Of the numerous species that once inhabited the world's oceans, only four species of horseshoe crabs still exist today. The modern species face threats to their survival that are unprecedented in their long history. However, these humble animals have immense value. Their role in the food web supporting other species makes them a keystone species in the ecology of the Delaware Bay and beyond. Furthermore, their biomedical uses have given them a key role in human health and well-being. Horseshoe crabs command our attention to ensure their survival. Take a closer look at these ancient mariners with us. ]]> Drag your cursor over this 3-D rendition of a horseshoe crab, and you'll find some interesting surprises! There are a total of eight video clips embedded in the background. Click on the hidden camera icons to launch the clips and learn more about this living fossil. Can you find them all? chitin, a cellulose-like compound that breaks down quickly. However, fossils have been found that indicate the first horseshoe crab appeared on Earth over 500 million years ago. The horseshoe crab of today is relatively unchanged from its prehistoric ancestor Mesolimulus, which lived 145 million years ago in Germany.

Most scientists think that the horseshoe crab is the closest living relative of the trilobite, a marine animal that has long been extinct. Scientists also believe that horseshoe crabs were among the dominant creatures some 300 million years ago. At the time, there were probably many different species of horseshoe crabs coexisting.

Of all the horseshoe crab species that once crawled in Earth's seas, only four species survive today. Three live in the western Pacific, from Japan to Vietnam. The fourth species -- Limulus polyphemus (shown here) -- is found along the western Atlantic coast, from Maine to Mexico, with the largest population concentration occurring in Delaware Bay. ]]> Horseshoe crabs are often called "living fossils" because they have remained virtually unchanged for millions of years. They are estimated to have inhabited the Earth for over 500 million years. Blurb about this video goes here pheromone, a natural attractant that acts as a sexual stimulant, into the water. However, most scientists believe that males locate spawning females by sight, using their compound eyes, rather than pheromones.

Upon finding a mate, the male uses his pedipalps to hook onto the shell of a female as she heads toward the beach. Sometimes additional males will attach themselves to the lead male, forming a chain.

The female drags the male to the water's edge. Once on shore, she uses her pusher legs to form a shallow nest where she deposits an average of 2,000 to 4,000 eggs. The attached male fertilizes the eggs as the female drags him over the nest to another location. Often, other males gather around the female and also fertilize the eggs. This process is repeated several times before the female returns to the water. Scientists estimate that a female may deposit 90,000 or more eggs during the spawning season and that fewer than 10 of these eggs will survive to adulthood.

The newly laid eggs are opaque, pastel-green in color, and about 1.5 mm (1.16 inch) in diameter. In a few days, the eggs double in size. The outer layer, or chorion, splits away, leaving the eggs transparent. Ideally, the moisture supplied by the tides and the warmth of the sun allow the eggs to mature and hatch in the two-week period between spring tides (the higher-than-normal tides that occur at the new and full moons). In reality, however, it probably takes three or four weeks and sometimes even months for the eggs to hatch.

Upon hatching, the horseshoe crabs dig their way out of the sand. They are approximately 3 mm (1/8 inch) across and resemble miniature adults. They lack a fully functional digestive system and swim around for about a week after hatching, absorbing their yolk sac as their digestive system matures. The tail of the larval crabs cannot move until after their first molt. In addition, the compound eyes of the larval crabs are not yet functional.

A horseshoe crab grows until it reaches sexual maturity at 8 to 10 years of age. It must periodically molt, or shed, its hard shell as it grows. All horseshoe crabs molt at least 18 times, three or four times a year until about age five and then once a year until they are fully grown. The males develop their pedipalps during their final molt. Scientists speculate horseshoe crabs have a life span of about 20 years or more. This enables horseshoe crabs to reproduce for a number of years—a key factor in sustaining a healthy population, since few live the 8 to 10 years needed to reach reproductive age. ]]> A female horseshoe crab lays an average of 2,000 to 4,000 pale green eggs per nest in the sand. Ideally, the moisture supplied by the tides and the warmth of the sun allow the eggs to mature and hatch within the two weeks between spring tides. Blurb about this video goes here
Shorebirds begin to arrive in early May. The numbers of birds soar upward during mid-month and usually peak between May 18 and 24 (in some years as late as May 28). They have traveled from the coasts of Brazil, Patagonia, and Tierra del Fuego, from desert beaches of Chile and Peru, and from mud flats in Suriname, Venezuela, and the Guyanas. After several days of nonstop flight, and having come as far as 10,000 miles, they reach the bay beaches depleted of their energy reserves. Luckily, nature provides an abundant food supply in this area at just this time of year: the eggs of hundreds of thousands of horseshoe crabs that have appeared on Delaware Bay beaches to spawn.

The shorebirds spend between two to three weeks gorging primarily on fresh horseshoe crab eggs, although worms and small bivalves are also plentiful. High in protein and fat, the eggs are an energy-rich source of food. This high-calorie diet enables the birds to double or even nearly triple their body weight before continuing on to Arctic nesting areas.

Each spring, scientists from the Delaware and New Jersey Divisions of Fish and Wildlife conduct weekly aerial surveys of migratory shorebirds on Delaware Bay beaches. In May 2001, scientists observed more than 775,000 shorebirds along beach habitat. Ninety-five percent of these birds were represented by four species: red knots, ruddy turnstones, semipalmated sandpipers, and dunlins. Migratory shorebirds are also known to utilize marshes and back-bay habitats. Thus, throughout their spring migration, the actual number of shorebirds using Delaware Bay as a staging ground may surpass one million.

Most of the horseshoe crab eggs consumed by shorebirds are found on or near the surface, from nests that have been disrupted by waves. Since these eggs would be unlikely to hatch, the hungry shorebirds are thought to have little adverse effect on the breeding success of the horseshoe crab. However, scientists and wildlife managers are concerned that declining horseshoe crab populations may adversely affect the shorebirds. During their stopover, the six most abundant shorebird species can consume almost 539 metric tons of horseshoe crab eggs! At least 1.8 million female horseshoe crabs must spawn on the shores of Delaware Bay to provide this food surplus. A recent decline in the populations of migrating shorebird appears to correlate with a decline in the horseshoe crab population. ]]> Over one million migratory shorebirds stop to rest and feed on horseshoe crab eggs along the shores of Delaware Bay en route to northern breeding grounds. The red knot, shown above, is one of the four species that are observed most frequently in Delaware Bay during the spring migration season. Blurb about this video goes here prosoma, the opisthosoma, and the telson, or tail. The prosoma is the large, semicircular part of the horseshoe crab and combines the head and thorax under a hard shell, or exoskeleton. Its shell also protects the gills and two genital pores. The spine-edged opisthosoma is attached to the prosoma with a hinge.

The top, or dorsal, surface of the shell has what look like strategically placed ridges and depressions. In actuality, these are locations where muscles are attached to the inside of the shell. Two large compound eyes are located on the prosoma, and eight other simple eyes and light receptors are distributed on the top and bottom of the shell.

Despite its threatening appearance, the horseshoe crab is harmless, and its long, spike-like tail is not poisonous, as people may think. The crab uses its tail as a lever to right itself when it has been overturned by a wave or a thoughtless human. The horseshoe crab may be harmed if handled by the tail. ]]>Photoreceptors throughout the length the horseshoe crab's telson, or tail, help set its circadian rhythm, or "internal clock," to daily cycles of light and darkness. While there are multiple light sensors on the tail, scientists count them as only one eye. Blurb about this video goes here Limulus alive in the laboratory, making it an ideal animal for eye research.

In 1967, Dr. H. Keffer Hartline received the Nobel Prize for his research on horseshoe crab vision. He discovered how sensory cells in the retina help the brain process visual cues, enabling horseshoe crabs to see lines, shapes, and borders. This mechanism, called lateral inhibition, allows horseshoe crabs to distinguish mates in murky water. Research of this type is helpful in understanding human eye diseases like retinitis pigmentosa, which causes tunnel vision and can lead to total blindness.

Building on Hartline's lateral inhibition research, Dr. Robert Barlow, a professor of ophthalmology at the State University of New York, is investigating the role of vision in potential mate selection. Using computer models, Dr. Barlow analyzed how the brain of a horseshoe crab processes signals transmitted from the eyes and optic nerve. In the future, decoding this pathway may provide valuable information for correcting human vision disorders. ]]> The answer is ten. However, only four can see images -- and at different times in the horseshoe crab's life. The two large, compound eyes (shown above) do not function until the animal is two or three years old. Until then, the two simple lateral eyes do the seeing. Blurb about this video goes here gnathobases, or 'shoulders of the leg.' Five pairs have claws; the claws of a horseshoe crab are not sharp and do not pinch very hard. The first clawed pair of legs are the chelicerae. This pair does not have gnathobases. The second pair of claws, or the pedipalps, are modified into a 'boxing glove' shape on the mature male for grasping the female during reproduction. The sixth pair of appendages are the pusher legs, and the seventh pair are remnant legs called chilaria. A horseshoe crab uses its clawed and pusher appendages to move along the seafloor. It pushes the sediment into little furrows like a farmer plowing a field. The chelicerae act as feelers to determine the presence of prey such as a clams or worms. The horseshoe crab does not have a nose. It 'smells' with tiny hairs on the gnathobases that act as chemoreceptors.

When the chelicerae find food, one of the claws will pick it up and push it toward the gnathobases, which are covered with tiny spines or bristles. As the gnathobases move, they grind and crush the food, which gets caught on the bristles. The food is then pushed into the crab's mouth by either the chelicerae, which are in front of the mouth, or the chilaria, which are behind the mouth.]]> The horseshoe crab is not a crab at all. In fact, its closest living relatives are spiders and scorpions. More than likely, it is called a crab because many of its physical features, such as its hard shell and leg-like appendages with claws, are similar to those of a true crab. Blurb about this video goes here
The horseshoe crab is rich in nitrogen, and when the crab is ground up and buried, the nitrogen is gradually released into the soil. This knowledge became the basis of a strong fertilizer industry in Delaware and New Jersey that lasted into the 1950s. The fishing industry has also found the horseshoe crab useful as bait for conch, eels, catfish, lobster, and other species.

However, the horseshoe crab has achieved much greater human value through its biomedical uses. Thanks to the horseshoe crab, medical science has made great strides in eye research, in the development of surgical sutures and wound dressings, and, perhaps most importantly, in the detection of bacterial contamination in drugs.

The latter use was discovered by Frederick Bang in the early 1950s. He found that the animal's blue, copper-based blood contains a clotting agent that would later be called Limulus Amoebocyte Lysate, or LAL. This clotting agent yields a fast, accurate method of testing a drug for the presence of infectious bacteria. Today, the LAL test is required for all injectable and intravenous drugs, as well as prosthetic devices such as artificial joints and heart valves. ]]> The horseshoe crab is the most researched marine invertebrate in the world. It has contributed to major advancements in human medicine -- in eye research, in ensuring that our medicines are pure and free of dangerous bacteria, in the development of wound-healing bandages, and more! Blurb about this video goes here
Extensive research has been conducted on chitin, the cellulose-like component in the horseshoe crab's shell. Although chitin is present in the shells of all arthropods, the horseshoe crab's chitin is very pure, making it desirable for research. It had been known since the mid-1950s that chitin-coated sutures enhanced healing time by 35–50 percent. Twenty years later, researchers with the University of Delaware Sea Grant College Program developed a method to spin pure chitin filaments. These chitin sutures can be absorbed by the body, which eliminates the need for physical removal.

A Japanese firm bought the patent rights, and chitin-based suture materials are now manufactured in Japan. In addition, this firm uses chitin to make dressings for burns, surface wounds, and skin-graft donor sites, which dramatically accelerate healing and reduce pain compared to standard treatments where the dressings must be removed.

Perhaps the horseshoe crab's most important role in human medicine was discovered by Frederick Bang in the early 1950s. He found that the cells of the blue, copper-based blood of the horseshoe crab contain a clotting agent that would later be called Limulus Amoebocyte Lysate or LAL. This clotting agent attaches to dangerous, fever-inducing toxins produced by gram-negative bacteria. Such bacteria cause a number of human diseases but were often difficult to detect. The discovery of LAL yielded a fast, accurate method of testing a drug for the presence of these infectious bacteria.

Today, the Food and Drug Administration requires an LAL test for all injectable and intravenous drugs, as well as all prosthetic devices such as heart valves or hip replacements. The test is also used to diagnose spinal meningitis and other bacterial diseases.

Four U.S. companies manufacture LAL and sell it throughout the world. Large horseshoe crabs are caught, examined for health, and bled using a stainless steel needle that is inserted into their circulatory system. The crab's blood is centrifuged to separate the blood cells (amoebocytes) from the liquid plasma. The amoebocytes are then freeze-dried and processed for use in the pharmaceutical industry. Horseshoe crabs are not seriously harmed during this process, and studies indicate that bled horseshoe crabs have a relatively low mortality rate. People in the LAL business carefully monitor their methods to guard their "golden goose."

Research on additional uses for the horseshoe crab's blood, such as in detecting other human diseases or as an aid to our immune system, is currently under way. ]]> In the early 1950s, Frederick Bang discovered that the horseshoe crab's blue, copper-based blood contains a clotting agent that would later be called Limulus Amoebocyte Lysate, or LAL. This clotting agent yields a fast, accurate method of testing a drug for the presence of infectious bacteria.
In the Delaware Bay, horseshoe crabs can be seen on almost any spawning beach during the high tides that occur in late spring. However, overharvesting and loss of habitat due to development or beach erosion associated with sea-level rise are of increasing concern. Overharvesting is being addressed by state managers through regulation, but the loss of habitat and beach erosion are more difficult to address.

An annual spawning survey of the horseshoe crab population is coordinated by the University of Delaware Sea Grant Marine Advisory Service at the height of each spawning season. The results of the survey show a peak in 1990, when 900,000 adults were counted, and then a decline in the population. From 1999 through 2005, the spawning population appears to have stabilized at less than 350,000 adults on the night of peak spawning. This number is relatively low compared with the 1990 population. Fisheries managers have attributed this decline to overharvesting.

Currently, all East Coast states have strict harvesting legislation in place. Legislation includes limiting the number of horseshoe crabs that may be harvested, requiring a license to harvest horseshoe crabs, and/or limiting the number of people allowed to harvest them. Contact your state department of natural resources for your state's regulations regarding horseshoe crabs. ]]> Only four species of horseshoe crabs exist today; all of them are facing population declines. Three species are found near the Philippines and the Indonesian islands; two of these also are present in the Bay of Bengal and one in the waters off Japan. One species, Limulus polyphemus, is found along the western Atlantic coast, from southern Maine to the Yucatán Peninsula.