Crabs are alien. Their eyes stand up on stalks, waving without apology. Their ten legs each grasp the world, but mostly they keep to their realms of tidelands and sea bottoms. The river crab, Potamon fluviatile, sneaks along streams throughout much of Italy, Greece and nearby Malta, Albania, Croatia and Macedonia. It is, as crabs go, both interesting and ordinary. It is one of the crabs Aristotle would have seen, Homer too. It is a crab millions of children have grown up running from and toward—screaming regardless of the direction. It is not a crab about which one hopes to make new discoveries; then someone did.

People have been keeping an eye on the river crab because like many species of crustaceans and nearly everything else in rivers and streams, its populations have been declining. We do terrible things to rivers and the species living in them. You have seen some of those things, and unfortunately, we do them nearly everywhere. Drop something into a river and it seems to disappear, and so we have dropped, dumped, poured and otherwise contaminated nearly every river in the world. In the Mediterranean we have been doing these things for thousands of years, especially in cities. As a consequence, crabs have gone locally extinct from most of the urban areas where they once lived.

Personally, I like crabs. You could not invent them if you did not know they existed. They are just a little too odd. Insects make do with six legs but crabs seem to need two extra sets. And then there are their eyes, which might as well be legs the way they can look up and around corners. They gather what falls toward them. Those humans interested in rebirth seem to like the idea of being reborn as tigers or eagles but by my accounting, a great many bodies are reborn as crabs.

They have their weaknesses though, their tragic failings. Males of some crabs fight with claws so oversized they seem dysfunctional for other tasks. Then there is the way they move; it is all their own. Some can swim, sure. Others can hop a bit, but the movement that belongs to the crabs is the one that is named for them. It does not mean anything to “ant” or to “beetle” though you can certainly crab along or move crabwise.

But the real joy of crabs is walking along a shore and chasing them with your kids. The real joy of crabs is having them run over your feet. The real joy of crabs is that they exist and might just pop out of the hole you happen to have found on the shore or, in the case of the river crab, along some lovely bank. For all these reasons, when crabs disappear from rivers or shores, I think something more than a species is lost; we lose a whole kind of being. Once crabs are gone, children just chase each other. Once the crabs are gone, crabbing stops being a way of walking and just becomes a complaint. Without real crabs, the only things left crabbing are our neighbors. The crabs don’t crab and somehow we are all lesser for it.

Then there is Rome. It came as a surprise to the world of crab-loving people (though perhaps not crab-loving Roman children) when in 1998 Massimiliano Scalici and colleagues at the University of Rome Tre saw the river crab beneath their city, the biggest city in Italy and one of the biggest cities in the world. One imagines their first temptation was to catch them, which is ultimately what they did, some of them anyway. Where no crabs were thought to exist, Scalici and colleagues found hundreds, looking up at them out of their long, stalk-eyes. The crabs, for their part, appear to have been there quite a while, unnoticed, perhaps glad for it.

The crabs were not everywhere under the city. They appeared (and appear) to live in only one place, in the center of Rome near the Roman Forum. A small river once flowed through Rome to the larger Tiber River, but in 20 BC the Romans altered this river, turning it into an underground canal of sorts, which continues to run through clay pipes beneath the city. This river, now called the “Cloaca Maxima,” is mostly but not totally underground. It is exposed for roughly three hundred feet, through the course of which it is just a few feet wide and inches to feet deep, depending on the rain.

If you look closely as it passes through that space, you can get a glimpse of an underworld—the realm of the crabs. It is perhaps merely a quirk of history that the crabs like to live in the sewers of ancient Romans more than they like to live in the rivers of modern Rome. Either that or the Romans were so much more clever and organized than we are that their sewers are cleaner than our rivers.

The crabs in the Cloaca Maxima appear isolated from other similar crabs, the nearest of which live twenty or thirty miles away. The questions become how and why: what on Earth is going on with these crabs? Either they were already there when the Romans (or the Etruscans before them) arrived, or they made their way afterward. However it happened, they arrived. I like the idea that these crabs ate alongside the Etruscans, but mostly because I am fascinated by the Etruscans who, before the Romans, made great art and engineered interesting cities but were a little less pretentious about it.

On its own, the story of the crabs in Rome is an interesting tale of the persistence of nature despite us. These crabs certainly make one wonder what else can be found up Rome’s cloaca. But there is more to the story.

When Scalici and colleagues described the biology of the Roman crabs -- having studied more than four hundred of them and compared them to populations from across Italy, they found that they were, well, “different.” They were slower growing but lived longer, and so ended up being roughly one and half times as big as other crabs of the same species. They were urban, modern, and fat. They also appeared to mate at a different time of year than do the other river crabs.

How could such differences have arisen, you might wonder. One possibility is that the conditions in the Cloaca Maxima are different than in the other rivers and so the growth and life expectancy of the crabs differs too. Biologists call this phenomenon plasticity, which makes it sound fancy. It is not. You see plasticity every day in your fellow humans; It is the reality that out of similar genes, given different environments and food, one can create very different forms. That your own weight waxes and wanes is a personalized demonstration of the phenomenon, however unwelcome it might be.

So maybe big, slow-growing, crabs just eat more or different foods. But the timing of the mating of the crab is more curious. When species mate tends to be relatively strongly genetically “hard wired.” Nothing spells evolutionary doom more quickly than trying to mate when no one is around to reciprocate, and so individuals who mate at unusual times tend to die alone and unfulfilled. Well, I don’t know that they unfulfilled, but they are less likely to pass on their genes.

One potential explanation for the timing of the crab’s reproduction is that since becoming isolated, the Roman crabs have begun to evolve, via natural selection, attributes (such as the timing of their reproduction and size) that make them better suited for the environment in which they now live. The authors of the study on these crabs thought this possibility to be unlikely. But let’s consider it in a little more detail. If the crabs have been in the Cloaca Maxima since it was built, they have lived there a little over two thousand years. For humans, two thousand years is one hundred and fifty generations or so, and in this time well-documented changes in human evolution have occurred -- not in our species overall, but in specific isolated populations. The crabs produce more crabs more quickly than humans produce more humans. Their age of first reproduction appears to be about one year, which is to say since the Cloaca was constructed, they have had two thousand generations to evolve, two thousand generations during which those individuals with genes better suited for Roman living had a chance to differentially pass on their genes. In fact, if these Roman crabs really are isolated from other populations of the same species, it would be surprising if they had not evolved adaptations to their unusual environment.

Fortunately, there is a good test of whether the differences between these populations are evolutionary (actually, there are several reasonable tests now). The simplest would be to raise crabs from other rivers and the Cloaca Maxima together in the lab, under similar conditions and over multiple generations to see if, once the difference conditions are removed, they are similar (scientists like to call such an endeavor a “reciprocal transplant.”). Or one could search to see whether there are genes that have been favored recently in the Roman but not the other crabs, which would be evidence of recent natural selection. Or there are quite a few other options. Unfortunately, nothing more has been published on these crabs since 2008. The mystery of their mating awaits resolution. Fortunately, a clever graduate student, or even a not so clever graduate student, could resolve this question. Unfortunately, it now appears that the crabs are off limits; the Cloaca Maxima is part of active archaeological work and cannot be accessed, at least for now.

Meanwhile, it is hard to avoid wondering about the other life in Rome. Rome showcases the wild life of Berlusconi, but I am thinking about even more feral creatures. Wild packs of dogs roam the streets. Wild cats reach extraordinary densities. Then there are thousands of smaller species, many of which reproduce even more quickly than the crabs. How many of these species have evolved attributes that make them very different from their closest relatives and better able to live, not just in cities in general, but in Rome in particular? My bet is many. And so if you go to Rome and visit the coliseum or anywhere else, I would look carefully at what is flying and walking around you. They may be creatures that exist nowhere else.

I am left at this point in the story unsure of how it will resolve. Are the crabs a newly evolved species? I bet so, or, to use the language of my people, that is my hypothesis. But the truth is no one seems to know enough yet to say. I hope they are (hoping is fundamentally unscientific, but inherently human). I also hope that crustaceans, be they these crabs or their kin, come back to many cities. They might need our help, whether that means cleaning up our rivers or making them more suitable to ten-legged life in other ways. It seems possible that we could help to stage their comeback. The ancient Romans appear to have done it without trying (damn them), so surely we could do it if we really put our hearts into it. What a joy it would be if it did happen. As I am walking through seaside and riverside downtowns, I would love to have the possibility of a crab popping up out of any untended hole. It might give new meaning to the term “to crab.” To crab could mean to rise again where you are least expected and then, of course, scurry from side to side while being chased by children and biologists.