The crabs of Rome.

According to the National Geographic, an ancient species of freshwater crab has been found living in canals dug by the Etruscans:

The Roman crabs—of the species Potamon fluviatile—were discovered in in 1997. Recent findings from an ongoing genetic study suggest the animals may have been around for more than a thousand years before the ancient complex was completed, around A.D. 112.

Researchers came to that conclusion after the discovery that the crabs’ genes are remarkably similar to those of Greek crabs.

"So it’s very likely that they were introduced by the Greeks 2,500 or 3,000 years ago, which means they were here even before Rome was founded in 753 B.C.," zoologist Massimiliano Scalici, of the University of Rome III, told the AFP news service. . . .

The crustaceans—which inhabit canals built by the Etruscans, a civilization that came before the Romans—are believed to be the only known freshwater crabs thriving in a major city.