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The Life and Times of a Ten-legged Cannibal
by Christopher Mims

"In writing this book about Crayfishes it has not been my intention to compose a zoological monograph on that group of animals.... I have desired, in fact, to show how the careful study of one of the commonest and most insignificant of animals leads us, step by step, from every-day knowledge to the widest generalizations and the most difficult problems of zoology; and, indeed, of biological science in general."
—Sir Thomas Huxley, 1879

The first time I went to a scientific meeting outside my field of invertebrate neuroscience, I discovered that a sure way to get colleagues to laugh is to tell them I study crayfish. The same trick works at parties. It's a nervous laugh, ha-ha weird and not ha-ha funny; the kind of verbal emission that follows non sequiturs.

Crayfish
There's more to crayfish than meets the eye. (Jessie Cohen/NZP)

Few scientists would bat an eye if I told them I work with vertebrates such as mice or monkeys, or even if I said I was creating a pig-bat chimera intended to fly deep into enemy territory and air-drop itself amid hungry refugees. Nor would they be surprised if I said that I work on Drosophila, the common fruit fly, or Caenorhabditis elegans, a tiny soil-dwelling worm, both of which are the bread and butter of developmental and molecular biologists the world over. But crayfish? They're neither here nor there. The strongest association most of us can muster upon hearing their name is a vague recollection of bayou cuisine.

Yet beginning with Aristotle, crayfish have afforded biologists some of their most basic insights into the workings of all higher organisms. In the late 19th and early 20th centuries the list of crayfish admirers reads like a who's who of physiology, neuroscience, and behavior. It includes Thomas Huxley ("Darwin's Bulldog"), Sigmund Freud, and Robert M. Yerkes (America's first and foremost comparative psychologist). In the modern era, the intellectual heirs of Conrad Wiersma, the godfather of crustacean neurophysiology, use crayfish to study everything from mechanisms of neural transmission to the formation of dominance hierarchies.

Take, for example, the neural circuit underlying the crayfish's escape tail-flip: a quick jerk of the tail that propels the animal backward and away from a predator. It is one of the best understood circuits in all the Kingdom of Animalia and so simple, elegant, and accessible for inquiry that we comprehend it at a higher level than almost any collection of neurons in any other organism. Because you and I have much in common with crayfish, an understanding of the crayfish escape circuit has proved useful in understanding the mechanisms of our own nervous system. Or, to put it another way, the neural circuit underlying the crayfish's escape tail-flip is a microcosm for understanding the workings of the human mind.

Plus, like most bugs, crayfish are creepy, weird, and possessed of that macabre chic that appeals to the kind of kids who weren't partying in high school, many of whom become scientists.

Swamp Thing
Crayfish, colloquially known as crawdads or crawfish, are decapod (meaning ten-limbed) crustaceans. They resemble lobsters in their outward appearance, but unlike lobsters, they adapted to fresh water and make occasional forays onto land. All crayfish have in common their lobster-like morphology, a dependency on fresh water, and not much else. They inhabit nearly every type of wet place imaginable including streams, lakes, caves, or areas with high humidity. They conquered these various habitats with a variety of adaptations, and perhaps the most important is their diversity of burrowing styles—each crayfish species has its own.

Crayfish are highly aggressive and use their powerful claws for both hunting and defense. They are predatory to the point of cannibalism. They're also filter feeders, and their diet includes no small amount of detritus: the muck, feces, and decaying garbage that sifts to the bottom of a body of water. Crayfish's teeth, all three of them, reside not in the mouth but within the stomach, where they rip and grind food as efficiently as any set of molars.

When two crayfish of about equal size meet, they rear up on their tails, spread their claws wide, whip each other with their long antennae, and engage in what is literally a pissing contest. Openings in their faces called nephropores eject streams of urine to communicate each individual's status, health, and identity.

Then the fighting commences. Gripping each other with crushing pincers, both crustaceans churn the water with their tails in a show of strength. Limbs may be lost in the heat of combat; like geckos dropping their tails, crayfish autotomize a damaged or entrapped limb by closing a sphincter of muscle at its base, and then regenerate it.

Procambarus clarkii, the species most likely to be found on your dinner table, prefaces mating with a protracted battle between male and female that ends when the male successfully pins the female, or she is killed.

The Thinking Man's Crustacean
But the salacious details of crayfish's life history are not what attracted the likes of Aristotle. More likely it is their ubiquity. In one of the earliest texts to mention the creature, On the Gait of Animals, Aristotle comments on the style of locomotion peculiar to ten-limbed animals, using as an example the crayfish he observed in the streams and lakes surrounding his home in Greece.

Crayfish are endemic to every continent save Africa and Antarctica. Most live in temperate, non-equatorial areas, and the largest number of species live in North America. As such, they have nearly the same geographic distribution as that of recent history's most prolific biologists. They are, as Huxley wrote, readily available and of a convenient size for scientific study.

Huxley recognized that living systems are incredibly complex, and that what is true for one may not be true for another. In the face of so much irregularity, he developed a special fondness for identifying phenomena that generalize across the animal world. This is especially apparent in his adoption of a then-heretical notion: Darwin's theory of evolution.

Huxley was a close personal friend of Darwin, who once described him as "the king of men." It was Huxley who publicly insulted Samuel Wilberforce, Bishop of Oxford, in a debate on evolution of such importance that its details were recorded in the London Times; Huxley who urged Darwin to publish The Origin of Species before Alfred Russel Wallace could issue his own work on the subject and rob Darwin of his role as sole progenitor of the notion of natural selection; and it was Huxley who coined the term Darwinism.
He believed that a thorough understanding of a single creature could provide far-reaching insight into the nature of all life on Earth. In this spirit, Huxley published The Crayfish in 1879, a work so thorough that it remains a useful reference today.

That same year, a 23-year-old Sigmund Freud embarked on a study of crayfish nervous tissue that marked a turning point in our understanding of all cellular life. In his paper titled "On the Structure of the Nerve Fibers and Nerve Cells in the River Crab," Freud wrote that he chose crayfish as his subject for the same reasons many others have: They are easy to work with. Crayfish are highly available, tough, and possessed of a giant nerve fiber running from head to tail that can survive for hours even after it is removed from the body.

Freud managed to see in crayfish what many, even Huxley, had missed. He recognized that the goop inside nerve fibers is not a uniform slurry, but contains many nearly invisible fibers. In time these fibers were called the cytoskeleton, which is the internal architecture of all eukaryotic cells, present in all living things except bacteria and cyanobacteria. A vast body of research has since shown their importance to nearly every cellular process, from locomotion to mitosis.

The giant nerve fiber upon which Freud conducted his experiments is part of the circuit underlying the aforementioned crayfish escape tail-flip. Four of these giant fibers run down the center of the crayfish nerve cord, and are so large they can be seen with the naked eye.

When a crayfish is startled by the bow wave that arrives milliseconds before the swift attack of a predator, or by a shadow passing overhead, the inmost pair of these giant nerve fibers discharges its accumulated energy and causes the crayfish's powerful tail muscles to contract and send the animal rocketing backward. Coincidentally, the same muscles devoted to escaping predators are also the ones that human gastronomes find delectable.

The sensations that inspire a crayfish escape are so different from our own that they're hard for us to understand. For instance, it is difficult to say whether crayfish have ears, mostly because the notion of an ear is an entirely anthropocentric one. Unlike human ears, which have fine sensory hairs inside structures called cochlea, crayfish have fine sensory hairs covering their entire bodies. Anyone illuminating a crayfish from behind can see these dense and almost perfectly transparent hairs wafting from every inch of the hard exoskeleton. Short hairs detect vibrations in the water that are similar to sound waves, while longer hairs record the changes in slower-moving currents of water such as the pressure wave that radiates from the leading edge of a predator's attack.

Crayfish also have a keen sense of smell, which they use for many things, such as detecting whether they have previously fought an enemy, or letting their young know where to find mommy. Rounding out their arsenal of sensory organs are highly developed compound eyes with both night and day modes that allow them to take advantage of whatever light is available. When they escape from their tanks (which they do a lot, and are notorious for), I sometimes find them walking purposefully down the hallway, clearly navigating by sight. As I approach, they rear up and spread their claws menacingly, as often as not falling backward because they are overburdened by the weight of their own giant pincers. Such actions make us wonder how these animals survive in the wild.

Yet survive they do, in their world of touch and smell and dim evening light, and have for nearly 300 million years. Today they use the same, nearly unchanged methods to manipulate their environments that they did millions of years ago. Fossilized crayfish burrows preserved in exquisite detail in rocks predating the dinosaurs bear evidence of the same claw marks and scoring with tails and bodies present in modern-day burrows. While we can only imagine how antediluvian crayfish used burrows, we know that modern crayfish use theirs principally to escape predators, be they fish or siblings. They also use burrows for egg brooding, overwintering, reaching the water table to stay hydrated, and regulating body temperature—all key to their survival.

A Resource in Peril
Although crayfish survived the largest extinction in Earth's history¾at the end of the Permian, 250 million years ago¾they are not guaranteed to survive the current one. Crayfish are one of the three most endangered groups of organisms in North America, along with mussels and stoneflies. Of the 313 species found between Panama and Alaska, 65 percent are rare, threatened, or already extinct, mostly due to habitat destruction.

In Europe crayfish have the dubious distinction of being one of the few organisms to displace and endanger themselves. Invasive North American crayfish have managed, like little crustacean Cortezes defeating the Aztecs with smallpox, to wipe out most of Europe's noble crayfish (Astacus astacus) by transmitting a deadly fungal plague.

Meanwhile, the steady flow of insights derived from the study of crayfish continues. In a twist bordering on the bizarre, scientists at the University of Missouri at St. Louis discovered that neurons attached to hairs on crayfish's tails employ an unexpected solution to a common problem in information theory.

It's a matter of signal and noise. Signal is the interesting part of a transmission—were we listening to the radio, it might be the voice of our favorite singer. The noise would be static threatening to drown out the signal. Simply turning up the volume might help the situation a little, but because both the signal and the noise would be increased by the same amount, it’s a less than optimal solution.

Crayfish face this problem every day: How can they discern an important signal such as a predator's attack, for example, from the everyday din of noise in the water? How can they make the incoming signal stronger than an interfering noise, when simply turning up the gain (as you would if you were to turn up the volume on a radio plagued with static) doesn’t work because the signal is too weak?

To solve this problem, crayfish have neurons on their tail hairs called mechanoreceptors that exploit a phenomenon known as "stochastic resonance." First described by physicists and mathematicians more than 20 years ago, stochastic resonance means that in some situations, doing the opposite of what we would expect—that is, adding noise to a system—may make an otherwise weak signal perceivable.

While crayfish mechanoreceptors are the first biological system in which scientists uncovered this phenomenon, it turns out that stochastic resonance is used by the sensory systems of all kinds of other creatures, and has even been applied to physical therapy in humans. Once again, the relatively simple, accessible, and well-characterized crayfish has allowed us a level of insight that would be difficult or impossible to discern in a more complicated animal model.

It's research like this that comes to mind when my dentist asks me what I do for a living and my answer, "crayfish neuroscientist," elicits a patronizing chuckle. Like Aristotle, Freud, and Huxley, I have discovered the pleasures of finding out how much I have in common with something as apparently unrelated to myself as an ill-tempered, ten-legged cannibal.

—Christopher Mims studies crayfish nervous systems in the lab of Donald Edwards at Georgia State University. He wrote about monogamous voles in the May/June 2004 ZooGoer.


ZooGoer 33(5) 2004. Copyright 2004 Friends of the National Zoo.
All rights reserved.

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