Horseshoe crabs (Scientific name: Limulus polyphemus) are not really crabs, but “chelicerates” There are more closely related to spiders and sea scorpions. Resembling a helmet with a spike tail, they grow slowly and molt around 17 or 18 times — reaching the size of a silver dollar when they are two and fit into the palms of a person’s hand when they are five.
Horseshoe crabs are one of the few living creatures with blue blood. This blood has the extraordinary ability to clot when harmful bacteria is present and thus is widely used in the pharmaceutical industry for testing drugs A substance called LAL, which is derived from the blood, is used in vaccines, allergy shots, nasal sprays, eye drips and medical equipment such as needles and tubes to make sure they are bacteria-free. On top of this, the study of a horseshoe crab's central nervous system processing functions have provided the principles necessary to understand information processes in virtually every other organism investigated.
The horseshoe crab has been around for almost half a billion years. They predate the dinosaurs by 200 million years and survived the asteroid that wiped them out. They have been called a “Swiss army knife of jointed limbs.” They have specialized pairs of legs for swimming, walking, grasping, shredding and defending themselves. They have been called “biological bulldozers” for the way they dig up the ocean floor or root out small shellfish and worms to eat. The structure of their eyes is similar to that of humans. They can see well in the dark.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
Horseshoe Crabs — Very, Very Old Living Fossils
Horseshoe crabs first appeared at least 480 million years ago during the Ordovician Period. They are a "living relic" of Merostomata, a class of anthropods that mostly went extinct millions of years ago. Mesolimulus walchi is an extinct species of horseshoe crab. In “The Extreme Life of the Sea”, Stephen and Anthony Palumbi wrote” “Like a Volkswagen Beetle, they stick out like a sore thumb but persist in sturdy practicality” and they are “double-living fossils “ because their larvae take the form of trilobites.
Jan Walker of the National Museum of Natural History wrote: Horseshoe crabs evolved from a now extinct order of giant water scorpions, which probably originated in the late Precambrian Era about 550 million years ago. Fossils of both creatures date back about 475 million years. These crabs have retained their basic physical characteristics since the Devonian Period of 408 million to 360 million years ago. Change apparently was unnecessary because the environment of coastal waters in which they live has remained fairly stable. Their high resistance to changes in salinity and to temperature fluctuations further favored their survival. The geographical distribution of the four species occurred “some 60 million years ago when all of the horseshoe crabs left European waters, never to return," one expert says. "The seas where Europe now stands were the center of this dispersal, one group migrating westward, the other eastward." [Source: Jan Walker, NMNH, Washington Post November 11, 1998]
Ian Frazier wrote in The New Yorker: “Horseshoe crabs are aliens from another planet, if we allow that the other planet was Earth about five hundred million years ago. Creatures much like horseshoe crabs go back that far or farther, to almost the beginnings of animal life. The earliest known horseshoe-crab fossils are four hundred and eighty-five million years old. The land then was bare rock without plants, except for some algae and tiny mosses along the watercourses. All animal life was in the oceans; millions of years passed before any animals lived on land. The oldest fossils of land animals date to between four hundred and twenty million and four hundred and ten million years ago, and they are of the horseshoe crab’s relatives — scorpions, spiders, and insects. All belong to the phylum Arthropoda, the most populous phylum, which has more than a million species. True crabs are also of that phylum, but their class is Crustacea, while horseshoe crabs are of the class Chelicerata. That is, horseshoe crabs are not crabs. [Source: Ian Frazier, The New Yorker, April 14, 2014]
Horseshoe Crabs Evolution
Ian Frazier wrote in The New Yorker: ““In the fossil record, the nearest ancient relatives of horseshoe crabs are the trilobites, a successful group for hundreds of millions of years. Because trilobites had shells made mostly of calcium carbonate, they fossilized better than horseshoe crabs, whose shells are composed of chitin. Most trilobites had ovoid, segmented bodies that would make good paperweights. The last species of trilobites disappeared in the Permian Extinction, between two hundred and fifty-two million and two hundred and fifty million years ago. Horseshoe crabs survived that extinction and the others — at least a dozen extinctions in all. Having exoskeletons of chitin rather than of calcium carbonate may have helped during times when the oceans acidified. Horseshoe crabs can tolerate low-oxygen water and other life-killing conditions. Animal life probably began in the bottom of the sea. Horseshoe crabs still live in the mud and sand at the bottom of shallow inland waters, and on continental shelves out to about a hundred feet deep. They are like our old neighbors who never left our home town. [Source: Ian Frazier, The New Yorker, April 14, 2014]
“In 2005, Dr. Dave Rudkin, a paleobiologist at the Royal Ontario Museum, in Toronto, found horseshoe-crab fossils on the shores of Hudson Bay near Churchill in central Manitoba. The fossil horseshoe crabs, to which he gave the name Lunataspis aurora, were no larger than two inches across. From tests of the rock’s isotopes and by comparing index fossils, he could date the horseshoe-crab fossils to the late Ordovician period, about 445 million years ago.
Horseshoe crabs saw the aeons come and go. In the Carboniferous period, when most bacteria able to digest wood had not yet evolved, a planetary reservoir of plant carbon was buried and became coal. The remains of Euproops, another early horseshoe-crab species, have been found in fossilized feces in coal deposits in England. A species of horseshoe crab that scientists named Limulus coffini because of its close resemblance to the Limulus of today became a fossil in eighty-million-year-old rock in Colorado. A horseshoe crab known as Mesolimulus walchi, whose fossils have been found in limestone quarries in southern Germany, preëxisted by fifty million years certain formations of ocean algae that died and fell to the bottom and occupied deeper rock and cooked at the right temperature for millions of years and eventually turned into oil; the kin of Mesolimulus may or may not still be here when all the planet’s oil is gone. Horseshoe crabs have been around at least two hundred times as long as human beings.
Horseshoe Crab Species, Habitat and Where They Live
There are four species of horseshoe crab alive today, all within the family Limulidae, found in waters off Asia and North America. They inhabit shallow marine areas, generally with clean sandy or muddy bottoms where they crawl or burrow just beneath the surface, preying on other animals or scavenging. They migrate to very shallow coastal waters often near beaches to breed in massive “orgy” events, with females laying many tens of thousands of eggs in the sand.
The four horseshoe crab species are: 1) Mangrove horseshoe crabs (Carcinoscorpius rotundicauda), found in South and Southeast Asia, in the western part of the Bay of Bengal, on both sides of the Malay Peninsula, and on the island of Borneo; 2) Indo-Pacific horseshoe crabs (Indonesian, Indian or southern horseshoe crabs, Tachypleus gigas), found in South and Southeast Asia, in a range that overlaps that of the mangrove horseshoe crab; 3)Tri-spine horseshoe crabs (Chinese or Japanese horseshoe crabs, Tachypleu stridentatus), found in Southeast and East Asia, in southern Japan, the Philippines, and on the central and southern coast of China; and 4) Atlantic horseshoe crabs (American horseshoe crabs, Limulus polyphemus), found along the Atlantic coast of the United States and the Southeast Gulf of Mexico
Atlantic horseshoe crabs are found along the Atlantic Coast and in the Gulf of Mexico, from Nova Scotia to the Yucatan Peninsula. Ian Frazier wrote in The New Yorker: “For no reason that scientists can explain, horseshoe crabs are found only on the eastern coasts of North America and Mexico and on the eastern coast of Asia and near environs. The other three horseshoe crab three species are found in smaller numbers in Asia. All the Asiatic species are in severe decline. The Tri-spine horseshoe crab seems seems to be the hardest hit of the three.
Delaware Bay is home to the world’s largest population of spawning horseshoe crabs by far. An estimated 19 million horseshoe crabs live there. Usually in May, as the water warms, the creatures start to appear see in Delaware Bay and Atlantic Ocean coastlines of New Jersey and Delaware. They are particularly numerous at Neptune’s Shark River Inlet. [Source: Fox Weather, May 24, 2023]
Horseshoe Crab Characteristics
Horseshoe crabs have specialized pairs of legs for swimming, walking, grasping, shredding and defending themselves. The structure of their eyes is similar to that of humans. They can see well in the dark. Research on their optic nerve won some scientists the Nobel Prize in the 1960s. Horseshoe crabs need to keep their primitive gills moist to survive. If they get tuned over their back they fold themselves in half, with their tails up in the air to conserve water. Their tail looks dangerous but it isn’t. It is used to steer, help them navigate and right themselves when overturned.
According to National Geographic: Horseshoe crabs are built to last — With spiky tails, shells shaped like combat helmets, and sharp pincers at the end of eight of their 10 legs. The gills are on the underside, and the dashes and indentations mark where they attach to the exoskeleton. The dark points beneath are minute spines
Ian Frazier wrote in The New Yorker: “ The front part of the horseshoe crab’s helmetlike carapace, called the prosoma, attaches to a lower part, called the opisthosoma, which would be like the helmet’s neck guard. The juncture of the parts is hinged, and the opisthosoma moves up and down, providing a swimming motion. The pointed tail resembles a file and is called the telson. It connects to the opisthosoma by means of an almost universal joint. If the animal becomes upside down, the telson moves all around in an attempt to lever it onto its legs.” One is supposed to hold a horseshoe crab on each side of the prosoma. You’re not supposed to pick up a horseshoe crab by the telson. [Source: Ian Frazier, The New Yorker, April 14, 2014]
On horseshoe crab’s underside six pairs of spidery, multi-jointed legs radiated from the center, where both the mouth and the brain are. The brain is doughnut-shaped and encircles the mouth. The legs did a kind of slow-motion random insectoid running-in-place. Its gills — called book gills, because they resemble bound pages — are also underneath, below the legs. There are large eyes on either side of the prosoma. Horseshoe crabs have a lot of eyes, and the species name, Limulus polyphemus, derives from some of them. The two large eyes can be construed as squinting; hence Limulus, which means “squinting or aslant” in Latin. A pair of smaller eyes on top of the prosoma are so close together they might be mistaken for a single eye; hence polyphemus, from the Cyclops.
Horseshoe Crab Behavior, Movement and Feeding
Horseshoe crabs use their long tails as rudders in the water and to flip themselves if they get overturned on the beach. They are tremendous reworkers of sediments and feed on a variety of marine worms and invertebrates, including some commercially important fish. They have been called “biological bulldozers” for the way they dig up the ocean floor or root out small shellfish and worms to eat.
Horseshoe crab inhabit shallow marine areas, usually on clean sandy bottoms where they crawl or burrow just beneath the surface, preying on other animals or scavenging. The generally walk along the sea bottom but swim if necessary — albeit awkwardly on its back by using its flap-like gills as paddles. In high waves, horseshoe crabs go through gyrations to keep their carapaces upward, and they generally don’t venture toward shore when the surf is rough.
The horseshoe crab feeds at night on worms, small molluscs, and algae. Food is picked up by the chelicerae and passed back to the bristle bases, where it is "chewed." The food is then moved forward to the mouth. They can go without eating for a year and survive doses of radiation that would kill a man. Among horseshoe crabs’ main predators are seagulls, which tear out their insides if they get turned over on their backs onshore.
Horseshoe Crab Reproduction
Horseshoe crabs migrate to shallow coastal waters to breed in massive “orgy” events, with females laying many tens of thousands of eggs in the sand. The first pair of the horseshoe crab’s six, flap-like appendages on the underside of the abdomen acts as a cover for the genital pore. The egg or sperm are released through this pore during spawning.
Females reach sexual maturity at around age 10. During high tides in the full-moon cycle of May and June, female horseshoe crabs come ashore and lay eggs. One female can lay around 34,000 eggs at one time and as many as 100,0000 in one season. Only a handful of larvae survive until adulthood. Many of those that don’t provide food to a number of animals.
“Orgy” evens often involve one female and a mass of ardent males, all vying to fertilize her eggs. Describing the event in the Delaware Bay, Dina Fine Maron wrote in National Geographic: It’s high tide on and Atlantic horseshoe crabs are clambering ashore to mate. For this shield-shaped arthropod, assignations are typically a group affair: one female, one male, and a lot of male hangers-on. [Source:Dina Fine Maron, National Geographic, April 28, 2022]
The trysts sometimes begin underwater. A male angling for fatherhood uses his boxing glove–like front legs to clasp onto a female’s abdomen and hitch a ride directly behind her through the surf. So situated, he stands ready to contribute sperm the moment she begins laying her eggs on the sand. However, “there’s a lot more competition on the beach,” says Jordan Zimmerman, a horseshoe crab biologist at Delaware’s natural resources agency. For reasons still unknown to science, some females are so appealing that, even if they’re already otherwise engaged, more males seek them out.
With spare studs in a polyamorous heap around her, the female releases her eggs. The attached mate deposits his sperm, and the third wheel — and fourth, fifth, and sixth — “pounces” to deposit his also, Zimmerman says. This waiting-in-the-wings technique can be surprisingly effective: Paternity tests have shown that satellite males sometimes father as many of the female’s brood as the attached male. Yes, mating is a battle. And after the animals spawn, ravenous shorebirds compete for the clutches of fertilized eggs. Yet after 450 million years, L. polyphemus is still here.
Why Do Horseshoe Crabs Mate During the Full Moon?
Horseshow crabs wait until the water temperature gets above 10̊C (50̊F) to spawn. They prefer do it when a high tide coincides with a full-moon or a new-moon night. On spawning even he saw, Ian Frazier wrote in The New Yorker: “Male horseshoe crabs are smaller than females, and when they spawn the male grasps the back of the female’s carapace with boxing-glove-shaped front pincers and hangs on. The female works herself into wet sand at the waterline, lays a cluster of some thousands of eggs, moves a short distance up, and lays more. The attached male fertilizes the eggs, as do other males, sometimes four or five or more, crowding around the pair. Mating pairs and satellite males now filled the shallows, moonlight glistening on their carapaces. [Source: Ian Frazier, The New Yorker, April 14, 2014]
Horseshoe crabs mating Joseph Trovosky posted on Quora.com: Horseshoe crabs spawn all throughout the lunar cycle, but their spawning activity peaks during both full moons and new moons. Timing their spawning with these moon stages is so important that horseshoe crabs have 3 entire eyes dedicated to tracking the lunar cycle. In Florida environmental conditions are optimal enough for horseshoe crabs to spawn pretty much year-round as opposed to certain seasons like up North, but even then you huge spikes during new/full moons because of how perfect the high tide is. [Source: Joseph Trovosky, Studied Marine Biology & Molecular Biology, Quora.com, May 7, 2022]
The reason this is so important is because during a full or new moon, both the solar and lunar tide combine forces to produce the highest high-tide levels. Horseshoe crabs come up to nest in the surf during high tide, so they want to make sure that they have the most optimal conditions to do so. They need to stay moist to breathe on land and lay their eggs, so they need to pick a time where the surf is going to be the most submerged with water. Horseshoe crabs rely more on these tidal conditions than the lunar cycle itself, and you can see horseshoe crabs spawning at any point in the lunar cycle as long as the tide is high enough.
Horseshoe Crab Blue Blood and Its Importance to Medicine
Horseshoe crabs are one of the few living creatures with blue blood. This blood has the extraordinary ability to clot when harmful bacteria and pathogen are present and thus is widely used in the pharmaceutical industry for testing drugs. A unique substance known as limulus amebocyte lysate, (LAL) causes the rapid clotting to even the tiniest amount of bacterial toxin. It is taken from the blood and used in vaccines, allergy shots, nasal sprays, eye drips and tests for medical equipment such as needles, I.V. fluids, surgical instruments, artificial implants and tubes to make sure they are bacteria-free.
The blood’s blue color comes from copper in its oxygen-carrying protein, hemocyanin, similar to iron-based hemoglobin in humans. Other animals, including snails, octopuses, and scorpions, have blood with hemocyanin, but not in the large, relatively easy-to- harvest amounts provided by horseshoe crabs.
The. horseshoe crabs’ toxin-sensitive blue blood was use to create COVID-19 vaccines and has been indispensable for biomedical companies that need to screen vaccines, IV fluids, and medical devices for bacteria that can be fatal in our bloodstream. The crabs’ blood coagulates instantly when it comes in contact with pathogens like E. coli and Salmonella thanks to proteins in cells in the blood that act like a primitive immune system. The test derived from the proteins are so sensitive they can detect amounts as small as one part per trillion — equivalent to one grain of sand in an Olympic-size pool.
Ian Frazier wrote in The New Yorker: “The strength of the LAL reaction may be an adaptation related to the animal’s shell. The ocean bottom breeds a lot of bacteria, and a chitinous shell punctures more easily than a mineralized, calcium-carbonate one. If a horseshoe crab’s shell gets a hole in it and bacterial toxins enter, amebocytes, or blood cells, instantly attack the bacteria by spewing enzymes that clot the blood. [Source: Ian Frazier, The New Yorker, April 14, 2014]
Horseshoe Crab and Scientific Research
Dr. Norman Wainwright, who worked with Dr. Jack Levin, one of the two discoverers of the LAL is the director of research and development at Charles River Laboratories, a major manufacturer of LAL, with headquarters in Massachusetts. He told Frazier that watching the cells under a microscope is amazing — the way the granules in the amebocytes suddenly pop like popcorn and make a clot around the bacteria. He explained that enzymes in the blood increase the reaction so that each step is a tenfold amplification of the one before, like a chain letter. [Source: Ian Frazier, The New Yorker, April 14, 2014]
Based on all this it would make sense to produce LAL in the lab. Frazier wrote: “Synthetic LAL does exist, and a biotechnology company called Lonza sells a version of it under the brand name PyroGene. Other synthetics have been developed and could, in theory, replace natural LAL entirely; for now, industry inertia and the cost of getting F.D.A. approval remain in the way. It also seems like it would make sense to raise a lot of horseshoe crabs in captivity like on a fish far. anyway,
In response to that John Tanacredi, a researcher at the American Museum of Natural History said: “You know why I love to work with horseshoe crabs? Because they’re the perfect research animal,” he said. “It may sound silly, but they don’t bite, they don’t take your finger off. They’re not slimy. They’ve got some sharp, nonpoisonous spines on their opisthosomas you have to look out for — that’s about it. Horseshoe crabs don’t harm anything except the small clams and worms they eat. That’s partly why LAL was discovered in the first place. The animals were readily available and no problem to work with. Back in the sixties, a Nobel Prize for research in vision went to optic-nerve research done on horseshoe crabs. The photoreceptors in their eyes are large and easy to study.
““So, yes, it would be great if we could raise them outside their environment,” he continued. “I have horseshoe crabs I’m raising in my lab right now. We’ve brought thousands past their first three or four instars — their first molts — and released them to the wild, but we have no way of knowing what happened to them. Scientists in Hong Kong and Taiwan have done the same. So far, though, nobody has raised a horseshoe crab to adulthood — to sexual maturity, which they reach at about ten years old. These animals can live to be twenty or even thirty. In an artificial environment, no one has kept them alive past the age of about five and a half. So what all this tells me is that we have to work harder to preserve them in the wild.
Milking Horseshoe Crab Blood
The biomedical industry catches about 500,000 U.S. horseshoe crabs annually and takes a portion of blood to use in lab safety tests and medical research. Most of them are returned to the water, but an estimated 15 to 30 percent don’t survive. The situation is worse in Asia, where they are drained of all their blood rather than just a portion of it.
The horseshoe crabs are harvested from their habitats and “bled” of about one-third of their blood, The crabs are collected annually along the U.S. East Coast under interstate regulations. In a lab the blood is drawn from the crab’s primitive equivalent of a heart, In South Carolina, about 20 percent of the crabs’ blood is collected in special lab before the animals are returned to the water.
Ian Frazier wrote in The New Yorker: “Horseshoe-crab blood is said to be worth fifteen thousand dollars a quart. Some horseshoe-crab experts consider fifteen thousand dollars to be too low an estimate, considering the amount of LAL that may be refined from a quart of blood. Biomedical companies take adult horseshoe crabs that have been caught in the wild, and then workers clean them, strap them to racks, stick needles in their hearts, drain them of about a third of their blood, and, eventually, release them. The crabs are supposed to be put back where they came from, but that doesn’t always happen, according to reports. Mortality rates from the bleeding procedure are unknown; they may be as high as thirty percent. [Source: Ian Frazier, The New Yorker, April 14, 2014]
Horseshoe Crabs, Their Ecosystem Roles and Conservation
Horseshoe crabs also are important ecologically. More than 20 species of migratory birds, loggerhead sea turtles, and a myriad of commercial and recreation fish and crab species rely on the crabs’ eggs as an annual boon to their diets. The International Union for Conservation of Nature (IUCN) Red List lists them “Lower Risk — Near Threatened.”
While Atlantic horseshoe crab numbers were declining in the 1990s, populations are recovering due to the regional management efforts of the states through the Atlantic States Marine Fisheries Commission. Delaware Bay has the largest population of horseshoe crabs in the world, and scientists from NOAA’s National Estuarine Research Reserves System help conduct annual horseshoe crab spawning surveys, which is a Delaware Bay-wide effort. However, habitat loss and high demand as commercial bait are still concerns for horseshoe crabs and migratory shorebirds.
Horseshoe crabs play a vital role providing food for migratory shorebirds. The red knot is a sandpiper-like bird that flies 15,000 kilometers between Tierra del Fuego and the Arctic. Critical to their trip is a stopover in Delaware Bay to feats on horseshoe crab eggs, The birds know when and where to find the eggs, in some cases doubling their weight after the feed on them. But these days horseshoe crabs and horseshoe crab eggs, by conch and eel fishermen as bait, are harder to find and a critical way station the red knots is threatened.
Humans and Horseshoe Crabs
“The main problem for horseshoe crabs in Asia is people eating them,” Dr. John Tanacredi, one of the world’s leading experts on horseshoe crabs, told the The New Yorker. “You can buy horseshoe crab in seafood markets in Taiwan and Hong Kong.People eat the eggs, too. A Japanese colleague tells me that in Vietnam there are snack carts that sell steamed horseshoe crabs on the beach.” [Source: Ian Frazier, The New Yorker, April 14, 2014]
“In many parts of Asia, the other disaster is habitat loss, of course,” Tanacredi said. “In Hong Kong, for example, there’s almost no undeveloped shoreline left. On the coast of China, the pollution is becoming too much even for horseshoe crabs. But the numbers here in America are not reassuring, either. Since we started doing local surveys, back in 2001, we’ve seen a decline of about one percent in the horseshoe-crab population on Long Island every year, for a total drop of about twelve percent. It’s a scary trend.”
“People’s tastes in seafood affect Atlantic horseshoe crabs, too. Some local East Coast horseshoe crabs are exported for tables in Asia. More are consumed at one remove, when fanciers of eels or of scungilli (conchs, also called whelks) buy those foods in stores or restaurants. The best bait for conchs or eels is female horseshoe crab; you take a female, cut it in quarters, and put the pieces in eel pots or conch traps. Suppliers ship conchs and eels internationally, so the global market drives local harvests of bait. Over the past decades, many millions of horseshoe crabs on the East Coast went for bait. When their numbers seemed to be dropping alarmingly, the Atlantic States Marine Fisheries Commission, or A.S.M.F.C., to which New York and the other Atlantic seaboard states belong, set harvest quotas.
The tri-spine horseshoe is flourishing in the muddy sediments off of the Pangatalan Island Marine Protected Area in the Philippines after a decade of restoration work there. According to National Geographic: Tri-spine horseshoe crabs have lost more than half their population in the past 60 years. But on the Philippine islet of Pangatalan, the species is an unexpected symbol of resilience. For years the island’s 11 acres were degraded: trees cut down for timber, mangroves burned for charcoal, and coral reefs overfished with dynamite and cyanide. By 2011 these horseshoe crabs, about 15 inches long, were among the biggest creatures left. Now a marine protected area, Pangatalan is starting to thrive again. Efforts to restore its reefs and plant thousands of trees have led many animals to return, including rare giant groupers that grow to some eight feet long.[Source: Amy McKeever, National Geographic, July 12, 2022
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023