Crabs in the Crossfire
by Alex Hawes
The horseshoe crab died slowly.
Along the far wall of an obscure Bavarian museum, at the center of a giant slab of gray rock, the fossilized creature stands frozen, flawlessly sculpted by time. Mesolimulus walchi reads the tag, identifying the body. Time of death: 140 million years ago. But look more closely. Pressed into the rough surface, a spiral of tracks encircles the crab, the creature’s final moments spelled out in stone.
During the late Jurassic Period--high times in the Age of Dinosaurs--our young Mesolimulus found itself swept across a tropical reef, and into a warm, sheltered, and highly toxic lagoon. Steeped in salt but devoid of oxygen, the pool exterminated most intruders in an instant, pickling their corpses for the ages; shriveled jellyfish and the feathered Archaeopteryx also rest in peace in this limestone cemetery.
Footprints prove, however, that the horseshoe crab arrived at the lifeless bottom alive and kicking. It survived long enough to make a few final, circling paces through the Jurassic mud before expiring at the spiral’s center.
Horseshoe crabs won’t go gently, the ancient rock teaches. From fishermen to pharmaceutical companies to migratory birds, many hope the rock is right.
The horseshoe crab is a survivor of a half billion years. Continents ripped apart and crashed together; asteroids riddled the planet with craters, vanquishing the dinosaurs; ice ages froze the seas and thawed; miserable shrew-like creatures evolved into monkeys and chemists. The horseshoe crab persisted unchanged.
But the animal’s quiet yet relentless march through time has run smack into an obstacle unseen throughout those millions of years: People. Humans have discovered rich treasures in the crab’s simple architecture, and stripped it down for parts.
Native Americans once used the horseshoe crabs’ telson tails as sharp tips for spearfishing. Today, we harvest the crabs for bait in eel and whelk pots; dissect their eyes and mount mini-cameras to their domes to understand both their vision and ours; grind up their shells to treat burns; and use their blood to detect contaminants in injectable drugs. We also spend millions of dollars each spring watching endless flocks of migratory birds feast on the crabs’ eggs to fuel the birds’ epic sprint north to the Arctic.
An explosion in the demand for horseshoe crabs as bait for eel and whelk is the chief suspect in a possible, but hard to prove, decline in both crab and migratory shorebird numbers. For fisheries, as well as bird observatories, state wildlife commissions, and biomedical firms, the crucial question still remains unanswered: How many crabs can we plunder before it all falls apart? While the horseshoe crab probably won’t soon go the way of the dodo bird or the passenger pigeon, there may not be enough crabs for everyone to share.
The horseshoe crab’s longevity offers a measure of hope. The ancestors of horseshoe crabs first emerged some 500 million years ago, and by 300 million years ago the horseshoe crab body plan, so familiar to beachcombers today, was in place. A half billion years ago, its closest relatives were giant water scorpions. Today, its modern cousins are arachnids: terrestrial spiders, scorpions, and ticks. The horseshoe crab, in truth, is not a crab at all.
Dozens of different horseshoe crab phyla arose and vanished through the eons. Four species now remain: the North American horseshoe crab (Limulus polyphemus), and three species native to coastal Asia, from India to Japan. The modern horseshoe crab’s tank-like appearance belies a peaceful existence. Its saber-like telson tail mainly helps the creature right itself in the sand after cinematic spawning amid crashing waves. It has nine eyes but sees poorly, for it spends most of its time blithely trolling the seafloor for unchallenging prey. Aside from the occasional loggerhead turtle or shark, adult horseshoe crabs have few natural enemies.
And so the horseshoe crab has survived the ages unscathed. A dietary generalist, it has managed to capitalize on the ecological niches nature has thrown its way over geologic time. Examining our fossil Mesolimulus, less than four inches wide, indeed offers a glimpse of perfection in adaptation: Its dome-like prosoma, head fused to thorax, protected its small, gear-shaped brain; its throbbing and contracting book gills, partially encased in its abdomen, propelled it--upside-down--through the sea; its telson helped guide it like a tiller.
Now travel to Atlantic beaches--anywhere from northern Maine south to the Gulf of Mexico’s Yucatan Peninsula. Try Tom’s Cove on Chincoteague Island, or the Great Bay of New Hampshire, or even the beaches of Key West. There, you’ll discover Darwin’s great lie: These creatures have diversified hardly at all.
But to appreciate fully the crab’s primeval nature, and critical relevance, visit Delaware Bay in late spring. During the peak high tides of the new and full moons, horseshoe crabs storm the beaches there by the hundreds of thousands to spawn. Perhaps 80 percent or more of the entire North American horseshoe crab population migrates to the inner curve of New Jersey’s Cape May peninsula, and Delaware’s shoreline on the opposite side of the bay. Beaches turn from light beige to a dark, crawling brown in one of nature’s most impressive, and bizarre, spectacles.
Strange shapes cloud the mottled waves as the assault on the shoreline begins. Many crabs hit the sand already hitched, literally, as males use specialized clasping pincers to attach themselves to their mates’ abdomens, and ride the waves to shore in tow. You may observe a female dragging three or four males slowly up the beach. She then deposits her eggs--some 4,000 at a time--in a nest dug a half-foot or more below the surface. As many as a dozen males promptly converge to fertilize the clutch of frothing, gray-green eggs before following their mate back to sea--or hitching onto another incoming female.
Each female horseshoe crab deposits up to 90,000 eggs during the two- or three-week spawning frenzy. Inevitably, crabs on the more crowded breeding beaches dig up previously laid nests, and kick the older eggs to the surface. This simple fact--an apparent footnote to the horseshoe crab mating story--may spell life or death for hundreds of thousands of shorebirds that rely on the eggs for essential sustenance.
Red knots, ruddy turnstones, sanderlings, semipalmated sandpipers, dunlins, dowitchers: names that might provoke perplexed looks in Atlantic City’s nearby casinos, but send birdwatchers into fits of jubilation. These small birds stop only briefly at Delaware Bay, but it’s often their only lay-over on migrations from wintering grounds in South America some ten thousand miles north to the Canadian Arctic.
A million birds or more arrive on the shores of Delaware Bay each spring, making it one of the largest and densest concentrations of migratory birds on the planet. Having been forced to burn muscle tissue to fuel their transit, some arrive at the bay weighing less than their fat-free weights. The birds must then as much as double their weight, or risk running empty during the final push north. To gain the necessary grams, the migrants gorge themselves on the scattered horseshoe crab eggs--several hundred tons worth dotting the shore. Because their short beaks can’t penetrate deep enough into the sand to reach the clutches of eggs, the birds survive on what the crabs accidentally excavate.
Humble keystones, the horseshoe crabs of Delaware Bay quietly bear the burden of the migratory birds’ survival. About 80 percent of the hemispheric population of red knots, and 30 percent of sanderlings, come through the bay each year to refuel. Thus it is bad news for Cape May birds that horseshoe crab egg counts on the New Jersey peninsula have declined by as much as two-thirds since 1990. Without dense aggregations of crab eggs, red knots, sanderlings, and perhaps other shorebird species would not be able to complete their epic treks each year. The delicate birds might then vanish to the winds as silently as their flocks dart through the salty air.
Exposed crab eggs on the surface don’t normally hatch, so foraging shorebirds have little impact on crab numbers. Biologists point the finger elsewhere to explain the feared decline of horseshoe crab populations: bait. That seemingly harmless term may stand between the horseshoe crab and the next five hundred million years. In recent years, as many as one horseshoe crab in four has wound up in fishermen’s nets, although the true numbers of both living and harvested crabs are little known, and largely contentious.
Harvesting horseshoe crabs--or "king" crabs as they’re known locally in Mid-Atlantic harbors--is nothing new. Fishermen from as early as the 1850s sold them to factories to grind up for fertilizer or animal feed. As many as four million crabs a year were reported pulled from the ocean or plucked off the beach in the latter half of the 19th century, although fisheries data are notoriously unreliable. Both the harvest and the total number of crabs slowly decreased in the ensuing decades. The bottom didn’t fully drop out of the crab population until after World War II; fortunately, chemical fertilizers began rendering crab formulas less cost-effective at the same time.
The horseshoe crab survived, barely, its numbers rebounding as slowly and steadily as the creature crawls through the benthic ooze. By the 1970s, the beaches again teemed with massive aggregations of the armored beasts. Carl Shuster, a marine biologist, made the first bay-wide survey in 1977. In conjunction with Mark Bottom, then a graduate student at Rutgers University, Shuster reported in 1985 that the estimated peak of spawning activity involved about 222,000 males and 51,000 females.
But the horseshoe crab has been swept up by the rushing tide of human commerce yet again. In the early 1990s, European and Asian eel stocks and Caribbean conch and New England whelk stocks ran dry. As the price of eel and whelk (commonly referred to as conch ever since watermen "fished out" the true conch population) soared, Delaware Bay fishermen shifted their focus to meet the new demand.
Word spread quickly that horseshoe crabs, seemingly limitless and ridiculously easy to collect, make perfect bait for eel and conch. Today, Atlantic watermen comb the beaches during the horseshoe crabs’ spring spawning season, collecting the creatures by the thousands into burlap sacks or rusty pick-up trucks. Boats trawl them from the sea in even larger numbers. Eelers and whelk fishermen buy the harvested crabs whole, then chop the crabs’ desiccated bodies into halves or quarters before tossing them into their pots and nets.
Eel fishermen prefer adult female crabs, which release a scent they believe attracts eels as well as amorous male crabs. An entire generation of crab mothers is winding up as bait in the bottom of eel pots--with the next generation years away from spawning age, for juvenile crabs take a full decade to reach sexual maturity. Even if the collection of bait crabs was halted immediately, the damage wrought by the last five years’ harvest could stretch well into the next millennium.
Until recently, the entire Atlantic coast horseshoe crab fishery rarely generated more than $100,000 per year. But in 1994 this bait industry grew to an estimated $229,000 business, and reached a record $1.5 million in 1996, with individual crabs selling for anywhere from 40 cents to two dollars each. Compare this, however, with the estimated $30 million Cape May County alone derives from birdwatching, an industry largely dependent on the migratory shorebirds that horseshoe crabs lure from as far away as Tierra del Fuego. (Another possible victim of this spiraling vortex of cause-and-effect is the popular and officially threatened loggerhead turtle, whose primary source of food in Chesapeake Bay is horseshoe crabs.)
Upwards of a half million horseshoe crabs were harvested each year from 1991 to 1995, and more than a million in 1996. That’s when state wildlife agencies finally decided to apply the brakes, and survey the wreckage.
Neither birders nor fishermen want to see the horseshoe crab disappear, and so the haggling over potential quotas has begun. New Jersey, Maryland, and Delaware already established harvest limits within their waters, regulating catches geographically, seasonally, and by weight (depending on the state). Last year, the Atlantic States Marine Fisheries Commission (ASMFC) set out to create a uniform management plan for the entire Atlantic coast. It consulted watermen, state fishery managers, wildlife biologists, and conservationists. It examined crab censuses, egg counts, bird surveys and landing records. It deliberated in various committees and subcommittees for many months. In the end, it changed nothing.
The ASMFC report ruled that New Jersey, Maryland, and Delaware--the states with the highest horseshoe crab concentrations--must maintain their current harvest regulations, while other states along the Atlantic coast are merely encouraged to adopt limits of their own. Each state, however, must gather monthly horseshoe crab landing records, which could ultimately support future harvest quotas. But without reliable data, the ASMFC report declared, the Commission can’t in good conscience authorize new regulations.
The plan confirmed conservationists’ worst fears. Fishery councils, after all, have traditionally been slow to react to population declines, as witnessed most notoriously in the collapse of the North Atlantic cod stock. To groups like the World Wildlife Fund, the horseshoe crab management plan ignored the precautionary principle that harvesting should be curtailed--or halted--until it’s proven that the practice doesn’t risk extinguishing the horseshoe crabs, or their shorebird dependents.
According to the only published estimate of crab numbers, from a decade ago, fewer than four million North American horseshoe crabs may exist, anywhere. Fishermen trust their own math, and their own suspicion that far more crabs are out there. Scientists simply aren’t looking in the right places, they say. Indeed, spawning crabs may favor the Delaware side of the bay one year, and the Cape May peninsula of New Jersey the next, making population trends exceedingly difficult to determine.
The watermen further point out that beach erosion, shoreline development, and global warming’s rising tides all can contribute to horseshoe crab fluctuations too. The livelihood of dozens of fishermen and hundreds of others involved in the lucrative eel and whelk industry, they complain, are at the mercy of paranoid conservationists, and of tourist interests whose priority is keeping the horseshoe crabs--and thus the birds, and thus the birdwatchers--on the New Jersey side of the Bay.
Although New Jersey, Delaware, and Maryland’s harvest restrictions seem to have slowed the total number of horseshoe crab landings since 1996, fishermen are quickly learning to exploit the loopholes. Other Atlantic states still have few or no regulations on crab harvests, so boats simply haul crabs trawled from federal waters--where there are no legal limits on take--farther afield.
Virginia’s number of horseshoe crab landings (that is, the number of crabs fishermen brought to shore in Virginia--not necessarily the number captured in Virginian waters, since many are taken off the continental shelf) skyrocketed from 25,000 in 1997 to more than 500,000 last summer. Along the Delaware River in Pennsylvania, harbormasters who in the past rarely saw the ancient beasts on dry dock found some 75,000 horseshoe crabs entering port at Chester in 1998. There, two-dollar-a-crab prices--boosted both by the high demand for unagi eel in Japanese sushi bars, and the decreasing supply of bait crabs brought about by state landing restrictions elsewhere--made it worth the extra commute for boats registered in harbors downriver.
Delaware has recently blocked access for boats carrying horseshoe crabs through their section of the Delaware River to Chester. And so New England and the Carolinas may be the next destinations for Cape May’s trawlers. Unless, that is, state governments there create landing restrictions, which the ASMFC seems hesitant to brood, much less hatch.
Yet even more is at stake in the horseshoe crab saga. For the creature has provided us with more than mere fodder for fields, farm animals, and fishing nets. Its anatomy, exquisitely honed by time, offers scientists food for thought as well. Three Nobel Prizes have gone to researchers drawing lessons from the humble crab. Our first understanding of the miracle of vision came from studies into its simple yet elegant optical apparatus. But the crab’s blood has proven the most valuable asset of all for mankind.
The ocean teems with toxic bacteria, as much as one million gram-negative bacteria per milliliter of seawater, and one billion bacteria per gram of sand near shore. At the first sign of toxic intruders, horseshoe crab blood forms a defensive clot to stave off infection. This phenomenon caught the attention of biologists in the 1950s--and of biomedical companies in the 1970s.
Toxic bacteria, known as endotoxins, survive even in purified and sterilized solutions. Needle-tips, intravenous drugs, and inserted devices like Pacemakers all can carry this poison directly into the human system, where endotoxins can provoke a "pyrotechnic response"--high fever, shock, even death. Drug companies once tested products for endotoxins using live rabbits. Feverish or dead bunnies revealed contamination. In January of 1973, the Food and Drug Administration (FDA) recognized Limulus amebocyte lysate (LAL), a derivative of horseshoe crab blood cells, as an acceptable alternative to rabbits for detecting endotoxins. All injectable drugs now pass this scrutiny.
Today a half-dozen companies, from South Carolina to New England, collect, process, and market LAL. In the laboratory of Associates of Cape Cod, live horseshoe crabs delivered each morning by the region’s fishermen are lined up in long assembly-line rows. College students hired for the summer puncture the crabs’ hearts with large-gauge needles, drawing as much as 30 percent of their rich blue blood drop by drop into small beakers, before sending the crabs back to the sea alive. (Anywhere from 85 percent to 99 percent survive the process, thereby not posing a serious threat to their populations.)
Centrifuges separate the amebocyte blood cells from the useless plasma. Once extracted, the LAL solution is freeze-dried in its final liquid form, which is stable for more than four years. To screen a drug--a flu shot, chicken pox vaccine, or anything else destined for a patient’s bloodstream or spinal fluid--the thawed lysate is mixed with an equal amount of test solution. If a gel clot forms after an hour of incubation, it is promptly discarded. The medicine is tainted--maybe as little as one-millionth of a gram, but dangerous nonetheless.
Biomedical researchers also employ horseshoe crab blood cells to detect spinal meningitis and gonorrhea, and to study AIDS, cancer, and other diseases. Science owes much to the horseshoe crab, and may yet repay it in kind. Researchers at the University of Delaware, working with local fishermen, hope to develop an artificial horseshoe crab scent to replace living crabs as bait. Such a solution not only would sustain the fisheries and spell relief for the crabs (and the starved birds in turn) but would allow vital biomedical research to continue unabated. Like much of nature’s bounty, the horseshoe crab’s unforeseen benefits are potentially limitless, but only if the creature survives in sufficient numbers.
Ten million generations or so after its ancient relative valiantly struggled through the toxic lagoon, a horseshoe crab beaches at Cape May under a pale moon. The crab ignores the curious onlookers toting cameras and birding telescopes, and plods up the sloped sand to spawn in silence, returning minutes later to the murky waters.
It might have laughed at us, if it could. "You think you can kill us off--grind us into manure, trawl us from the sea, bleed us dry? We’ve been around longer than you’ve had backbones!" But the crab spares us the commentary, mute on the sea floor, confident its kind will outlive the strange creatures peering down through the filtering surface, their stilted houses teetering upshore.
This meek creature has not exactly inherited the Earth, but we stand to inherit much from it. Like remoras on a shark, many humans have leeched onto the horseshoe crab to survive, whether for food, medicine, or money. Humankind may ultimately determine whether the ancient mariner and its plentiful gifts last ten million generations more.
Alex Hawes is a science writer based in Washington, D.C.
ZooGoer
28(3) 1999.
Copyright 1999 Friends of the National Zoo.
All Rights Reserved.
