Potamon fluviatile crabs' unusual size, up to , and longevity (up to 15 years) are also interpreted as evidence of a long-established population, by analogy with island gigantism. On the island of Malta, "Potamon fluviatile" is rare and restricted to a few locations in the west of the island. On Gozo, there is a single population which inhabits a stretch of river only long. In the Balkan Peninsula, "Potamon fluviatile" is known to occur in Croatia, Montenegro, the Republic of Macedonia, Albania and Greece. There are four species of "Potamon" in the Balkans, and "P. fluviatile" is replaced by "Potamon ibericum" in
Potamon fluviatile fluviatile" may reach a carapace length of , with females being generally smaller than males. As with other crabs, the body is roughly square, with the reduced abdomen tucked beneath the thorax. The thorax bears five pairs of legs, the first of which is armed with large claws. The life span of "P. fluviatile" is typically 10–12 years. Moulting does not occur in winter. Mating lasts between 30 min and 21 hours, with spawning usually taking place in August. Females carry the eggs on their pleopods (appendages on the abdomen) until they hatch directly into juvenile crabs, having passed through