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Article for November -December 2011

For Wildlife Watchers: Horseshoe Crab
by Rob Simbeck
photograph by Phillip Jones

Horseshoe crabs aren't true crabs at all. They are more closely related to spiders, scorpions, ticks and mites.

Horseshoe Crab - photograph by Phillip JonesI'm one of those people hoping that we eventually make contact with an alien civilization or two. It’s an endlessly fascinating subject; what are the possibilities when it comes to extraterrestrial life? Who among our scientists and science fiction writers got it right? Fortunately, I'm also one of those people who are awestruck by the endless variations of life played out right here on earth. I've been writing "For Wildlife Watchers" for seventeen years now, and I have yet to run across a creature, no matter how quotidian, that didn’t offer something wondrous upon the least amount of real reflection. The horseshoe crab, for instance, may well be the creature best suited to giving us a foretaste of the wonders that lie beyond this lovely but troubled little planet. It is a remarkable assemblage of features, a testament to the variety possible among living organisms and to their potential stability as well.

First, the variety. Like Mr. Spock of Star Trek fame, the horseshoe crab's blood contains a copper-based oxidizing agent called hemocyanin, rather than an iron-based hemoglobin, so they bleed dark blue or green. Horseshoe crab blood also contains a substance used to test for bacterial contamination, making it valuable enough to the pharmaceutical industry — it can bring as much as $15,000 per quart — that the crabs are harvested, and in this case released again, after about a third of that blood is removed.

A horseshoe crab has nine eyes, two of them complex and the rest simple, on its large helmet-like head or prosoma, including two on its underside. A tenth consists of rudimentary light receptors spread along its tail. Those eyes were the subjects of research into the physiological and chemical processes controlling vision that earned the 1967 Nobel Prize in medicine.


Atlantic Horseshoe Crab -
  Limulus polyphemus

Description: Up to a foot across, oval-shaped, helmet-like prosoma, or head, smaller abdomen, sword-like tail.

Range and Habitat: From Maine to the Yucatan. Sandy, muddy beaches.

Reproduction: Females lay eggs in excavated depressions in the beach, where they are fertilized, often by several males.

Viewing Tips: Horseshoe crabs are nocturnal. Spring spawning brings them ashore on seaward beaches in places like the ACE Basin.

And there's more: Both the horseshoe crab’s brain and its heart lie in the greenish-gray to brown prosoma, which is shaped somewhat like a horseshoe, and its gaping, jawless mouth is set amid the legs on its underside. Its long tail, which acts as a rudder, is strong enough that the crab can use it to right itself should it be flipped onto its back, and, like a small and fortunate number of creatures, it can regrow limbs. The stability of that unusual design is impressive. Horseshoe crabs date to the Ordovician period and are, in that sense, living fossils; little-changed since well before the dinosaurs.

Limulus polyphemus is one of four species of horseshoe crabs, and the only one that occurs in the Western Hemisphere; it can be found from Maine to southern Mexico. But in fact, horseshoe crabs aren’t true crabs at all. They are more closely related to spiders, scorpions, ticks and mites. They live in the sand and mud of the ocean bottom, where they burrow for worms and mollusks, spending winters along the continental shelf and moving in late spring to shallow waters to reproduce. Males arrive first and wait for the much larger females.

"The timing is pretty critical," says Dr. Elizabeth Wenner, a marine scientist retired from the S. C. Department of Natural Resources. "Water temperature and beach composition are both important, and much of the mating happens during the spring tides at full moon."

Males use glove-like claws on one of their seven pairs of legs to grasp a female's shell, and the females pull them along when they leave the water to dig a series of five to seven small holes and deposit between 3,000 and 4,000 tiny green eggs into each one. Only ten or so from each hole will survive to adulthood, says Wenner. A female may lay anywhere from 60,000 to 100,000 eggs in one season. The male fertilizes them as he is pulled along, and since males greatly outnumber females, multiple males often fertilize one female’s eggs.

"One of the most amazing things I've ever witnessed," says Wenner, "is thousands of horseshoe crabs in the water on a spawning beach. There was a frenzy of activity, with many satellite males forming a daisy chain, latched on, one to another to another, with one male latched onto the female. The power of those pheromones to draw them ashore at that particular time when all the conditions were right was awesome."

Shorebirds, especially red knots, which stop during their 12,000-mile migration from Tierra del Fuego to the Arctic, gorge themselves on those eggs, often doubling their weight. In fact, reductions in the number of horseshoe crabs have had major impacts on the numbers of red knots and other birds.

"It brings up what a great web of interconnection we're part of," says Wenner, "how a creature millions of years old provides eggs on which birds are dependent and provides in its blood a component depended on by Homo sapiens. How cool is that?"

The larvae molt four times inside the egg, then hatch after two to four weeks and dig their way out of the sand. They will molt three or four times a year for a few years, and then, at age five or six, molt annually. Males become sexually mature in their eighth or ninth year, females in their tenth. By then, they are sixteen to twenty inches in length and ten to twelve inches across. They are thought to live for as long as thirty years.

Horseshoe crabs are nocturnal, which helps them avoid predators like shorebirds, as well as sharks and loggerhead turtles. When the latter are present, horseshoe crabs often bury themselves in the sand, using the leaf-like ends of the rearmost of their seven pairs of legs. The five pairs between the rear pair and the front pair (which are used for feeling) are used for locomotion. In juveniles, the gills also aid in movement. Those gills, arranged in five pairs under the abdomen, absorb oxygen from the water and can also be used for breathing on land if they remain moist.

Besides research and medical use, humans use horseshoe crabs as bait and in fertilizer. The biggest impact humans have on them, though, comes via habitat loss and degradation. They are not yet listed as either endangered or threatened in South Carolina's waters, but concern for their numbers has led to a ban on harvesting outside of biomedical uses — and then only with a permit — in place for the past twenty years.

A species of horseshoe crab common to Japanese waters is endangered, and groups such as the Atlantic States Marine Fisheries Commission are working to try and prevent that from happening here, seeking to conserve a species thats existence has been of untold value to our own, and whose unique blend of features offers us such a compelling glimpse of life’s myriad possibilities.

—Rob Simbeck

Rob Simbeck is an award-winning freelance writer living in Nashville, Tennessee.

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