Overwhelmed by the crabs, archaeologists hauled buckets of them out of the earth during excavations. Known scientifically as Potamon fluviatile, these crustaceans are the only large freshwater crabs indigenous to Italy. Researchers suspect that the population’s heritage is ancient, dating back to when Rome wasn’t much more than a marshy valley.  

The crustaceans have not been extensively studied and have been largely forgotten about over the last 15 years. These crabs may have seen empires rise and fall, but new monitoring data suggest that this once plentiful population could be at risk of disappearing.  

How ancient are these crabs?

Unsurprisingly, the crabs’ habitat has changed significantly over the last 2,000 years. The area on which the Colosseum and Roman Forum now stand was once a marshy valley fed by the Tiber River’s waters, explains naturalist Gianluca Damiani. The crabs’ ancestors likely used the river as an aquatic highway to move around the area. During the construction of the ancient Roman Forum between the first century B.C. and the second century A.D., the Romans built the Cloaca Maxima, a drainpipe system still in use, to drain the area and funnel its waters into the Tiber.

The timing of these construction efforts is one reason why researchers believe the crabs have inhabited Rome’s sewers for centuries. Urbanization cut the crabs off from the Tiber, isolating them from other members of their species and trapping them in the middle of the developing city. “They have been stuck in the center of Rome since antiquity. What we see today is a relic of an ancient population that once lived there,” explains Damiani.  

Over the millennia, the crabs have continued to use this ancient sewage system to move beneath the city, particularly beneath Trajan’s Forum, in canals and tunnels that are largely inaccessible to humans. The critters rarely emerge on the surface and only at night to feed on human refuse and animal remains. (Learn more: Who was Trajan?)

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“Rome’s underground, especially in the area of the ancient Roman Forum, is very rich in water, with many passageways and hiding places to survive,” says Marco Seminara, an environmental biologist at the Sapienza University of Rome. “It doesn’t surprise me that these animals have lived there for so long.” The collapse of the Roman Empire and the disuse of the Forum gave these crabs uninterrupted reign over the territory, which could explain the population’s persistence, he adds.

What do we know about these crabs?  

The local scientific community has been vaguely aware of the crabs’ existence for nearly a century, explains Seminara, who remembers hearing about the population in passing as a student. The excavations in 2005 revealed the scale of their numbers and briefly generated some interest in studying the population.

Between 2004 and 2006, researchers at the local Roma Tre University caught and identified nearly 500 individual crabs. The team reported in 2008 that Rome’s crabs were between 13 and 20 percent larger than other members of the same species. The average Roman crab reached just over 2.7 inches (seven centimeters) in carapace length—a possible example of gigantism, where a population increases in size compared to its relatives.

The study also noted that the Roman crabs grew more slowly and were expected to live up to five-and-a-half years longer than other members of their species. These differences in size, growth and life expectancy would have taken centuries to develop.

The same 2005 excavations that sparked that study also unearthed a network of underground canals that had protected the crabs for centuries, exposing the population to   predators such as seagulls and crows. Seminara increasingly finds more crab body parts than live crabs. And, as the surface becomes hotter and less humid due to climate change, the crabs have tunneled further down to remain cool and undisturbed. “Now they’ve gone even deeper. They’re impossible to find,” says Damiani.

In 2020, the Archaeological Park of the Colosseum recruited naturalists to help monitor the diverse fauna inhabiting the area between the Colosseum and the Roman Forum. Damiani and Seminara were part of a group that tracked the crab population at night with flashlights, deployed fiber optic cameras to peer into shallow dens in the mud and set traps to catch and record individuals for current population estimates. The results have been underwhelming—in the last three years, only six new crabs have been identified, says Damiani.

While this doesn’t necessarily indicate that the population is on the brink of extinction, it suggests that their numbers are rapidly declining. Seminara believes that the crabs may still have a chance at long-term survival if they can adapt to living an even more sheltered life deeper underground. But, he adds that “seeing how Rome has developed, how the Tiber is no longer a water highway for them, and because of climate change, the population certainly does not have a rosy future.”

Will the crabs survive?

So far, the city has denied petitions from Damiani and his colleagues to implement grates and signs informing passersby of the crabs’ existence. The reasoning, he says, is that doing so would mar the view of the ancient monuments: “They don’t care about the crabs, they care more about tourists going to see the Colosseum.”

While the future of Rome’s ancient population of river crabs is nebulous, Seminara hopes that one day there will be enough funds and interest to further investigate the population. For now, crab fans must wait and see if the reclusive creatures can continue to eke out an existence deep beneath the city’s ruins.

“Their existence is certainly folklorish. It would be like finding a crab living in Central Park,” he says. “This is a unique animal. The fact that in the Roman Forum there is the only large freshwater crustacean that we have in Italian waters, has and will always generate a certain amazement.”