Horseshoe Crabs Changed Human Healthcare! cover

Horseshoe Crabs Changed Human Healthcare!

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Did you know that every time you receive an injection or an IV, you should thank a horseshoe crab? These marine animals play an important role in human health care. A substance extracted from the blood of wild-caught horseshoe crabs is used to prepare a reagent to test vaccines, intravenous drugs and fluids, and implanted medical devices for bacterial contamination. If certain bacteria are present, a clot forms. The horseshoe crabs are tagged and returned to the wild. Individual horseshoe crabs are collected for blood donation once a year.


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Horseshoe Crabs Changed Human Healthcare!

Ocean Animal Changes Human Healthcare

Did you know that every time you receive an injection or an IV, you should thank a horseshoe crab?

These marine animals play an important role in human health care. A substance extracted from the blood of wild-caught horseshoe crabs is used to prepare a reagent to test vaccines, intravenous drugs and fluids, and implanted medical devices for bacterial contamination. If certain bacteria are present, a clot forms. The horseshoe crabs are tagged and returned to the wild. Individual horseshoe crabs are collected for blood donation once a year.

Before this unique property of horseshoe crab blood was discovered, live animals were used to test the safety of injections. A contaminated sample would result in the illness and potential death of the test subject.

Warding Off Infection

Horseshoe crabs live in ocean waters teeming with bacteria. Although they have no immune system, when injured, they are able to ward off infections due to a substance produced by the amebocytes in their blood that causes clotting of the wound.

LAL

Scientists extracted this substance and develop a reagent called Limulus Amebocyte Lysate (LAL) for use in testing intravenous medicines, vaccines, and medical devices used in the treatment of humans and other mammals.

The US Food and Drug Administration (FDA) now requires the use of the LAL test instead of the previous more time-consuming, less sensitive test that used rabbits to detect contamination.

Atlantic Horseshoe Crab

(Limulus polyphemus)

The horseshoe crab is not really a crab. It is more closely related to scorpions, spiders, and mites than to true crabs, lobsters, and shrimp. It is often described as an armored box that moves or a blue blood living fossil or ancient mariner because its basic body shape that has not changed in over 420 million year. It is a “blue blood” in the truest sense. Its blood turns blue when exposed to air due to presence of a copper containing molecule, hemocyanin. Hemocyanin carries oxygen in the crab’s blood as hemoglobin does in humans.

Geographic Distribution & Habitat

Geographic DistributionThese animals are found along the East coast of North and Central America from Nova Scotia to Mexico’s Yucatan Peninsula, including part of the Gulf of Mexico.

Habitat In the spring and summer these crabs are usually found in the sand or mud flats, estuaries, and mangrove swamps. In the winter they are found at depths to about 30.5 m (100 ft).

Physical Characteristics

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Physical Characteristics

The horseshoe crab has a horseshoe shaped hinged, hard shelled body that is divided into three parts; the carapace or head region, middle abdominal region, and the telson tail). Its upper side is smoothly curved. There are six pairs of appendages on the underside of the carapace. The first pair is used to pass food into the mouth.

Physical Characteristics (Cont.)

The second pair is the walking legs.

In males they are tipped with the claspers that are used during mating to hold onto the female’s carapace. The remaining four pairs of appendages are also used in locomotion.

The middle abdominal region has six more pairs of appendages. The first contains the genital pores and the other five are modified plates called book gills. The color of these crabs ranges from light brown to black.

Size and Diet

Size Female horseshoe crabs can reach 60 cm (2 ft) in length and weigh up to 4.5 kg (10 lb). Males tend to be about 20 percent smaller. As their body size increases, the crabs shed their too small exoskeleton (shell). Males molt about 16 times over a nine year period and females, 17 times over 11.

Diet Horseshoe crabs eat small mollusks, (mostly clams), but also feed on worms, crustaceans, and dead fish. Our horseshoe crabs are fed a mixture of previously frozen restaurant quality fish, shellfish, shrimp, and squid.

Reproduction

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Reproduction

Spawning occurs in late spring or early summer at high tides typically during new and full moons. Adult crabs migrate from deeper water to shallow flats or beaches in large numbers, females first. Females then secrete a pheromone to attract a male who uses the hook-like claspers at the tips of his first pair of walking “legs” to attach to the female, hitching a ride to the high tide line.

Reproduction (Cont.)

Often other males will attach to the first male until a chain has formed that the female has to drag to the spawning area.

The female digs four to five nests in the sand every few feet in which she deposits a cluster of as many as 3,600 eggs that are externally fertilized by the male. Following fertilization, the female covers the eggs with sand. Tailless green, free-swimming larval typically hatch two to four weeks later. They settle to the bottom in about six days where 20 days after hatching, they molt for the first time.

Baby Horseshoe Crab

Baby Horseshoe Crab

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Behavior

Sexually mature crabs migrate annually from deep to shallow waters during the breeding season, not moving very far from the beach where they hatched.

Molting begins during the egg stage. Embryos moll four times at 4, 8, 11 and 13 days after egg fertilization. The next molting occurs 20 days after the larvae emerge. At each molt they grow about 25 percent in length. There are 16 molts in the male and 17 in the female before adulthood is reached. Occasionally, there is one more molt during a horseshoe crab’s life span.

Adaptation and Longevity

Adaptation The horseshoe crab has 10 “eyes” or light sensing organs scattered about its body from its head to and including its tail. It has one set of compound eyes like those of a fly, one set of simple, rudimentary eyes, and multiple photoreceptors on its telson. In spite of all these “eyes”, the crab’s vision is poor! Only movement is detected in the daylight and perhaps a blurry image by the compound eyes. At sundown, signals from the tail plus other factors give the crab improved night vision.

Longevity Horseshoe crabs can live up to about 19 years.

Conservation

The numbers of horseshoe crabs are declining.

These crabs have many predators, migratory birds, some species of sharks, loggerhead sea turtles, and sea gulls, but their most serious predator is humans. Since the mid 1800’s large numbers of horseshoe crabs have been caught for bait in the eel and conch fisheries, cattle feed, and fertilizers.

The decline of horseshoe crabs on the Atlantic coast has led to state and federal regulations on their harvest. Delaware regulations require that that the crabs be hand captured and returned to the area in which they were caught, and crabs may only be harvested for LAL. Harvesting for use as eel bait or fertilizer is prohibited.

Conservation (Cont.)

But other states bordering Delaware Bay have not instituted similar restrictions.

Regulating harvesting continues to be a controversial issue among fisheries, environmentalists, the pharmaceutical industry and state regulators responsible for natural resources. The impacts of climate change may affect horseshoe crab, and loss of spewing areas due to sea level rise and ocean acidification’s affecting shell development.

Horseshoe crabs have adapted to environmental changes for millions of years. Will they have time to adapt to climate change?

To The Ocean

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To The Ocean

Predators include some species of sharks, loggerhead sea turtles, sea gulls and migrating shorebirds. At the same time that horseshoe crabs are spawning, huge flocks of shorebirds pause in their migration from South America to northern breeding grounds as distant as the Arctic to feast on crab eggs.

Amazing Facts

Horseshoe crabs are often referred to as living fossils.

There is some debate about whether these crabs should actually be called living fossils, because of the lack of anthropological evidence. Remains preserving the structure of the hard body parts of horseshoe crabs closely resemble the fossilized structures found in rock – many of them over several hundred million years.

In this way, they appear to be living specimens of long dead, fossilized species. However, this is not to say that they have not evolved at all since then. There is no evidence of changes that may have occurred in the soft tissue and biochemical mechanisms that can be studied in typical fossils. I suspect that very close attention to detail could even show minor changes to the exoskeleton of the horseshoe crab.

Getting Close

Photo by Aquarium of the Pacific

Getting Close

Horseshoe crabs are on view at the Aquarium of the Pacific in Long Beach, California, in a touch exhibit near the Aquarium’s Molina Animal Care Center. The exhibit helps people learn about the important role these animals play in human healthcare. Caring for the ocean and coastal environments these animals call home is just as important for our health as it is for theirs.