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Old Hossfoot
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U.S. Fish and Wildlife Service photo |
Fire Island, 1988
If you've taken a walk along the bay shore at low tide lately, perhaps you've found the tiny shells cast off by young horseshoe crabs as they molt. They may be no more than an inch or two in diameter, and when you hold one up to the light, you'll see that it's translucent, with a delicate beauty that may not be so easy to appreciate when the animal reaches full growth.
Not many people consider the horseshoe crab beautiful; children find them frightening and adults say they are repulsive. Yet to my mind, these harmless creatures are among the most mysterious and even romantic inhabitants of the barrier beach environment. How can one not be awed in the presence of an animal that predates the dinosaur? The Atlantic horseshoe crab (Limulus polyphemus to biologists, Old Hossfoot to baymen) has roamed the waters unchanged for more than 150 million years. The species was already ancient when our ancestors began their first, tentative attempts at walking upright; it is many times older than Fire Island itself.
The horseshoe crab is not really a crab; instead it is an arachnid, a distant relative of spiders. A close look at an upturned specimen reveals several pairs of legs and a long, pointed tail. The legs differ in appearance, and serve multiple functions. Since the horseshoe crab has only a primitive mouth, the first pair of legs is small and spiny and serves to grind food into small pieces; by this method, the crab eats small mollusks, crustaceans, and worms. The remaining pairs are the walking legs, but on a male the first of these is larger than the others, and is used to grasp the shell of the female during mating. The last pair of walking legs has been compared to a set of ski poles; these legs are used to push the crab along on soft surfaces. Finally, there is the tail: when waves tumble the horseshoe crab about, and deposit it upside-down on the beach, the animal can use the tail to right itself. It must do so quickly, or else it will be set upon by hungry gulls.
Though the horseshoe crab may seem sluggish and unresponsive- its brain is small and primitive- it is equipped with not one but two pairs of eyes. Both of these are found on the larger portion of the shell: a large pair situated high on the back enables the crab to see to the side, while a smaller pair is found toward the forward edge of the shell. The nerves that connect the eyes to the animal's brain are located in a way that makes them easy to observe and study, and in recent years scientists have learned much about human vision by examining the structures of the horseshoe crab's visual apparatus.
The horseshoe crab has been useful to humanity in other ways as well. In earlier times, farmers from Long Island would cross Great South Bay to gather bushels of horseshoe crabs to use as fertilizer. Baymen would collect them too, then chop them up for eel bait. Today, horseshoe crabs have assumed even greater value to mankind: their blue blood is a vital ingredient in the production of diagnostic testing materials. Extracts from horseshoe crab blood enable doctors to diagnose such diseases as meningitis quickly, cheaply, and accurately.
The value of the horseshoe crab to humankind may count for little in the eternal scheme of things; we must remember we are mere upstarts compared to this venerable creature. In the late spring, if you walk to the marsh on a night when tides are high and the moon is full, you can watch with humility as the horseshoe crab performs its mating ritual. Female crabs by the thousands will make their way to the shore; the tides will help them reach a high place where they scoop out a hollow in which to lay thousands of eggs. Meanwhile, the male crabs ride on the backs of the larger females, jockeying for position to release their sperm into the water so the waves will carry to to the eggs. If you are far enough away from the sounds of mankind, you will hear the clicking of the shells, sounding just as it has through all the centuries of centuries.
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