A Living Fossil Faces Uphill Climb

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A horseshoe crab nears the ocean at sunrise. Dell Cullum photo

 

By Stephen J. Kotz

Let’s face it, the sight of hundreds, if not thousands, of horseshoe crabs crawling up onto the beach from dark bay waters under the light of a full moon to spawn could be the stuff of childhood nightmares.

It turns out the animals, which may look like interstellar aliens but have been calling the Earth their home, in some form or other, for the past 450 million years, are facing their own nightmares in the form of overharvesting and rising ocean temperatures as they fight for their very survival.

“This is a species that has survived the test of time,” said Dr. Matthew Sclafani, a marine biologist with Cornell Cooperative Extension who has studied horseshoe crabs extensively. If Earth is 4.5 billion years old and one created a timeline comparing human beings to horseshoe crabs, “human beings would be on that timeline for 1 second,” he said. “Horseshoe crabs for two-and-a-half to three hours.”

“They are called the living fossil,” he added. “They have lived through five mass extinctions.” Now, though, with a huge population crash dating back to 2000 and the lack of a significant rebound since then, their fate is in question.

In a world, where it’s either eat or be eaten, horseshoe crabs haven’t survived just because they don’t look particularly appetizing, although sea turtles and sharks, which do feed on them, might disagree. They have obviously developed some winning coping mechanisms like a hard shell, and a tail-like appendage that not only helps them steer, but provides leverage, along with a hinged shell, to flip them back over when they are rumbled in the surf. Under their shells, they have sensory organs that help sniff out food (they are partial to clams, by the way), pincers to help grab food and bristle-lined mouths that grind food as their legs move.

Then there are the eyes. The horseshoe crab has two large eyes on the side of its shell, which are each equipped with thousands of lenses. There are another five eyes that are sensitive to light or ultraviolet rays on top of the head, plus two more under the shell near the mouth, which scientists believe are used to help them navigate. The tail itself contains light sensors, which are thought to help them regulate their bodies in relation to night and day.

If the horseshoe crab is starting to sound like a spider, that’s because it is. It’s technically not even a crustacean and is more closely related to other arachnids. And parents who have ever battled an outbreak of head lice in their children might notice that when a horseshoe crab is turned over, it resembles the giant head louse from hell. Distant relatives, according to Dr. Sclafani.

“The Original Sex on the Beach”

Every year, during lunar cycles starting with the first full moon in May (next Thursday, May 11) and extending through the full moon in June, horseshoe crabs crawl up on beaches along the Eastern seaboard to spawn. One male will attach itself to the female’s shell by means of a cleverly evolved hook, and hang on for two months or longer, thus being in the best position to fertilize some of the approximately 80,000 eggs she will lay. When the female pulls herself up on the beach, other males crowd around to also help fertilize the eggs and guarantee genetic diversity.

“It’s the original sex on the beach,” said Dr. Sclafani. “The original hook up.”

A pair of mating horseshoe crabs.

The female buries eggs a few inches under the surface of the sand. “Maybe five survive,” Dr. Sclafani said. “If they all survived, we’d be walking to Connecticut on a bridge of horseshoe crabs.”

Most eggs don’t survive because they are eaten by other animals like the red knot, a robin-sized shorebird that makes an incredible annual migration of more than 9,000 miles from Antarctica to the Arctic. Not surprisingly, as the horseshoe crab population has declined, so has that of the red knot as well as other birds that feast on their fatty eggs like the American oystercatcher and ruddy turnstones.

“If you see a red knot on the South Fork today, it’s pretty rare,” said Frank Quevedo, the executive director of the South Fork Natural History Museum in Bridgehampton. “They used to be plentiful on Long Island. But if you take away one species, you have an impact on others.”

So, why have horseshoe crabs gone into such precipitous decline to the point where beaches that were once thick with them at spawning season, are now virtually empty?

“In the 20 years since I’ve been here on the East End, I’ve seen a tremendous change, especially on the South Fork,” said Mr. Quevedo. “Where I used to see them at Napeague and Three Mile Harbor, I’m not seeing them as much. And while I still see some adults, I’m not seeing the juveniles.”

Mr. Quevedo theorized that perhaps the loss of eelgrass, which has largely disappeared due to frequent harmful algal blooms caused by rising nitrogen levels over the past 30 years, might be affecting the horseshoe crab population by eliminating places for the younger crabs to hide.

Dr. Sclafani said baymen have traditionally taken a large number of horseshoe crabs, which they use in traps as bait to catch both eels and whelk, or conch, which apparently think of horseshoe crab meat as a treat.

Serving a Medical Purpose

Now, though, they are coveted for a much more valuable purpose: saving lives. It turns out the clotting agent in the horseshoe crab’s blue blood can detect certain harmful bacteria, such as E. coli. Bio-medical companies use the clotting agent to create a substance called Limulus Amoebocyte Lysate which is used to detect harmful bacteria in injectable drugs like insulin, implants like artificial hips and hospital equipment that must be sterilized like scalpels and other surgical tools.

And LAL is not cheap. It costs about $10,000 a quart, driving up the demand for horseshoe crabs, hundreds of thousands of which are collected each year and brought to laboratories, where technicians draw about a third of their blood. Although the crabs are returned to the water after their blood is drawn, the mortality rate is thought to be as high as 50 percent because they are often left on docks or in trucks for a day or two afterward, causing major stress and unknown impacts on breeding habits of the survivors.

Dr. Sclafani said medical demand for the crabs has driven their price up astronomically. While you could buy a couple horseshoe crabs for about a penny during the 1920s, the price reached a peak of $5 apiece in the early 2000s, meaning an enterprising youth who wanted to make some extra spending money could pocket $1,000 by simply picking up 200 crabs at the height of the spawning season.

Another Casualty of Climate Change?

But something more pervasive than loss of habitat and overharvesting is also at play. Scientists say climate change, specifically a rise in the average temperature of the Atlantic Ocean has led to a decline in the horseshoe crab population.

Dr. Robert Cerrato, a marine scientist at Stony Brook University, said scientists discovered a correlation between ocean temperatures and the population of horseshoe crabs and other marine species in 2012, thanks to a long-term sampling program conducted by the New York State Department of Environmental Conservation.

Since 1987, the DEC, originally in an effort to learn more about weakfish populations, conducted regular trawler drags of local waters. Its findings showed that as of about 2000, the population of horseshoe crabs and several others species, including windowpane flounder, winter flounder, oyster toadfish, and the calico crab, declined significantly. During that same period, butterfish, spider crabs, smallmouth flounder, scup, and summer flounder, or fluke, saw their populations increase.

Tyler Abruzzo, a Stony Brook student, working on her master’s degree, took the DEC’s data and compared it to two gauges of atmospheric and ocean temperatures over the long term, and she was able to show a direct correlation.

“These are offshore changes,” Dr. Sclafani said. “They could result in an increase in top-down predators or it could be bottom up” in the form of nutrients or some other cause. “It’s a major regime shift.”

“Whatever is happening is on a large-scale basis,” added Dr. Cerrato.

Be a Crab Counter

So you fancy yourself a citizen scientist, or maybe just an amateur naturalist, who would enjoy helping Cornell Cooperative Extension monitor horseshoe crabs when they begin hauling themselves up Long Island beaches to spawn starting this week.

The good news, according to Dr. Sclafani, a marine biologist who studies the crabs, is that Cornell can use your help. The bad news is you have to commit to showing up for 12 nights between May 8 and June 26 through the entire spawning season that runs through two full and two new moon phases. Oh, and by night, we mean late, like from 10 p.m. to 2 a.m., so you are on site for the high tide.

Spawning monitors are tasked with counting, measuring and tagging the crabs they find on the beach. A detailed 8-page protocol is available at www.nyhorseshoecrabs.org as is information about monitoring sites. (The closest ones to Sag Harbor are at Squire’s Pond East Landing Road in Hampton Bays and South Harbor Road in Southold, although additional monitoring sites could be established.

If you are into more casual monitoring, you can download the Horseshoe Crab App for your iPhone. The app allows you to report your observations about spawning horseshoe crabs to Cornell. Information about the app is also available at www.nyhorseshoecrabs.org .

Cornell says there are many reasons for collecting data on horseshoe crabs, but given the crash in their population and the related decline in migratory shorebirds that feed on their eggs, it is important to get accurate census data.

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