By Federico Betti – Reef Check Italia Associate Program Manager
Percnon gibbesi is a beautiful crab belonging to the family Grapsidae, easily recognisable by the extremely thin carapace, the long and flattened legs, and the colourful coat, brown with red lines and yellow circles close to the articulations.
It may be found in the first centimetres under the surface, on ravine rich rocks, where it quickly hides if scared.
This crab come from American Atlantic coasts, from Florida to Brazil, and African ones, from Madeira to the Gulf of Guinea (Galil et al., 2002),
but it’s now several years that its presence is recorded also along American Pacific coasts, from California to Chile.
In 1999 it had been observed for the first time in the Mediterranean Sea, along the coasts of Linosa (Relini et al., 2000); it probably reached the island by ballast waters of ships coming from the American continent.
Since then, its spread in our sea has been quite quick, with many sightings coming from Balearic Islands (Garcia & Reviriego, 2000), Malta (Borg & Attard-Montaldo, 2002), from the Sicily coasts to Campania, from the Southern Sardinia and, now, from Elba Island. Let see how long it will take to reach also Ligurian coast.
Right now it seems that this invasion is not creating ecosystem’s variations, maybe thanks to the fact that Percnon gibbesi is
decidedly herbivorous, while the largest part of Mediterranean crabs is detritivorous or carnivorous. So, the new species found a free ecologic space, and it has been able to easily settle down
and, in the meanwhile, do not compete with the already present species.
However, interactions between different species within the ecosystems are so complex and rich of implications that is often difficult, if not impossible, to understand the effects of the introduction of new species or other modifications since they do not create huge variations; so, it is probably to early to sigh with relief.
Borg, J.J., Attard-Montalto, J., 2002. The grapsid crab Percnon gibbesi (Milne Edwards, 1853) (Crustacea, Decapoda, Brachyura), a new addition to the marine fauna of Malta. Central Mediterranean Naturalist; 3: 159–160.
Galil, B., Froglia, C., Noel, P., 2002. Crustacean Decapods and Stomatopods. Frederic Briand (ed.) CIESM Atlas of exotic species in the Mediterranean Sea. CIESM Publ., Monaco: 192 pp.
Garcia, L., Reviriego, B., 2000. Presència del cranc subtropical Percnon gibbesi (H. Milne Edwards, 1853) (CRUSTACEA, Decapoda, Grapsidae) a les Illes Balears. Primiera cita a la Mediterranea occidental. Boll. Soc. Hist. Nat. Balears 43: 81-89.
Relini, M., Orsi, L., Puccio, V., Azzurro, E., 2000. The exotic crab Percnon gibbesi (H. Milne Edwards, 1853) (Decapoda, Grapsidae) in the Central Mediterranean. Sci. Mar., 64 (3): 337-340.