An updated look at an ancient wild animal that helps us test the safety of countless vaccines (including the COVID19 vaxxes) and other drugs but now, thanks to our unsustainable overexploitation, it is endangered
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The Atlantic horseshoe crab, Limulus polyphemus, is an ancient creature that has inhabited Earth mostly unchanged for longer than 450 million years. The species vastly predates humans and even dinosaurs. It lives along the east coast of North America, and hundreds of thousands leave the sea to spawn in the sandy beaches from New England down through the Mid-Atlantic, with the largest numbers of them clambering ashore on Delaware Bay.
“You can think of a horseshoe crab as living halfway between the marine world of a trilobite and the terrestrial world of a pill bug”, writes the author, marine biologist and science writer William Sargent. (p. 21.) “Horseshoe crabs come onto land only once a year to lay their eggs. If disturbed while spawning, they curl up much like pill bugs to protect themselves. As long as they keep their gills moist, they can survive for several days in this defensive curled-up posture. The blood of both animals is based on copper, not iron. Some scientists think their common ancestor evolved this blue copper-based blood when copper was more available in oceans than iron.”
It is their blue blood that makes Limulus vitally important: their blood contains critical immune cells that are exquisitely sensitive to toxic gram-negative bacteria. These bacteria are encased in a thick sugar-like outer coat composed of endotoxins, which are poisons that cause burning fevers associated with lethal diseases like typhoid, spinal meningitis, gonorrhea and toxic shock syndrome. When gram-negative bacteria invade a horseshoe crab, their primitive immune cells, known as amoebocytes, form a clot around the bacteria, thereby inactivating them and protecting the horseshoe crab from deadly toxins.
Several clever and observant scientists noticed this peculiar trait and devised a way to adapt horseshoe crab amoebocytes to test vaccines and other drugs for bacterial contamination. This test, the Limulus Amoebocyte Lysate (LAL) endotoxin test, was a major breakthrough that has been used around the world since the 1970s to protect people from a variety of dangerous diseases. More recently, it also was used to detect bacterial contamination on laboratory equipment, on the Mars Rover, and even in humans. Of course, horseshoe crabs probably have a vastly different view of the situation: they are captured and bled for their astoundingly valuable and medically useful cobalt blue blood.
In this fascinating book, Crab Wars: A Tale Of Horseshoe Crabs, Ecology, And Human Health (Brandeis University Press; 2021: Amazon US / Amazon UK), we learn the story of Limulus and their key place in East Coast ecology from one of their most eloquent life-long fans. Dr Sargent alternates between his own recollections, many from his childhood, and noteworthy scientific discoveries about Limulus, and the multimillion dollar industry that grew up around it. The book details the discovery and development of the LAL endotoxin test, the careless over-exploitation of these iconic animals as fish bait by fishermen, as a blood donor by pharmaceutical corporations — and even by some scientists — and the many environmental, political and legal entanglements this has inspired over these animals’ ultimate fate.
A proven lifesaver, the LAL endotoxin test is the basis of a multimillion dollar pharmaceutical industry. For example, every vaccine and drug certified by the FDA must be tested using LAL, and LAL is solely sourced from just a single species of wild animal. So it is deeply troubling that Limulus populations are declining up and down the East Coast. As the author notes, this sounds like the recipe for disaster:
“…if we were to lose all the lobsters, striped bass, or shrimp on the East Coast, it would be an environmental tragedy. But if the East Coast were to lose all its horseshoe crabs, it would be a major medical disaster. While we protect our valuable food species with stringent regulations, to date we have only started to protect our far more valuable populations of horseshoe crabs. Thus, it is time that all the East Coast states follow the lead of South Carolina, and only allow horseshoe crabs to be used for biomedical purposes.” (P. 129, Crab Wars, second edition, by William Sargent, 2021.)
In addition to global use by the medical industry, Limulus are an irreplaceable foundation upon which the East Coast ecosystem is built. Hundreds of thousands of shorebirds are dependent upon gorging on Limulus eggs to fatten up before embarking on the last segment of their northward migration. Birds that do not eat enough Limulus eggs before arriving on their breeding grounds are too thin and weak to lay their own eggs. Many die.
Furthermore, even as Limulus attract shorebirds, the shorebirds attract thousands of birders and ecotourists from around the world to watch and photograph the spectacle. And where ever people go in vast hoards, their money also travels with them, infusing local economies with much-needed cash.
As a result, the conservation of Limulus is an extremely relevant issue for many essential reasons, and these competing interests are the impetus for ongoing efforts for researchers to develop recombinant lysates to supplement or replace LAL as diagnostic test products in the future.
This beautifully written book is nonfiction but it reads like a particularly gripping novel that I simply could not put down. It’s filled with natural history, scientific discovery, pharmaceutical developments, and plenty of politics, and yet, Dr Sargent tells the horseshoe crabs’ tale with enormous compassion and balance. But we cannot forget that Limulus populations are still dwindling, and this has profound implications not only for the future of the animals themselves but also for the ecosystems that depend on them.
Recently updated from the original book that was published in 2002, this second edition includes five additional chapters and a new epilogue that features surprising information regarding the COVID-19 vaccines. Although published by a university press, this book is refreshingly accessible for the non-specialist.
In the end, the fascinating story of the horseshoe crab is a thought-provoking and sobering reflection on the unintended consequences of scientific progress and the dangers posed when self-regulating industries control a limited natural resource.
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