It is called Percnon gibbesi. It is an “alien” crustacean spotted in the Mediterranean, for the first time in 2009, in Linosa, but probably already present in many other areas without being recognized. Not having a common name we called it Yellow-legged crab. We met him in various areas of the south / east of Sardinia, practically everywhere on rocky bottoms and composed of pebbles and stones. We report below some information found on the web.
Many reports were followed in almost all the seas of the Mediterranean and many scientific works were produced. The crab with a carapace (central body) up to 25-30 mm is present mainly in shallow seas from a few centimeters of depth up to a maximum of 30 meters.
The Percnon has a large number of privileged non-inhabited environments, is exclusively herbivorous and can compete only with sea urchins (disadvantaged by fishing, acidification and climate) and a few other species.
The penetration of this tropical species in the western and central Mediterranean is linked according to some authors to a transport of eggs or larvae with the currents through the Strait of Gibraltar, according to others through the ballast waters, favored by the resistance from the egg and the species, this would also justify secondary movements of an alien in the invasion area.
For its diffusion and rapid distribution there is a proposal to include it among the 100 most dangerous alien species in the Mediterranean, even if there is no evidence of environmental damage.
Percnon gibbesi is a medium-sized crab, native to the shallow coastal waters of the western Atlantic, from Florida to Brazil. In 2000 it is reported for the first time in the waters of Linosa. Since then it has spread very quickly going up the Tyrrhenian coast of our peninsula. At the same time, a series of reports in locations distant from each other makes us think that it has been transported by ships.
A parenthesis: the transport with ballast water, the water that is pumped into the holds of merchant ships when they travel empty, has probably been exploited by many of these animals moving from one area to another. In the larval, planktonic phase, they can easily be sucked in here and deposited there, and the game is done: they unknowingly have crossed the oceans.
Percnon gibbesi (H. Milne Edwards, 1853) is a crab of the Plagusiidae family, originally widespread in the coastal areas of the Atlantic and the Pacific, but has also been reported in the Mediterranean Sea for some years.
Distribution and habitat
Percnon gibbesi is a widespread species along the coasts of the Atlantic Ocean (from Florida to Brazil and from the island of Madeira to the Gulf of Guinea) and the Pacific (from Baja California to northern Chile).
In 1999 it was also reported for the first time in the Mediterranean Sea, on the island of Linosa. After this initial report it was also found in the Balearic Islands, in Pantelleria, in Malta, in the Egadi Islands in Sicily, Sardinia, in the islands of Ischia, in the Pontine archipelago and in other places of the Tyrrhenian Sea, in the coasts of Calabria, in those of Salento (Puglia), Greece, Turkey and Libya.
It populates the rocky ravines of the infralittoral plan.
It has a flattened disk-shaped carapace, which in the adults reaches 3-4 cm in diameter, of a brownish-red color with bluish veins. The legs, whose anterior border has a row of thorns, have yellowish rings at the joints. The massillipeds also present small spinous processes.
It is a species exclusively herbivorous, or better to say algivora, a characteristic that differentiates it from most of the crabs of the infralittoral plan of the Mediterranean Sea and that at least partly explains the ease of propagation in this area. Recently, however, there are some sightings that would also testify to their omnivorous nature. In the coasts of Sicily, especially on the beaches between Cefalù, Finale di Pollina and Tusa, it is not rare to see them with small fish / other crustaceans, in the clutches of their claws.
Despite the different eating habits it is a potential competitor for some native Mediterranean species such as Pachygrapsus marmoratus and Eriphia verrucosa, with which it shares the habitat.