Potamon fluviatile Herbst, 1785
Common names: Freshwater crab [En], Crabe des fleuves, Crabe d’eau douce d'Europe [Fr], Granchio di fiume, Grachio d’acqua dolce [It], Κάβουρας του γλυκού νερού [Gr]
IUCN Status: NT (Near Threatened)
Platania, ILIA ● Greece
In Greece, four species of Potamon can be found:
• P. ﬂuviatile (Herbst,1785) is considered to be widely distributed in mainland Greece south and west of the Axios River in the Macedonian Prefecture;
• P. ibericum (Bieberstein,1808) occurs in Greece from the Axios river east to Evros river at the Greek-Turkish border;
• P. potamios (Olivier,1804) is endemic to selected South East islands (e.g. Crete and Karpathos);
• P. rhodium (Parisi,1913) is restricted to the islands of Ikaria, Kos, Samos, and Rhodes (Brandis et al., 2000; Hogarth, 1989).
Description: Adult of P. ﬂuviatile may reach a carapace length of 50 millimetres (2.0 in), 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.
Biology: Crabs are most active between May and October, after which (i.e., from November to February) they are less active and hibernate either in natural refuges or inside burrows. Crabs feed at night either in water or on land, and they are opportunists that consume a wide variety of food items, that is either dead or alive, plant of animal. Food items include vegetable matter, tadpoles, insects, small frogs, and fish.
Young crabs are more aquatic than adults, and females mate in late spring and release hatchlings in summer. Crabs are semi-terrestrial and spend time out of water either at night or in the day when it is raining. Crabs are capable of dispersal by walking on land across relatively short distances to reach nearby streams.
Habitat: This species is found in rivers, streams and lakes throughout its range. In streams and rivers crabs shelter under stones or among vegetation, or they rest in their burrows dug into the banks; burrows are up to 50 cm deep and have water at the bottom.
Distribution: Potamon fluviatile has a highly fragmented geographic distribution over a wide area in a number of countries that have a Mediterranean coastline. This species is found in Italy (including Sicily), the Maltese Islands (also considered as a subspecies, P. fluviatile lanfrancoi), the Balkan Peninsula (Croatia, Montenegro, Macedonia, Albania), and Greece (western mainland, Ionian Islands and western Aegean islands).
The origin of the species was on the Balkans and the colonisation of Italy proceeded from the northern Balkans via southern Italy in the Otranto Strait region. The population expansion associated with this invasion was estimated to have taken place 15,000 years before present (95% c.f. 10,000-24,000 years BP). An anthropogenic introduction is therefore implausible and a natural expansion likely.
Threats and protection: Population levels of Potamon fluviatile have declined dramatically in recent years throughout its range. Despite the wide distribution of this species (E00 800,000 km²), some of its populations (especially those on islands) are discontinuous and highly fragmented, and the threat level is high for some of its isolated subpopulations. Populations of this species may under immediate and long-term threat from rapid anthropogenic changes affecting its habitat such as water diversion, drainage, habitat disturbance, over-harvesting, and pollution. It is considered to be close to qualify as VU (Vulnerable).
Many populations of river crabs are now in urgent need protection, especially those populations that are found on the Mediterranean islands. Potamon fluviatile is associated with streams and rivers and it is important that these habitats be protected from pollution and drying out. The conservation of these freshwater crabs depends heavily on habitat protection and the preservation of freshwater ecosystems.
Perennial rivers, and those rivers that normally flow for only part of the year, should be protected against extended seasonal drought because these are vital habitats for aquatic freshwater animals. Stream and river beds should not be concreted to make channels and water pollution should be prevented. Water pollution should be prevented, and the collection of crabs for food should either be banned or regulated.
Cumberildge N., 2008. Potamon fluviatile. In: IUCN Red List of Threatened Species. Version 2012.2.
Maurakis E.G., Grimes D.V., Mcgovern L. & Hogarth P.J., 2004. The occurrence of Potamon species (Decapoda, Brachyura) relative to lotic stream factors in Greece, Biologia, Bratislava, 59/2:173|179
Jesse R., Pfenninger M., Fratini S., Scalici M., Streit B., Schubart C.D., 2009. Disjunct distribution of the Mediterranean freshwater crab Potamon fluviatile – natural expansion or human introduction?, Biological Invasions, 11(10): 2209-2221.