Hundreds of "giant" crabs have invaded the ancient heart of Rome, threatening to interrupt the work of archaeologists excavating the city's subterranean treasures.

Zoologists discovered the colony of freshwater crabs when they examined water quality in a channel running under the Imperial Forum. After their initial surprise at the unlikely discovery of about 550 potamon fluviatile scuttling around the centre of the Italian capital, they also found that the crabs so enjoy their urban environment that they are growing larger than they would in their natural habitats in Sicily and Tuscany.

The size of the average fully grown male is 5cm, but the Roman crabs grow to a "giant" 7cm. Scientists attribute this to good quality water and a lack of natural predators in the water channel that runs under the Palatine Hill and surfaces in the Trajan Market.

Negotiations are now under way with Rome's archaeological authorities excavating the Forum site to find a way to preserve the colony without halting their work.