 Learning with Limulus A Model for Teaching Marine Education "There is no animal more maligned and at the same time more worthy of attention than the horseshoe crab." (John Hay - The Sandy Shore.) FOCUS GRADE LEVEL FOCUS QUESTION LEARNING OBJECTIVES Students will use familiar items as analogies to "construct" and study a horseshoe crab. MATERIALS (See attached)   AUDIO/VISUAL MATERIALS TEACHING TIME SEATING ARRANGEMENT MAXIMUM NUMBER OF STUDENTS KEY WORDS BACKGROUND INFORMATION Horseshoe crabs have been called walking zoos and living museums, and if there were only one creature to bring into a marine biology lab for study, Limulus should be on the top of your list. Although they are the most intensively studied marine invertebrate, scientists are discovering new things about them all the time. And, they are an important marine resource that has been harvested by the millions for bait, fertilizer and medical research. Jonathan Swift wrote that…naturalists observe, a flea hath smaller fleas that upon him prey; and these have smaller fleas to bite 'em, and so proceed ad infinitum. Modern naturalists have observed dozens of invertebrates hitchhiking on horseshoe crabs - turning older specimens into moving reefs - at times completely covering the crab's shell. There are even some animals like the Limulus Leech, a flatworm, which are found only on the crabs. Shaking an adult horseshoe crab in a bucket of water also will reveal dozens of crustacean, mollusk and bryozoan epibionts for your class to study. Horseshoe crabs save people's lives everyday. "Blue"
blood of the horseshoe crab is processed into Limulus Amoebocyte
Lysate (LAL) which is extremely sensitive to the presence of
bacterial contamination and reacts if exposed to it. LAL is used
to screen for purity, any injectable medicines and devices that
are put into the human body. What came first, the chicken or the egg? Hundreds of thousands of shorebirds migrating between South America and their Arctic breeding grounds have spent millennia synchronizing their pre-nesting feeding binge with the massive spring spawning of horseshoe crabs in Delaware Bay. Without the bountiful harvest of the greenish horseshoe crab eggs at the mid-point of their journey, it is thought that nesting attempts of many shorebirds like the red knot might fail and threaten the species. There is no animal more maligned and at the same time more worthy of attention than the horseshoe crab. Decades ago, Massachusetts horseshoe crabs had a bounty on their tails and were killed because they were considered pests by shellfish farmers. And until recently, millions were harvested in the Mid-Atlantic for fertilizer and animal feed. Today, fishermen harvest large numbers of crabs for conch and eel bait, and the biomedical industry "bleeds" crabs for producing LAL. Both industries compete with shorebirds and dozens of species of fishes and other creatures that feed on horseshoe crab eggs. The management of horseshoe crabs as a marine resource with multiple users is a complex topic that involves biologists, ecologists, fishermen, the public and politicians. For educators and their students, there are limitless opportunities for learning with Limulus.  Limulus polyphemus - The Side-looking Cyclops "When they drop The Bomb - two things will survive:
cockroaches and Nothing helps demonstrate this survivability concept than the shape and body parts of Limulus. Evolutionary biologists call Limulus a "generalist." That is a species that with a basic design that changes little over time. (Some compare the phenomenon to the dependable "ol' Chevy" as opposed to the latest model of sports car.) Such creatures tend to appear longer in the fossil record than specialized animals like the giraffe or dodo bird. Form follows function. Like an insect with its three body parts (head, thorax, and abdomen) that youngsters are familiar with; the crab also has three distinctive body sections: front and back carapaces - the prosoma and opisthosoma; and a long tail (telson). These help it lumber along the bottom, plowing through benthic communities from Maine to the Yucatan, filling the niche of the dominant benthic predator of clams and worms in Mid-Atlantic estuaries. Students will often draw a tank or bulldozer to illustrate their observations, especially when the crabs crowd bay beaches in May to dig simple nests at the high tide line. The helmet-shaped prosoma is rounded in front like a shovel and arches up gracefully so that movement through the water or sand is efficient; plus there is a lip across the base that facilitates digging in like the blade of a snow plow. Two nicknames of Limulus are soldier and helmet crab, and this too is often reflected in children's descriptions and pictures. Lift a crab out of the sand and the imprint of the prosoma is the horse's foot or shoe that gives it the two best known common names. The prosoma is hinged to the opisthosoma and not only does this facilitate graceful swimming that any backstroker's coach would be proud of, but when flexed, automatically closes up the animal, including the vulnerable book-gills, when it is turned over or stranded on the beach during nesting. A row of movable spikes protrude from the sides of the opisthosoma, and one wonders what predators they deterred over the 350-million year history of the crab. Today they may still afford some protection against an adult's few estuarine predators - dusky sharks and loggerhead turtles; and they may also serve as sensory organs, detecting the physical conditions of the water. Perhaps the most common misconceptions about Limulus involve its tail. It is not poisonous and this xiphosuran arthropod cannot slash it around like a gladiator's xiphius or sword. (Although coastal Native Americans reportedly utilized it as a fishing spear point.) The tail does not function as rudder or propeller for swimming, but more importantly, if overturned, the crab can wedge it in the sand like a pole-vaulter to slowly flip back over. (We remind students that they should never pick up crabs by the tail because damaging the muscle or breaking the shaft may leave it without a crutch or cane to help right itself when stranded on the shore.) The exoskeleton of Limulus is made of a pure form of chitin. Besides protecting the crabs internal structure, it has anti-fouling properties that inhibit bacteria and other fouling organisms from growing on the shell of young crabs. Chitin has also been used as a suture and coating material to repair wounds in humans. If ever a creature needed a podiatrist to explain its movements, it is the horseshoe crab. Limulus has thirteen pairs of appendages and simple modifications of its jointed (Arthropod) legs perfectly suit its life on the bottom of the bay. The five pairs of legs that wave around and startle people when a crab is picked up also do most of the work for Limulus. The pincher ends of the legs are as smooth as chop-sticks. Although they can firmly grip a finger like a pair of kitchen tongs, they are not sharp-edged or serrated, and instead function like tiny trowels or post-hole diggers to gather prey out of the sediments and work them up to the mouth. People say horseshoe crabs "eat on the run." Besides digging and walking, the base of the legs surrounding the mouth of the crab move to grind baby clams and other food items between the brush-like gnathobases around the mouth. It is sometimes said of the four species of horseshoe crabs that comprise the Class Merostomata (legs surrounding the mouth) that they must "walk" to eat, although it or more like treading in place, the way some people sometimes harvest clams. In mature male crabs the first pair in the series, or "fist" pair, as I like to describe it (pedipalps) is fashioned to lock on to the female's shell while spawning at the shoreline. This "boxing glove" and smaller shell size helps student surveyors identify the sex of individuals at the spawning beaches. The back legs of Limulus are called swimmerets. Splayed like the bottom of a ski pole, they provide (If you'll pardon the pun) the horse[shoe]power to push the crab through the sand while burrowing and especially digging a spawning pit. The short front appendages or chelicera are like forceps and assist in picking meat out of clamshells and moving food to the mouth. These also help give the crabs a scorpion-like appearance when turned over, and are a clue that they evolved from similar ancestors of the second largest Class of Arthropods, the (70,000 species strong) Arachnids. The coordinated movement of all of the legs assists the most interesting appendages, the gills, in allowing the crab to swim. Although the crab is not a long distance traveler, it can swim elegantly when necessary, especially as a youngster. The paddle-like operculum or cover of its "book-gills" pushes most of the water; while at the same time protecting the more delicates "pages" that allow it to breath underwater. The simple structure of book-gills on the crab keeps the tender tissues moist but exposed to air and allows it to remain alive out of the water for extended periods if stranded. The synchronized movements of the legs and gills are also crucial for the crab to spawn successfully. As legs excavate, swimmerets shove and prosoma plows, the book gills pump water across the eggs to aerate and fertilize them. Scooped from the moist sand, the tiny clumps of eggs look like a handful of tiny living green marbles. Limulus is a real, live "blue-blood" and this is because the element that is present in its blood protein to bind oxygen is not iron, like in humans; but copper. When oxidized, an iron nail turns rusty and red; but when exposed to the air, the blood of Limulus turns a greenish-blue, like an old copper penny. Processed blood in the lab can look as rich as food coloring. Like our blood, the horseshoe crab's blood also seals off wounds and acts like a natural Band-Aid and antiseptic when the shell is punctured. "Seeing is believing." Limulus has "eyes" all over; ten in all! They form a fantastic array of sensory devices that assist the crab day and night. The two largest and most obvious are lateral eyes on the side of the prosoma. These are compound eyes like those on a dragonfly and each has 1,000 light detectors (ommatidia). So when Limulus looks at you, it sees hundreds of human images moving towards it; much like the view through a toy prismatic lens. Signals from two other light sensors allow these eyes to become so sensitive to the faintest light that the crabs can find their mates in almost total darkness along the shore. These two median ocelli near in front of the crab can also form an image, and detect ultraviolet light from the moon and stars (We cannot see UV light, but it is what causes sunburns in humans). This sensory system helps regulate the crab's circadian rhythm, like the automatic timer on a nightlight. Five other light sensitive organs, including the centrally-located one responsible for the "Cyclops" moniker, are found at the front of the crab; two of which are on the underside near the mouth. The tenth "eye" is on the tail where photosensitive organs also help synchronize the day-night light cycle of the crab. When I introduce Limulus to students and teachers, I like to paraphrase Charles Kingsley's comments about the lobster in The Water Babies: "All the ingenious men, and all the scientific men, and all the fanciful men in the world, with all the old German bogy-painters into the bargain, could never invent anything so curious, and so ridiculous as"…a horseshoe crab! LEARNING PROCEDURE 1. Research and review horseshoe crabs and their classification. Suggestions: " The Directed - Guided, Open-ended Approach. Guided learning may be the best approach for classes outside the East Coast region that don't have access to live or preserved specimens. " The Constructivist Learning Model. Proceed by relating the known to unknown - building upon existing knowledge (Example: insects, crabs, etc.) " Using Analogies - Compare and contrast the horseshoe crab with other creatures, including humans, to advance towards higher order learning skills.   Nematode worm on shell. Two month old juveniles. THE BRIDGE CONNECTION THE "ME" CONNECTION Have students research the biomedical uses of horseshoe crabs and their blood. (Consider: "How is my health and well-being directly related to the horseshoe crab?") Have students research the ecological niche horseshoe crabs
occupy in the estuarine environment and the global link they
create along with migrating shorebirds. CONNECTIONS TO OTHER SUBJECTS EVALUATION EXTENSIONS Students can research creatures that rely on horseshoe crabs
(Epibionts) http://www.brookdalecc.edu/staff/sandyhook/tripdata/creature/horseshoe/index.htm RESOURCES www.dnr.state.md.us/education/horsehoecrab/raising.html www.lsc.usgs.gov/2065/index.asp www.ocean.udel.edu/horseshoecrab http://sssp.fws.gov www.hup.harvard.edu/harvard.edu/catalog/SHUAME.html www.ufrsd.net/staffwww/Stefanl/Crab%20Booklet.PDF www.k12.de.us/warner/introhsc.html Brookdale College's Ocean Institute at Sandy Hook (New Jersey) provides field trip and teacher training programs, and has produced activity books and the DVD: Learning With Limulus as resources for schools. The Ocean Institute Articles: Dunlap, Julie, 1999 Extraordinary Horseshoe Crabs. Carolroda Nature Watch Books, Lerner Pub., Minneapolis. 1-57505-293-8 Grant, Dave, 1998 Living on Limulus Underwater Naturalist, Vol. 24, No. 2 pp. 13-21 American Littoral Society, Highlands, NJ 07732, September 1998. Grant, Dave, 2001 (In Limulus in the Limelight: A Species
350 Million Years in the Making and in Peril? Edited by John
Tanecredi; Kluwer Academic/Plenum, 2001) Grant, Dave, 2002, Hitchin' a Ride (In Naturalist at Large) Natural History Magazine, June 2002, pp. 66-69. Shuster, Carl N. et al, 2004 The American Horseshoe Crab,
Harvard University Press NATIONAL SCIENCE STANDARDS Content Standard F: Science in Personal and Social Perspectives
ACKNOWLEDGEMENTS Dave Grant is the director of Brookdale College's Ocean
Institute and has relied on horseshoe crabs as a teaching
tool for over twenty years. Honors for his programs in which
Limulus deserves credit include: Environmental Quality Award
(USEPA), Innovative Environmental Programming (Water Resources
Association), Excellence in Environmental Education (Grossinger
Award); and personal citations from: NJ Marine Education Association,
Audubon Society, and Alliance of New Jersey Environmental Educators
(Educator of the Year) Special thanks to: These materials were produced by Dave Grant with partial funding
ADDITIONAL NOTE |