Mary Syrett - June 2011

 

The Tidewater's Living Fossil
by
Mary Syrett

 

When a new moon rises in June, thousands of prehistoric creatures emerge from the depths of the sea, their heavy armor clanking as they clamor over one another in the shallows of Chesapeake Bay. Here they mate, leaving millions of eggs in sand near the shore. Then they seemingly disappear.
Of all the animals that inhabit this planet, the horseshoe crab, also known as “horsefoot” and “saucepan,” is my favorite. There is something special about a creature that has remained unchanged for hundreds of millions of years, surviving mass extinctions that at various times wiped out much of life on earth.
The horseshoe crab (Limulus polyphemus) belongs to the Anthropod class of animals that have legs with joints and a body divided into two or more parts. The American horseshoe crab is found from Maine to Mexico and throughout Florida. The largest concentration in the world is found in the Delaware Bay/Chesapeake Bay region. The name originates from the rounded shape of the head. Just like the shoe on a horse’s foot, the head is round and U-shaped.
Ancestry. The horseshoe crab first appeared some 500 million years ago, during the Ordovician Period, and has changed very little since then. Half a billion years ago, tiny horseshoe crabs crept along shorelines much like today’s larger versions do. Two fossil specimens recently discovered in Canada reveal a new genus of the creature, pushing its origins back to at least 100 million years earlier than previously thought. Dubbed Lunataspis aurora, the ancient horseshoe crab is estimated to have been just 1.5 inches long from head to tail tip. That’s much smaller than its modern-day relatives, some of whom span 20 inches. Horseshoe crabs are the closest living relative of the long extinct trilobite, which was a marine creature of the Paleozoic era having a segmented skeleton divided by grooves.
Consider the real possibility that long, long ago a horseshoe crab resting on a beach saw a dragonfly with a three-foot wingspan pass by, or witnessed enormous dinosaurs in the immediate vicinity. Today, dinosaurs exist only as fossilized exhibits, and live dragonflies have three-inch wingspans. Still, horseshoe crabs live on without any appreciably noticeable evolutionary changes.
One reason why the horseshoe has been able to live so long is because it can tolerate different variations of salinity in the water. Also, the creature can go an entire year without eating. Trying to survive with marked variations in salt water is enough to limit the lifespan of most sea creatures, but surviving for a year without intake of food is something few animals can do.
Physical Characteristics. Horseshoe crabs breathe through gills that are attached to the underside of a pair of abdominal legs. They lack jaws. The mouth is essentially a hole in the esophagus with muscles that open and close.
The first pair of legs is covered with thick, hard bristles pointing inward toward the mouth. These bristles act like a set of grinding tools as the animal is walking; they are used to “chew” food and direct it toward the mouth. Because horseshoe crabs grind food with their spiny leg segments, they must be walking in order to properly chew.
The long tail is not used as a weapon. Rather it is employed as a rudder for steering.
Distinctly different horseshoe crab populations inhabit major estuaries along the Atlantic Coast. Populations in one estuary can be differentiated from others based on the relative size of adults, the shell’s color and by pigments present in the eyes.
Horseshoe crabs eat worms, mollusks, crustaceans, and small fish. When eating clams, a horseshoe places a clam near its mouth and crushes the mollusk with the burr-like sections of its legs.
In the ever-shifting tides, horseshoe crabs are coveted real estate. Over 20 different organisms live on horseshoe crab shells, including barnacles, blue mussels, sponges, flatworms and leeches.
No one can say with certainty how horseshoe crabs have been able, with “no modern improvements,” to survive in competition with other creatures that are more highly evolved. Their survival may reflect a lack of specialization. Horseshoes can inhabit shallow sandy, rocky or muddy shores. They can walk or swim by flapping their primitive abdominal appendages. They have six pairs of legs, two in the front for seizing prey and five more for walking. Their shell provides protection, and the tail helps put them back upright when they are turned over by a tidal wave. Really not too bad a life, when you think about it.
Horseshoe Crabs & Medicine. If you’ve ever received a flu shot or medication from an IV line, or if you have a pacemaker or another surgically implanted medical device, odds are that a horseshoe crab has saved you from a life-threatening infection. The blood of the horseshoe crab, colored blue by the presence of copper, clots in the presence of toxic bacteria. Pharmaceutical companies test the purity of IV drugs, vaccines, medical devices and the water used to produce them with Limulus amebocyte lysate (LAL). This chemical is made from horseshoe crab blood cells that have been separated from the plasma, then broken down to release a toxin detector. LAL is unique. Researchers have been unable to synthesize its equivalent in the laboratory, making the blue blood of horseshoe crabs the gold standard for quality control of pharmaceuticals.
No IV drug reaches your hospital pharmacy without a blood test. If you or someone you care about has ever been hospitalized, you owe more than you will ever know to a creature whose lineage extends far back in the planet’s primordial history.
Each year, horseshoe crabs donate (involuntarily) their blood for the production of LAL. Some die in the process. Increased understanding of how much blood a crab has, how it can be safely extracted, and how to reduce the animals’ stress during time spent out of water can help sustain the population of this unique creature.
Along with people, some birds owe much to the horseshoe. Millions of shorebirds migrate each year between the southern tip of Argentina and breeding grounds in Alaska. Delaware and Chesapeake bays are one of the refueling stops of these shorebirds in their very long distance migratory flight.
After a four-day non-stop flight from South America, the birds are hungry in the extreme by the time they arrive along the Atlantic Seaboard. Here shorebirds gorge themselves on horseshoe crab eggs in order to gain the strength needed to continue flying northward to Alaska. While regaining their strength on Delaware and Chesapeake bays, it has been estimated that each bird eats 10,000 horseshoe crab eggs a day.
Threats to the Horseshoe Crab’s Survival. Human development threatens the long-term survival of the horseshoe crab, both in the United States and in other parts of the world where the creatures live. Human disturbances can adversely affect spawning activities. For example, beach development and shoreline modifications oftentimes prevent horseshoe crabs from reaching traditional spawning areas. Recreational vehicle traffic on beaches can crush the crabs and destroy prized nesting habitat. Oil spills also pose threats to breeding grounds and to young horseshoe crabs.
We often only learn the real value of a species after its population has been decimated. However, in the case of horseshoe crabs, we already know their importance to the biomedical industry. Today man has an opportunity to protect horseshoe crab populations at sustainable levels and ensure sufficient numbers of the creatures will be around for both ecological and commercial uses, now and in the future.
The most decisive factor in the animal’s long-term survival may involve future changes in sea level and water temperature. Even if they occur only on a minimal scale, such changes may negatively impact the crabs’ distribution and reproduction patterns.
Maryland’s Department of Natural Resources is seeking your help in identifying shoreline used as horseshoe crab spawning habitat around the state. When fishing, should you notice horseshoes spawning on the shoreline, please document the location, using GPS coordinates if possible. For further information, contact Mark Topolski at 410-260-8263, or by e-mail at mtopolski@dnr.state.md.us.
The horseshoe crab is the sort of fanciful, out-of-this-world creature that appears more frequently in science fiction stories than in zoology texts. It is almost impossible to comprehend how long this incredible animal has lived on earth.
For some people, the horseshoe is a childhood acquaintance, first introduced when we saw a fierce-looking skull embedded in sand on a Chesapeake beach. For ecologists, historians and scientists, the horseshoe crab is a significant creature – an adaptable generalist that pre-dates most every species now living on the planet. For all of us, the creature is an intriguing puzzle that offers insights into evolution, bacteria and species interdependence.

Mary Syrett is a freelance writer and an avid student of nature.