Betting on horseshoes: High-tech research in a high-stakes game
by Creed Taylor
It's close to midnight in late
May. The first butter-yellow full moon of spring hangs in a sky the color of
octopus ink. At the high-tide line on this section of Delaware beach are
perhaps 100,000 horseshoe crabs, each glistening shell reflecting its own lunar
orb. With a spawning instinct that is as old as the beach itself, the crabs again
have ascended the sand at flood tide, continuing the ritual of renewal they
began one night 4.5 billion moons ago.
Watching the drama unfold is Jim Berkson, director of Virginia Tech's Horseshoe Crab Research Center (HCRC), the world's largest center devoted to multi-use management of the species. Although they are the most-researched marine invertebrate on the planet, little is actually known about the habits and population dynamics of horseshoe crabs--except that their numbers are going down as interest in them is going up.
That interest has been fueled by declining numbers of Eastern shore birds, which depend upon the horseshoe crab's eggs for food; the growing value of the horseshoe crab fishery industry; and increasing awareness of the curative value of the creature's blood. As a result, the once overlooked arthropod has crawled into the spotlight. Berkson notes, "The battle over this ecologically, economically, and medically essential species has become one of the most heated environmental issues on the East Coast in recent years."
The Limulus polyphemus
"A month after we started studying these animals," says Berkson in a quietly excited tone, "my partner in all of this, Steve Smith, from the vet school, said, 'We're convinced these things come from outer space.' They don't follow any of the rules of any animal we've ever looked at over the years."
Their closest planetary relatives are trilobytes, imprinted in stone during their hasty departure half a billion years ago. Fossils of animals essentially unchanged are part of the makeup of the very bedrock of the earth. They were here 150 million years before the first dinosaurs, and at the end of the Paleozoic era, when a cataclysmic force of nature wiped out 95 percent of all marine species, the horseshoe crab trundled through the cooling silt.
The horseshoe crab's life history is unique among earth's creatures. Not actually "crabs" at all, horseshoe crabs--sometimes called horsefoot, or soldier, crabs--belong to the phylum Arthropoda, which includes insects, arachnids, and crustaceans. Among arthropods, horseshoe crabs are closest to scorpions, spiders, and ticks, yet they belong to and are the sole owners of the class Merosomata, meaning "legs attached to the mouth."
Of the four species of horseshoe crabs, three are Indo-Pacific, spanning from the cold Bering Sea currents of the North Pacific to the South China Sea and warm Indian Ocean of the Indonesian archipelago. The species most familiar to the Western World, Limulus polyphemus, inhabits North and Central American waters from the warmth of the southern Gulf of Mexico's Yucatan Peninsula to the brisk North Atlantic, just below Maritime Canada. The greatest concentration of animals is found in the tidal sweep of the Mid-Atlantic states of Virginia, Maryland, Delaware, and New Jersey.
Adult crabs can live out of water for months. They've been known to go without food for an entire year. Bite marks that sometimes show up on the shells of spawning crabs bear the unmistakable leer of sharks, which occasionally eat the crabs whole, and it's not uncommon to see propeller scars on the backs of adventurous crabs.
Although a steady crawl is their primary means of locomotion, horseshoe crabs are good swimmers. In fact, they swim upside down in the open ocean, using their front set of legs to guide a rear set of branchial appendages, called "book gills," as propellers. To keep on schedule through it all, they use synchronizing light sensors on their spear-like telson (the non-poisonous tail) to help cycle the brain through night and day.
Exactly where they go as they crawl and swim the sea is not yet understood. "We know so little about where crabs go," Berkson says. "One of the studies we're doing at Tech is looking at their movement patterns. We're tagging crabs every year out of Chincoteague, and we're getting re-sights as far north as New York in as little as one year. The thought is that they get swept up in the currents, sometimes carried for distances we previously thought impossible."
Like their insect relatives, horseshoe crabs wear their skeletons on the outside. The exoskeletons, shells composed of chitin, are shed often when the crabs are young and with less frequency as they age. Crabs emerge from each molt about 25 percent larger, averaging around 16 shell cycles. They are considered to be fully grown at age 10 and wear their last molt for the remaining decade or so of their lives.
A mature, 20-year-old animal may carry a submarine forest, its chitinous dome teeming with hitchhiking wildlife. Plants and animals attach to the shells in a process called "fouling." Seaweeds and algae provide food and shelter for mud snails, baby spider crabs, skeleton shrimp, and periwinkle. Firmly footed mussels carry their own freeloading freight of barnacles. And there are countless others. The oldest horseshoe crabs haul around so many fouling organisms that a single shell becomes a complex biological community complete with interactive relationships, including creatures that will make short work of cleanup when their host inevitably keels over.
The "Crab Shack"
Hundreds of miles inland, horseshoe crabs have found a home on the rural edge of the Virginia Tech campus. At the end of a country road through woods and hayfields, sits the HCRC, or what Jim Berkson affectionately calls the "Crab Shack." When he's not organizing coastwide trawl surveys, checking radio telemetry releases, or giving presentations on sustainable harvest imperatives, Berkson is working in the Crab Shack, part of the university's Aquacenter, a colony of low-slung, high-tech, living aquatic research labs.
Like a bungalow at a summer science camp for arthropods, the Crab Shack sits in a copse of hardwoods in an expanse of pastureland that stretches to receding ridges of blue. Totally climate controlled, the aquaculture system contains eight 750-gallon fiberglass tanks, arranged like bunk beds for a revolving procession of camping crabs. State-of-the-art biological and ultraviolet filtration, saline, and dissolved oxygen sensors ensure complete crab comfort.
Berkson points to the uniquely synergistic benefits available only at Virginia Tech. "The Department of Fisheries and Wildlife Sciences has a record of 30 years of continuous study of fisheries management issues. We've got Jim Fraser, one of the nation's top shorebird ecologists, doing research on the needs of migratory birds. Between Bioinformatics and CMI [the high-tech Conservation Management Institute], we're building a store of information that's available nowhere else."
Interest in horseshoe crabs is most acute in the biomedical industry. The eyes, shell and blood have undergone continuous research for many years. Several Nobel Prize awards have been based on the horseshoe crab's compound eyes, which share similarities in the physics of scale with the human eye.
The crab's chitinous shell is revealing new secrets about post-operative tissue regeneration. Surgeons and prosthetics designers now understand more about material interaction in artificial organ construction and transplantation. Ophthalmologists found chitin perfect for the manufacture of contact lenses, and the material also is used in a sheet form as a wound dressing for burn victims and to make medical sutures that work to accelerate healing in human tissue. Among its many newly discovered applications, chitin is used to remove lead from drinking water and in purifying wastewater.
The horseshoe crab's blue, copper-based blood is perhaps the most noteworthy extract from the arthropod. One Virginia Tech partner, BioWhittaker, uses extracted blood cells to make the medical-purity testing agent Limulus Amebocyte Lysate, or LAL, which is crucial for testing vaccines and intravenous solutions and as an aid in tissue acceptance for hip replacements and artificial hearts.
Berkson says that LAL has recently become useful in the war against bio-terrorism. "All vaccines, including the anthrax and smallpox vaccines, must be tested using LAL to prevent any possible contamination, whether intentional or unintentional." Another project looks for alternative ways to produce LAL to ensure supply while developing sustainable management of the existing resource.
As the nation's largest producer of LAL, BioWhittaker has a stake in sustaining healthy crab stocks. The company's extraction procedure boasts high survival rates, with more than 90 percent of freshly bled crabs returned to the ocean alive. The crabs reabsorb lost fluids within three days of the bleeding and regenerate all of the blood cells within a couple of months of their return to the sea.
The HCRC is in a high-speed relay race, representing the factions who "need fair and sustainable management options," Berkson says. "This is a real multiple-use resource." Expertise across multiple disciplines is helping to formulate the first realistic projections in population dynamics.
Berkson is asked to present findings to state and federal commissions, including Virginia's Terrorism Task Force, and the Commission on Security and Preparedness. To monitor stocks, the HCRC is designing coastwide trawl surveys, designing strategies and systems for radio telemetry tracking, and using airplanes equipped with infrared nightscope cameras to index spawning numbers with increased speed and accuracy.
Since the 1800s, Mid-Atlantic farmers have reaped enormous benefit from the crabs. "Soldier" crabs were collected by the millions on the beaches of the Delaware and Chesapeake Bays. They were dried, ground, and used to fertilize peach orchards, corn, onion, and bean fields, and as a high-protein supplement in livestock feed. Annual harvests from the 1870s to the turn of the century remained at around five million, but by the early 1920s, the numbers began to dwindle. From the early 1940s to the mid 1950s, annual harvests plummeted to 500,000, then 250,000, one-sixteenth of original harvests. The horseshoe crab fishery bottomed out by the 1960s, with an average per year teetering at a telltale 42,000.
The once populous army of seemingly indefatigable soldier crabs was commercially exhausted by 1970. After the horseshoe crab fishing industry collapsed, the crustacean's numbers gradually rose. Today, with the advent of more sophisticated equipment, huge numbers of crabs are again being taken. The number of horseshoe crabs landed per year during the 1990s was up around 2 to 3 million, and although that number recently has taken a downward pitch, there is still concern over whether the crab stocks could sustain a second round of depletion.
In addition to their use as feed and fertilizer, horseshoe crabs are now gathered by Chesapeake Bay watermen for use as bait in a newly lucrative market for eel and conch, although the "conch" are actually giant bay snails. Showing up as sushi and scungeli on the tables of domestic Japanese and Italian restaurants, both eel and conch are also fetching high prices in the overseas Asian market. Female crabs are preferred over males for eel bait, while crabs of either sex are split and used to bait a pair of whelk pots. The new whelk fishery augments rose from 75,000 pounds in 1994 to 750,000 pounds in 1995, pressuring already weakened crab stocks.
The horseshoe crab population also has an impact on the environment. Its eggs are vital to migrating bird populations, and the Delaware Bay is the single most important stop for migratory shorebirds in the lower 48 states. Numbers of the many migratory shorebirds dependant on crab eggs for food are spiraling downward at critical velocities. The red knot, a threatened migratory shorebird, has seen population crashes up to 54 percent in just two years. That these declining numbers coincide with the meteoric rise in horseshoe crab landings is probably no accident.
The crabs also factor into the diet of the endangered loggerhead sea turtle. Recent analysis of the stomach content of loggerheads in Mid-Atlantic coastal waters found that three-quarters of them had recently eaten horseshoe crab. Taking the necessary measures toward the maintenance of a healthy stock of horseshoe crabs is therefore a smart start in helping to ensure viable numbers of sea turtles in the Chesapeake Bay area.
While the horseshoe crab is neither endangered nor threatened, its numbers recently have taken a nosedive. It's unknown just how fast diminishing stocks might be replenished by natural reproduction, although Berkson thinks that "there are enough crabs out there for shorebirds and for biomedical bleeding, with a degree of harvesting that might yield self-replenishing stocks."
A three-sided debate
Given the concerns over migrating shorebird populations, an increased demand in the whelk fishery, and biomedical requirements for meeting the needs of public health, the debate over competing needs for the horseshoe crab seems like a no-win situation. The crab's population dynamics contribute significantly to the biomedicine, commercial fishing, and ecotourism industries. The U.S. Fish & Wildlife Service (USF&W) values the horseshoe crab fishery at $11 million per year and the eel and conch fisheries--dependant on the crab for bait--at $21 million. The USF&W also puts a value of $150 million on the non-lethal collection and use of adult horseshoe crab blood for the biomedical industry. A single report to the New Jersey Department of Environmental Protection valued the 1998 ecotourism industry of Delaware Bay at $34 million.
At the HCRC, Berkson explains, they're not taking sides in the debate. "If you're doing good, unbiased science, with the goal of benefiting the resource and the resource users, you can get everybody's support. It's when you try to say more than your data says, when you infer too much for a political purpose--that's when you lose support. We're not doing that here. Just good science."
Plenty of data exists for most commercial marine species resources, but when it comes to horseshoe crabs, Berkson says, "We're playing a big game of 'catch up.' We should have 20 years of [data]. If we had 20 years of this, we could actually manage the resource. We got the first year last year, and we now have a baseline."
"We're not saying we're 'for the birds,' we're 'for the crabs,' we're 'for the fishermen,'" he continues. "We're saying, 'we want to do good management and good science.' We're not going to be able to protect the fishery, migratory birds, or the public health until we know how to manage the resource."
Right now, there is a five-year budget proposal before Congress for $700,000 per year to conduct horseshoe crab population dynamics research. "That money could specifically come to Virginia Tech to do four studies that need to be done," says Berkson. "It's been approved by the Senate Appropriations Subcommittee. We've got support from Congressman Boucher and senators from Virginia, Delaware, and South Carolina; we've got 15 of the big environmental groups; biomedical industry backing; and the states of Virginia, Maryland, Delaware, New Jersey, South Carolina, and Georgia behind us."
"In the four years since Tech started its research, we've identified what is known, what is not known, the methods needed to get the information we still need, and we've started collecting it," the researcher says. "We've got the right people involved. We're getting the information we need to develop the right solutions. Virginia Tech research will lead to an effective management plan for horseshoe crabs. Virginia Tech is helping people to realize just how important this species is."
They were here before metamorphic events of fire and ice destroyed and replaced much of life on earth. And 350 million years later, they are still here. The horseshoe crabs ascending this moonlit beach tonight--perfect genetic echoes of animals long ago turned to stone--are fulfilling a promise made at the beginning of time. And with a little help from the researchers back at the Crab Shack, someone will be watching this ritual in the moonlight 350 million years from now.
Creed Taylor is a web designer in University Relations and a writer.