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f01u5p3 - Unit Five COAST/Horseshoe Crabs COAST W ritten by...

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Unit Five COAST/Horseshoe Crabs Project Oceanography Fall 2001 81 COAST Written by Willie Heard On the cutting edge… There are only four species of horseshoe crabs in existence in the world today. These are Limulus polyphemus, Tachypleus gigas, Tachypleus tridentatus, and Carcinoscorpius rotundi-cauda. These creatures are sometimes called “living fossils” because they have changed little from their fossilized relatives; the earliest species identified is approximately 450 million years old. Horseshoe crabs are a valuable resource, commercially as a fertilizer and as a source of calcium for enriching fowl grains and medicinally in identifying endotoxins. The most persistent study on these animals has focused on the properties of their blue blood. In 1977, The Food and Drug Administration of the United States approved a new test for identifying endotoxins using Limulus Amoebocyte Lysate (LAL) purified from horseshoe crab blood. In spite of their commercial and medical importance, horseshoe crabs are threatened by the loss of living and breeding habitats. This habitat degradation has resulted in a rapid population decline over the last few decades. Horseshoe Crabs Lesson Objectives: Students will be able to do the following: Describe the difference between a true crab and a horseshoe crab Briefly discuss the life history of the horseshoe crab Give some medical and commercial uses of the horseshoe crab Key Concepts: chitin, Limulus Amoebocyte Lysate (LAL), chitosan, exoskeleton, pedipalps, molt, arthropod, decapod, chemoreceptors, pheromone What is a Horseshoe Crab? Even though the horseshoe crab has a hard shell and numerous appendages with claws, it is not really a crab. Horseshoe crabs belong to the phylum, Arthropoda, along with crabs, insects, and other invertebrates with jointed legs, but their closest living relatives are spiders and scorpions. True crabs have two pairs of antennae and a pair of mandibles, or jaws; horseshoe crabs lack these structures. Further, comparing the legs of a true crab with the legs of a horseshoe crab reveals another significant difference. True crabs classified as decapod crustaceans, have five pairs of legs, which include a pair of claws. Horseshoe crabs have seven pairs of legs under their helmet-like shells; five of these seven pairs of legs are equipped
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Unit Five COAST/Horseshoe Crabs Project Oceanography Fall 2001 82 with claws. In adult males, the second pair of claws ( pedipalps ) has a “boxing-glove” appearance and is used to grasp females during spawning. Horseshoe crabs also have four simple eyes on the top of their carapace instead of two as with the true crab. Our North American species has been named Limulus polyphemus – Limulus meaning “a little askew or odd” and polyphemus after the giant cyclops in Greek mythology.
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