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Emanuele Biggi's photograph 'Gladiator Crab' shows one of the resident freshwater crabs that live beneath Trajan's Forum in Rome. The river crabs choose not to inhabit the Tiber, but the ancient channels of the Cloaca Maxima Roman sewer system. Since their discovery in 1998, these Roman sewer crabs have perplexed scientists - growing slower, living longer, and reaching one and a half times the size of other crabs of the same species. They also mate at a different time of year to other river crabs. One theory suggests that since moving into the isolated habitat of the Roman sewer 2,000 years ago, the crabs have undergone an accelerated form of evolution, essentially diverging into a new sub-species.