Limulus polyphemus remains a thriving species. Because of the time it takes for horseshoe crabs to reach sexual maturity, it is important that population densities remain high. In addition, the eggs of the horseshoe crab are an important part of the food web along the Atlantic coast as they provide food for many shore bird communities. To this end, horseshoe crab harvests for bait and biomedical purposes have been restricted in the last decade and protected areas have been established to ensure the viability of Limulus polyphemus along the Atlantic Coast.

Horseshoe crabs, Limulus polyphemus, are listed as Lower Risk/near threatened (LR/nt) on the IUCN Red List of Threatened Species:

A taxon is Lower Risk when it has been evaluated, does not satisfy the criteria for any of the categories Critically Endangered, Endangered or Vulnerable. Taxa included in the Lower Risk category can be separated into three subcategories:

  1. Conservation Dependent (cd). Taxa which are the focus of a continuing taxon-specific or habitat-specific conservation program targeted towards the taxon in question, the cessation of which would result in the taxon qualifying for one of the threatened categories above within a period of five years.
  2. Near Threatened (nt). Taxa which do not qualify for Conservation Dependent, but which are close to qualifying for Vulnerable.
  3. Least Concern (lc). Taxa which do not qualify for Conservation Dependent or Near Threatened.

References & Further Research

Limulus Site - MBL
Horseshoe Crabs "A Living Fossil" - Maryland Department of Natural Resource

Research Limulus polyphemus » Barcode of Life ~ Taxonomy ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species + Database ~ WoRMS

Search for Horseshoe Crabs » ARKive ~ Flickr ~ Google ~ Creative Commons search ~ Wikipedia ~ YouTube

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