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Understand Him: He's an Old Timer Horseshoe Crab - Limulus polyphemus Prentice K. Stout
2. The Limulus is a "living fossil" whose origin dates back to Triassic times, over 200 million years ago, a time when the first dinosaurs and primitive mammals appeared. Few other well-known animals can claim that record. It is not easy to be a living fossil. To apply for this category, that animal can only have, at most, a few close relatives. The body form should not have changed over the years. A true living fossil's family tree must be tens, or preferably hundreds, of millions of years old. 3. Its tail is NOT a weapon. The animal almost never carries it in an upright position so there is little chance that people might step on the upright tail. This tail, or telson, serves one real purpose: to assist the animal in turning over should a wave tip it on its back. 4. Limulus is found along the Atlantic coast, spending summers in the shallow coastal waters and winters offshore in the mud. They usually walk along the bottom, feeding on sea worms and young clams. Because of this diet, professional clam diggers have claimed that the Limulus can destroy hundreds of clams as they feed. However, it has fed this way for thousands of years and we still have clams. 5. Limulus can reach maturity in nine to 11 years. At maturity, the female is larger than the male. This is one way you can tell them apart. 6. Because Limulus has a hard outer shell (called an exoskeleton), it has to shed (molt) its shell periodically in order to grow. Many of the "dead" Limuli you find on the beach are not dead, but the castoff shells of molted Limuli. Once a Limulus sheds its old shell, it has a new, soft one that hardens in about 12 hours. 7. Limulus has four eyes - two small, simple eyes up forward and two larger, compound eyes (much like a fly's eyes) on either side of the shell. 8. To breathe, the Limulus has "gill books." Turn one over and take a look at these structures which do much the same things as your lungs. Five in number, each gill book contains 100 "leaves" resembling parchment paper. The total surface area is large enough to permit the exchange of oxygen and carbon dioxide. In addition to permitting exchange of these gases, the gill books can sometimes be used by the animal as paddles while swimming upside down in the water. 9. Limulus is a true "blue blood" for, while human blood is red, the blood of this creature is a light blue. Human blood is red because it has a red pigment called hemoglobin which contains iron. The Limulus blood contains copper rather than hemoglobin thus giving the blood its blue color. The animal's blood also contains fantastically sensitive chemicals used by researchers in discovering harmful bacteria called endotoxins, sometimes found in human blood. In short, the blood of this ancient animal might well save your life some day. (See update below.) 10. Limulus has one pair of feeding legs, four pairs of walking-feeding legs, and one longer pair of walking legs that shove the body forward. 11. One final word: This ancient animal is in danger of becoming extinct. Its numbers are being greatly reduced due to extensive use as eel bait, for blood research, and by fearful people who throw the animal up high on the beach, leaving it to die in the broiling sun. Spawning time is its most visible and vulnerable period. The females come ashore dragging the males in tandem. They fall prey to capture and needless death. If killed before the thousands of eggs can be laid and deposited in the sand, the population will continue to decrease. Perhaps these animals should be protected during the breeding season which occurs during the new moon in May and June.
References Further reading: UPDATE 2004 New Sea Grant/NOAA Horseshoe Crab website http://www.ocean.udel.edu/horseshoecrab/ UPDATE 2001 (from Modern Maturity magazine, March/April 2001): Because they are so important in testing drugs for purity, in the past few months, every Atlantic coastal state has restricted by 25 percent the number of horseshoe crabs that can be caught (they are used by fishermen as bait). "When a solution made from the crab's amebocytes—similar to human white bloos cells—is combined with a small sample of a batch of drugs, the mixture immediately clots if the drugs contain certain impurities ... The best minds in medical research have tried to create manmade substitutes, but nothing comes close. And while it can't be comfortable for the crabs to be caught, transported to a special laboratory, and bled for five minutes, at least they are then returned safely to their home waters within 48 hours of their capture." UPDATE 1998 (from National Wildlife Federation): The horseshoe crab has a blood clotting system which produces proteins used to detect gram negative sepsis, a potentially life-threatening bacteria affecting more than 10,000 people each year. These proteins are used by the food industry to test for bacteria in canned food, in medicine for clinical testing of gram negative sepsis in humans, and by the pharmaceutical industry to test if products are free of toxins released from bacteria. The horseshoe crab is endangered in Japan due to coastal development and pollution, and destruction of wetlands and water pollution is threatening U.S. populations. UPDATE 1999: The Horseshoe Crab-Putting Science to Work to Help "Man's Best Friend" LEWES, DE. Dog and cat lovers continually debate about which animal is man's best friend. But ask Bill Hall, marine education specialist for the University of Delaware Sea Grant College Program, his opinion, and he will tell you that the hands-down winner is the horseshoe crab. While few may know it, this prehistoric creature with the helmet-shaped body and spear-like tail has saved countless human lives. "Horseshoe crabs are critical to the welfare of migrating shorebirds that stop along the Delaware Bay each spring to fuel up for the flight north to Arctic nesting grounds. Some of these birds double and even triple their weight by feasting on horseshoe crab eggs," Hall says. "Yet the horseshoe crab is just as important to humans as it is to wildlife," he notes. "This animal's blood contains a unique clotting agent that the pharmaceutical industry uses to test intravenous drugs for bacteria. No IV drug reaches your hospital pharmacy without its horseshoe crab test. So if you or someone you love has ever been hospitalized, you owe a lot to the horseshoe crab." "Delaware Bay is the world's population center for horseshoe crabs, but during the past few years, we've noted a significant downturn in the animal's population, from 1.2 million spawning females down to about 400,000," Hall says. Hall helps organize a regional census of the Delaware Bay's spawning horseshoe crab population. Now in its tenth year, volunteers from Delaware and New Jersey conduct the census on selected bay beaches each May and June. This year, the census will be expanded to additional beaches along both sides of the bay with support from the Atlantic States Marine Fisheries Commission. "Scientists believe the decline is due to overfishing of the crab for eel and conch bait and to the loss of the sandy beaches it needs for spawning," he notes. "The census is designed to help resource managers and scientists gain a better understanding of the horseshoe crab's status and what we can do to guard our 'golden goose.' " Developing an Artificial Bait to Reduce Fishing Pressure On the Horseshoe Crab In Sea Grant research at the University of Delaware, marine biologist Nancy Targett has been working to minimize fishing pressure on the horseshoe crab through biochemistry. She has made significant progress toward identifying the stimulant in female horseshoe crabs that makes them such an irresistible bait for eels and whelks. With this information, she wants to develop an artificial bait that will attract eels and whelks just as well as female horseshoe crabs do. Targett and her graduate students are in the home stretch in chemically characterizing the attractant. The next step will be to incorporate the compound into a variety of artificial bait types and test their effectiveness. Several commercial fishermen in Delaware have contacted Targett, offering to test the baits when they are ready. "The fishing industry is very supportive of this effort," Targett says. "With their help, our goal is to develop an artificial horseshoe crab bait that will work as well as the traditional one. The result should be a win-win situation for the fishermen as well as the horseshoe crab, resulting in more horseshoe crabs for spawning and sustainable uses in medicine." Horseshoe Crab Facts Scientists have learned a lot about the human eye by studying the horseshoe crab's large compound eyes. The horseshoe crab also has numerous smaller eyes called photoreceptors on its top shell and along its tail. Sensitive to light, they help synchronize the crab's internal clock with daily cycles of light and darkness. Return to Rhode Island Sea Grant Fact Sheets posted 4/98 |
1.
This is a crab that isn't a crab! It is distantly related to spiders
and scorpions. To avoid confusion, let's call it by its correct
name, Limulus polyphemus. (Pronounced Lim-u-lus poly-feem-us)