Friday, February 18, 2011

The Atlantic Horseshoe Crab! by Connie Gould

The Atlantic Horseshoe Crab! (Limulus polyphemus)

Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Merostomata
Order: Xiphosura
Family: Limulidae
Genus: Limulus
Species: L. polyphemus
The Atlantic horseshoe crab lives primarily in shallow ocean waters on soft, sandy, or muddy bottoms. They do, however, come ashore for mating in the late spring, and they spend their winters on the continental shelf. There are four species of horseshoe crab and are the only remaining members of the order Xiphosura, one of the oldest classes of marine arthropods. Despite their name, they are actually not crabs, but are related to arachnids (e.g. spiders, scorpions, ticks, and mites). The name “limulus” means “askew” and “polyphemus” refers to the one-eyed giant in Greek mythology.
The Atlantic horseshoe crab has three parts to its body: the prosoma (the anterior region), the opisthosoma (the abdominal region) and the telson, a spine-like tail. They have a hard outer shell (an exoskeleton), 5 pairs of jointed legs (the first four are used for walking, while the last pair are located near the gills and are used for pushing. They also have a pair of pincers. They can grow up to 2 feet long and weigh up to 10 pounds. The horseshoe crab sheds its skin as it matures. The male horseshoe crab is smaller than the female; being two-thirds the female’s size. The horseshoe crab has two pairs of eyes; the first pair is compound, and the other pair is simple.

Form and Function
Horseshoe crabs are scavengers and feed on molluscs, worms, crustaceans, and small fish. They obtain their food by using chelicerae, which are feeding appendages which are jointed and sometimes will have claws. The chelicerae are used to push food into the mouth while mandibles cut or grasp the food. The food passes from the mouth into the esophagus then into a crop and a gizzard that grind the food, before finally passing into the stomach and intestine and ends in the anus which is located ventrally in front of the telson. Behind the horseshoe crab legs, they have book gills, which exchange respiratory gases and can sometimes be used for swimming. One interesting thing about horseshoe crabs is that they have no hemoglobin in their blood, and instead must rely on hemocyanin to carry oxygen. Because of the copper found in the hemocyanin, their blood is blue. Their blood contains cells called amebocytes, which are similar to white blood cells in humans. The horseshoe crabs protects itself against predators with its hard shell covering, and its spiked tail (the telson) is used as a rudder, so say the crab finds itself flipped upside down, it can bend it abdomen at the point where it meets the main shell, and can dig into the sand (the tail serving as an anchor) while it turns itself over. To reproduce, horseshoe crabs come ashore to shallow coastal waters. The male finds a female and clings onto her back. The female will then dig a hole in sand, lay her eggs, and the male will fertilize them. The eggs (if they survive the wrath of the many hungry shore birds) take two weeks to hatch. The horseshoe crab excretes waste through an anus located on the ventral side near the telson.

Impact on Humanity
Horseshoe crabs are frequently used as bait for eel and conch fishing, especially in the USA. It is important to conserve the horseshoe crab and prevent their endangerment because of the time it takes for them to reach sexual maturity, which is not reached for 9 to 12 years, and their eggs are a very important part of the food web, as they are sources of nutrition for many shore bird communities. Horseshoe crabs are also used in the biomedical and pharmaceutical industries. Horseshoe crabs have blue blood that clots when exposed to endotoxins (a dangerous class of chemicals). This clotting feature serves as an important alarm system to pharmaceutical companies which need to test the sterility of fluids intended for use on human patients.

Journal Article Review
The article discusses how the decline in the numbers of horseshoe crabs could signify climate change. While most people attribute the decrease in the horseshoe crab population to overharvesting for fishing bait, new research suggests that climate change also plays a role in modifying the numbers of horseshoe crabs. The decline in horseshoe crabs draws parallels to climate change experienced near the end of the last ice age. The decrease in horseshoe crabs could impose serious problems for nearby wildlife that feed on their eggs, such as Atlantic loggerhead sea turtles.

"Climate Change Implicated in Decline of Horseshoe Crabs." Welcome to the USGS - U.S. Geological Survey. Web. 17 Feb. 2011. .
"Horseshoe Crab." Assateague Animals and Plants. Web. 17 Feb. 2011. .
"Horseshoe Crab." Enchanted Learning. Web. .
"Horseshoe Crab." Wikipedia. Web. 17 Feb. 2011. .
"Limulus Polyphemus." Smithsonian Marine Station (SMS) at Fort Pierce. Web. 17 Feb. 2011. .

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