The following resource was first published by the National Aquarium in Baltimore's Department of Education in the winter of 1991 as a "Puffin Report" to schools. 
 
It's late May, and you have just arrived with your family to spend a weekend at the shore before the summer vacation crowd invades the resorts. After settling in at your cottage on Delaware Bay, you head down to the beach to watch the sun set and see t he strangest sight you've ever laid eyes on!

 All along the beach, for as far as you can see in either direction, hundreds upon hundreds of bizarre marine creatures are emerging from the depth of the Bay. Looking like miniature army tanks, they clamber up onto the wet sand with slow jerks of their gl istening shells, or patrol the shallow water where waves are breaking gently. There are so many of them that they climb over one another or collect in small clusters. Some cling to the shells of others and are carried along by the lead animal. It's a fasc inating, but almost frightening, sight to you. What are these strange animals? What are they doing? How can anyone hope to wade or swim with all this incredible activity going on? To local residents, the event you witnessed is just as fascinating, but is also a familiar and expected occurrence. They would have no problem identifying the animal for you as the harmless horseshoe crab.

 The horseshoe crab belongs in the large group of animals called Arthropoda, which includes lobsters, crabs, insects, spiders and scorpions. Even though it looks crab-like, the horseshoe crab is more closely related to scorpions and spiders.

 Over 300 million years ago, long before the dinosaurs appeared on earth, there were hundreds of kinds, or species, of horseshoe crabs and their relatives, the sea scorpions. Today sea scorpions are extinct, and only four species of horseshoe crabs remain. Three of these are in the Far East, from Japan through Vietnam; the fourth is found along the Atlantic Coast, from Nova Scotia south to Mexico's Yucatan peninsula. The scientific name for the Atlantic Coast horseshoe crab is Limulus polyphemus.

 A "Living Fossil"
Because its basic body design has remained almost unchanged for millions of years, the horseshoe crab is often called a "living fossil". The horseshoe crab gets its common name from the "U" or horseshoe shaped of its shell, which is called a carapace. The carapace is the color of sand or mud. This helps the animal blend in with the muddy and sandy bottoms on which it lives.

 Two pairs of eyes are on the rounded, front part of the carapace. The largest pair is located near the top, one on each side. These eyes are compound, like those of insects. They allow the animal to see in all directions and are good at detecting movement . Two very small eyes are located on each side of a small spine found on the front of the shell.

 Beneath the shell are seven pairs of appendages, four of which bear claws. The first pair, called chelicera, are small, and are used to push food into the mouth. The mouth is located at the base of the legs. Then come five pairs of long walking legs. The last set of appendages, the chelaria, are also small, and are used to help the first pair move food towards the mouth.

 Horseshoe Crabs are for the Birds!
The scene described at the beginning of this article is really only part of the fascinating spectacle seen every year on the Bay, for when the crabs arrive, so do the shorebirds. Hungry birds which are flying northward from their southern wintering habita ts in Central and South America stop along the Bay shores to feast on the horseshoe crab eggs. Red knots, sanderlings, ruddy turnstones, sandpipers, as well as gulls, song sparrows, grackles, mourning doves, and even pigeons are just a few of the kinds of birds that find a feast. Raccoons, foxes, diamondback terrapins, moles, and even small fish and mollusks join in. During the last two weeks of May the bay beaches may host between 500,000 and 1,500,000 shorebirds alone! The birds gorge on the eggs and ma y double or triple their weight before moving on.

 Good for People, Too
Though the eggs and flesh of Limulus polyphemus are not toxic to people (that of the other three species are), they are not eaten by people today. Years ago, however, Indians did eat the lump of meat in the abdomen that moves the tail. They also us ed the shells to bail water out of their canoes, and the tails as spear tips.

 More recently, horseshoe crabs have been used as fertilizer and as feed for chickens and hogs. Though a few farmers still till horseshoe crabs into their fields today, their use as fertilizer has largely been replaced by chemicals. Chicken and hogs fed on horseshoe crabs developed a bad taste, so this was discontinued.

 Today horseshoe crabs are important to people for their use in medicine. For over 50 years they have been used in eye research. They are easy to study because they have large eyes and a large optic nerve (the nerve that sends signals from the eye to the b rain). Scientists have learned a great deal about how human eyes function from research on horseshoe crab eyes.

 Chitin is Excitin'
Chitin is a substance found in the shells, or exoskeletons, of horseshoe crabs, as well as other arthropods, such as lobsters, crabs, shrimps, spiders, beetles, and mosquitoes. It has received the attention of scientists because is non-toxic, biodegradabl e, and when processed to produce another substance called chitosan, can be used to produce a variety of important products.

 Contact lenses, skin creams, and hair sprays can be made from chitin. It can be used to remove lead and other harmful metals that may be dissolved in drinking water, and clean certain harmful chemicals from wastewater. Chitin joins the fight against fat w hen it is added to foods. It has the ability to bind with fats and then passes them through and out of the body without being digested. Chitin can also be made into string used to sew up wounds and used in wound dressings. People do not have an allergic r eaction to the stitches, which dissolve slowly, and the dressings actually promote healing. Many of these products are now available in Japan and are under development in the United States.

 To find a meal of its favorite foods - worms, mollusks and dead fish, the horseshoe crab crawls along the bay bottom, using its small first pair of legs as feelers to detect the presence of prey. When it comes upon a worm or clam the small claws pick it u p and move it to the bristly area near the base of the walking legs. The horseshoe crab has no jaws and uses these bristles to crush the food as it moves its legs. This means that a horseshoe crab can only eat while it walks along the bottom!

 Behind the mouth and the walking legs is a body segment called the abdomen. The abdomen is connected to the rest of the carapace by a flexible joint, allowing it to move up and down. Located on the underside of the abdomen are the horseshoe crab's gills, called book gills. These are broad and flat and look like the pages of a book. The horseshoe crab gets oxygen from the water using these gills. Also, as long as the gills are moist, the horseshoe crab can get oxygen from the air while it is on the beach. Young horseshoe crabs can swim upside down! They flap their book gills and flip their abdomens up and down to propel themselves along.

 The horseshoe crab ends with a long tail, or telson. The telson is used to help the animal flip over when lying upside down on its carapace. The tail is not poisonous and does not have a stinger as some people believe. A horseshoe crab, though it look mea n and fierce, is a very gentle, harmless creature.

 Like many animals with shells, eventually a horseshoe crab outgrows its own, and must molt. This means it grows a new shell and leaves its old one. To do this, the old shell splits around the front edge, and the crab crawls out. At first the new shell is very soft, but it soon hardens, and the horseshoe crab is about one-quarter larger than it was before. Females are larger than males, and can grow to a length of 60 cm (24 inches).

 Reproduction is a Big Event
During the cold months of the year, the crabs lie half-buried in the bottom of the Delaware Bay and Atlantic Ocean. As spring returns, an unknown signal triggers the crabs to stir and begin an annual migration to the shore. By late May, over one million h orseshoe crabs are crawling in the shallow waters along the Delaware By, the center of the Atlantic Coast population.

 First the males arrive, then the larger females. To attract a mate, the female releases a chemical called a pheromone into the water. A male attracted to a female attempts to hook onto her abdomen using special claspers located on the end of his first pai r of walking legs. Sometimes a chain of several crabs is formed as one male clasps onto another behind a female! Once the female has a male in tow, she slowly leads him to the edge of the water, where she scoops out a nest in the sand and deposits up to 2 0,000 BB-sized eggs. As she drags the male over the nest, he in turn fertilizes them. Two weeks later the eggs hatch, and out emerge tiny horseshoe crabs about 3.mm in size. Minus a tail and called trilobite larvae, because of their similar appearance to the ancient, extinct trilobites, they swim feebly about in the water as plankton, feeding off a yolk sac while their digestive system matures. Finally, after several molts and the development of a tail, they come to rest and live on the bottom.

 Young horseshoe crabs continue to molt and grow for the next ten years until they reach maturity. How long can horseshoe crabs live? No one really knows. A few horseshoe crabs have been kept in aquariums for 15 years.

 True Bluebloods!
Perhaps the greatest use of horseshoe crabs is found not in their shells, but in their blood. In the early 1950's, scientist Frederick Bang discovered that the blue-colored blood of the horseshoe crab contains special cells that help kill certain kinds of bacteria. When a crab receives a wound, the cells swarm to the area , form a clot, and kill the invading bacteria. Bang was able to separate the chemical in the blood cells that formed clots in the presence of bacteria, which can be harmful to people as well as horseshoe crabs. He called the substance LAL - Limulus amoebocyte lysate. LAL is now used as a fast, effective way of testing drugs to make sure they are free of harmful bacteria before they are given to people. Blood is collected from hors eshoe crabs during the summer months, where they are easily caught in the shallow waters off the Atlantic coast from Cape Cod through North Carolina. After collecting some of the blood, the crab is returned to the water. In one bay off of Cape Cod, over 8 0,000 crabs are bled over the course of a season. Even so, the product is rare enough to be very expensive, up to $15,000 a quart!

 What's in Store for Horseshoe Crabs?
If you were to actually see the springtime arrival of horseshoe crabs to the Delaware Bay, you might think that there will always be plenty of Limulus in the waters of the Atlantic. But because they have become important sources of products for pe ople, there is some concern that we may use them to the point where population numbers go down. This has happened already with the species that live off of Japan. In that country horseshoe crabs were declared endangered and are now a protected animal. In the United States the importance of the horseshoe crab was recognized in 1986 when the governors of New Jersey and Delaware declared a strip of beach where the horseshoe crab mate as a sanctuary to protect the crab eggs. The Food and Drug Administration i s also setting stricter rules for the bleeding of horseshoe crabs, so that they may be returned to the water in a shorter time, and subjected to less stress.

 Horseshoe crabs have successfully survived for millions of years. Their future depends on how much people understand and appreciate the importance of horseshoe crabs to wildlife and humans, and on the conservation practices undertaken to preserve them.


:::back :::