Diary: Laura Marris, on Limulus polyphemus, the Atlantic horseshoe crab

Photograph by the author from a community science horseshoe crab survey

Limulus polyphemus is the official name for the Atlantic horseshoe crab. These creatures and their ancestors have been on Earth longer than dinosaurs. They’ve lived through all five mass extinctions, surviving on this planet for some 445 to 475 million years. Though everyone calls them crabs, they’re actually a different kind of arthropod—more closely related to scorpions and spiders. As a child, I became fascinated with limulus and the way they carry other species, becoming itinerant microcosms of aquatic invertebrate life. Over time, mature horseshoe crabs accumulate small cities of marine creatures on their shells, including colonies of bacteria, snails, worms, slipper shells, barnacles, sponges, fringed ghost anemones, and the quick, pink tentacles of sea strawberries. Naturalist Dave Grant writes that some members of this “moving menagerie” rely on the horseshoe crab to hitch a ride out of the silt. Other species simply settle on the shells of these crabs as they move across the bottom of the sea, making the horseshoe crab a “living fossil” with the contents of a tide pool on its back. They’re comfortable in the depths of Atlantic bays, but they can survive temporarily in air. When they come up on land to mate, they have book gills whose folded, page-like membranes can keep them alive for as long as the book stays damp.

I wish I could say that limulus translates to “liminal,” an appropriate word for an animal that troubles the threshold between the terrestrial and watery worlds. In fact, the first part of the name means “askance,” or “oblique,” which is less interesting than the second part, Polyphemus, the Cyclops of The Odyssey, who is literally robbed blind in the myth, stabbed in his one eye by Odysseus, who sneaks into his cave by hiding his men under the pelts of a stolen flock. Polyphemus reflects a misperception that limulus has only one eye, though it’s more appropriate when you consider Limulus polyphemus’s bizarre relationship with humans—whose theft of horseshoe crabs has made us perennial wolves in sheep’s clothing.

Despite their Cyclopean name, horseshoe crabs have ten eyes, remarkable ones, that allow them to see as well by moonlight as they do under the noon sun. Their biggest eyes, the lateral ones, have light receptors that are easy to notice, even without a microscope—they look like tiny black pinpricks in the silvery mirror of their eyes. Or like the pebbled shadows of a reflector on the back of a bike. A strong circadian rhythm adjusts these receptors to become a million times more photosensitive at night, and they have been known to preserve this inner clock even when cloistered in a darkened tank.

 As creatures from the tidal zone, theirs is a world of rhythms not easily forgotten or undone—of waves, of seasons, of migrations, of the moon, of the piling up of dunes and their erosion back into the sea. Their oldest ancestors were alive at moments when inland oceans dried up and sea levels rose over islands—when the world warmed and there were vast changes in the salinity and temperature of the tides. As Rachel Carson writes in The Rocky Coast,

To understand the shore, it is not enough to catalogue its life. Understanding comes only when, standing on a beach, we can sense the long rhythms of earth and sea that sculptured its forms and produced the rock and sand of which it is composed; when we can sense with the eye and ear of the mind the surge of life beating always at its shores—blindly, inexorably pressing for a foothold.

 In the summer months, horseshoe crabs arrive with the moon. The light receptors along their bodies can sense when the moonlight changes and shifts, orienting their circadian rhythms to the actual clock of dawn and dusk. At the full and new moons when the tides are highest, these arthropods come up on the beaches to spawn along the coast. Though their movements are associated with the cycles of the moon, horseshoe crabs likely navigate by the slope of the beach and the currents, rather than by moonlight itself. They sometimes gather on tidal flats just off the beach and wait for the tide to rise, preferring the moment when the water is highest.

 Other creatures wait for them. Deborah Cramer has written about the vital connections between bird populations and limulus, especially in Delaware Bay, where the beaches are massive breeding sites for the crabs, whose abundance and predictable timing have nourished many migrations of red knots. They feed on limulus eggs on their way north to the Arctic. The eggs are also food for fish, American eels, and sea turtles, making horseshoe crabs a keystone species, an essential part of other animals’ survival strategies.

Humans use limulus to bolster our own resilience. Over the last 150 years, we’ve relied on them in moments of scarcity and crisis, when our tolerance for changing resources was nearing its limits. The first example of extreme harvesting of limulus began in the mid-nineteenth century, when farmers began to use them in significant numbers as fertilizer for their fields.

 By the first half of the nineteenth century, landowners had depleted the soil of the mid Atlantic, pushing it to exhaustion through plantation-economy practices of over-farming cash crops. Many landlords used enslaved labor to work the fields, or divided their holdings between sharecroppers, requiring their impoverished tenants to grow mostly corn. In places, crop yields became so poor that tenant farmers resorted to stripping the bark off oak trees and selling it to tanners to eke out a living, an unexpected consequence of soil exhaustion that decimated the area’s black oaks. At the time, the agricultural reformer William Huffington wrote: “Under such a system, destruction of the soil is rapid and certain.”

 Something, officials realized, had to be done about the dirt. At first, they tried importing seabird guano all the way from Peru and excavating marl, a clayey rock containing the lime of crushed seashells. But importing guano was impractical, and marl was heavy to transport. So they turned to a local fertilizer, which, like many of the extractive “solutions” applied by the colonists, had its origins in sustainable Indigenous practices.

 In the early 1600s, French explorer and imperialist Samuel de Champlain  observed that the Indigenous people who planted corn along the Maine coast would take dead horseshoe crabs and bury them in the corn mounds to increase the crop. By the mid 1800s, these farmers had begun to do something similar. Initially, people who harvested horseshoe crabs would just walk out into the tidal flats and marshes and pick them up by their shells. And at first the horseshoe crabs were abundant enough to provide nutrients for the soil. But by 1870 photographs depict stacks of dying and dead limulus rising out of the sand. The sepia tint makes the carnage look gentler and yet somehow more strange—great curving masses along the shoreline that slowly register as thousands of individual shells. In one summer of that decade, over four million horseshoe crabs were collected. The process was quickly optimized. People captured crabs in their breeding grounds in structures called “pounds” and stacked them to desiccate, then carted them to a factory, where they were run through giant blenders and turned into powder. This powder was sold as a fertilizer and spread over fields across the Northeast. The product was christened Cancerine.

The overharvesting of limulus for fertilizer tapered off in the early twentieth century, and by 1960 the market had all but disappeared. The man who ultimately made the pulverizing of horseshoe crabs obsolete was Fritz Haber, a German chemist who ultimately became known as the father of chemical warfare. But in addition to formulating poison gas, he also discovered that nitrogen could be drawn from the air. By the mid 1900s, Haber’s innovation was successful on a commercial scale, alleviating soil exhaustion and famine. Haber won the Nobel Prize in Chemistry. But his invention also allowed the system of extractive farming to continue, so that it no longer meant the end of the soil’s fertility. With synthetic fertilizer, nitrogen could be spread across the earth in amounts far greater than the original soil concentrations.

After Cancerine became obsolete, there was a brief period when horseshoe crab populations rebounded. These days, up and down the Atlantic coast, biomedical companies harvest the blue blood of horseshoe crabs for medical testing, because it has the property of alerting us to even tiny amounts of gram-negative bacteria, a dangerous and common contaminant in medical products. In the US, every batch of intravenous drugs—vaccines, chemotherapy medications, dyes for imaging, vitamins, anything that comes out of a drip or is implanted in a human—must be proven safe with horseshoe crab blood before it can be sold.

As we bolster our immunity to covid through the use of vaccines, it’s because horseshoe crabs have been our poison testers. We can talk about saving the horseshoe crab, and scientists like the late Carl Shuster have done important work to preserve their habitat. But it’s equally important to recognize that it’s not just about whether biomedical companies, bait fishermen, and regulators want to offer Atlantic horseshoe crabs some clemency. The illusion of human control masks the question of whether clemency is even ours to offer—when humans have been in dire straits, over and over again, Limulus polyphemus has rescued us. When humans have been in dire straits, over and over again, Limulus polyphemus has rescued us.

People often describe the horseshoe crab as a resilient animal that can bounce back from both prehistoric challenges and human harvesting. I’ve even heard them called “old soldiers.” Why this projection of toughness onto the mysterious life of an arthropod? Besides the usual charge of anthropomorphizing a crab, doesn’t emphasizing the strength of these creatures hide our guilt at needing so much from them?

 If they carry out their full life cycle, horseshoe crabs don’t die alone. By the end of their time, they resemble gardens of seaweed and shells, snails attached to their undersides, barnacles encroaching on their eyes. Eventually their backs are so crowded with other creatures that they become sluggish with the drag. Plants and animals grow over their eyes, like a thicket over a ruin, and they go blind. But when crabs enter a biomedical facility for their blood to be harvested, one of the first things technicians do is scrub everything off their shells.

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Laura Marris is a writer and translator. This post is drawn from her book, The Age of Loneliness, soon to be published by Graywolf Press. She is the recipient of a Robert Silvers Grant for Work in Progress. She teaches creative writing at the University of Buffalo.

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The author and her subject. From The Age of Loneliness: “There is a photo that my father must have taken of me as a child, emerging from ankle-deep muck, the scion of a prehistoric arthropod clasped in my fist by the lip of its shell. I would often land these horseshoe crabs from out of the marsh and then return them, at his orders, lingering until the dazed animals once again began to move.”