Limulus Polyphemus

Spring, the new moon: The littoral of the city is filling with mating Atlantic horseshoe crabs. The males of Limulus polyphemus, sometimes several at a time, are clamoring aboard the larger females, who come ashore at high tide to lay their eggs at the wrack line. A female may lay 90,000 eggs in a season. Many of these eggs are eaten by a least two dozen species of hungry shorebirds; the red knot in particular depends on them on its long migration from the South Atlantic to arctic Canada.
The three main parts of the body are the telson, or tail; the opisthosoma, or abdomen; and the prosoma, or cephalothorax. Horseshoes have numerous optical sensors, including compound eyes, median eyes, ventral eyes, and photoreceptors on the telson to tell the time of day. Their species name comes from the Cyclops blinded by Odysseus, because of the central eye on the forehead of the prosoma. Additionally, an astonishing 7 million neural fibers allow the animal to feel and sense its world.

This is a shed shell of a young horseshoe. You can tell because of the tan color and the fact that it’s hollow inside; every little jointed part is empty now. To grow they must molt: males may molt 16 times over nine years, females 17 times over 11 years, before reaching sexual maturity.

Now here’s the stunner: You may owe your life, or the life of a loved one, to the horseshoe crabs and not even know it. Even if this extreme isn’t part of your personal experience, you still have horseshoes to thank for the safety of intravenous drugs, vaccines, and medical devices, because for 40 years a substance extracted and refined from the blue blood of horseshoes, called Limulus amebocyte lysate (LAL), has been used to test medical equipment for bacterial contamination. You see, sterilization isn’t good enough when it comes to bacteria; the bacteria may die, but components of it, called endotoxins, can survive, and that’s grim news for you and I. But horseshoes live in the littoral, which turns out to be awash in microbial trouble. They don’t have immune systems as we would think of them, but they need a defense against potentially harmful bacteria, viruses, and fungi hanging out in the goo. Their blood clots around these potentially deadly forms, deactivating them and helping to seal wounds. An elegant solution in the sea.

So scientists at the Woods Hole, MA, Marine Biological Lab figured out that LAL would make a great test for endotoxin. It replaced the rabbit test. To this day, horseshoes are bled up and down the coast for this purpose. Most survive the procedure (studies show a range of mortality: the industry says 3%; non industry sources say 10-15%), but if ever a synthetic were necessary, it would be here.

And for this noble work, shamefully unnoticed by most, how have we treated the horseshoe crab? Why, with typical human consideration, of course: there’s the usual befouling and destruction of habitat, which escalates up a complex chain of animals species who depend on horseshoes for food and shelter (the shell can be festooned with dozens of species). And there’s the wholesale slaughter of millions of horseshoes for bait by the conch (whelk) and eel fisheries. Yes, they are chopped into chum. How fucking obscene. At least they aren’t ground into fertilizer anymore.

In our childlike, vain, and preening view of the world, horseshoe crabs –- more related to spiders and mites than crabs — are considered “primitive” life forms, low down on the “tree of life.” The notion of a tree of life seems to me to have been one of the compensations of the shocking post-Darwinian world, when many humans still had to feel that we were top animal, that evolution led higher and higher to us, up there in the high branches, waving our thumby paws with smug satisfaction. What vanity, to dethrone a god and put ourselves in its place!

One day we too will be extinct, like the great majority of all life forms that have existed on this planet. And unlike all the others, we will have been responsible for the destruction of great numbers of our own and many other whole species. It behooves us to consider the horseshoe crab, polyphemus scuttling across the seas for 20 million years, its relatives for 200 million, perfectly suited to its life, amazing and beautiful, and learn a little humility. Should we not use our fatty brains, our vaunted consciousness, and our handy tools in service to the planet instead of towards its dismemberment? Why is this even a question?

(Leap into the future for a another post about horseshoes in action.)

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