The current study investigated basic biological features for two co-occurring shore crabs Pachygrapsus marmoratus and P. transversus in the eastern Mediterranean Sea. Differences in population structure, reproductive periods and settlement patterns were studied along the coast of Lebanon. Sizes of sampled individuals ranged from 4 to 37 mm carapace width (CW) for P. marmoratus and from 3.5 to 24 mm CW for P. transversus. Males were larger than females in both species, each of them exhibiting different size frequency distributions. Both male and female proportions were similar for P. marmoratus while males dominated the population of P. transversus. Fecundity and egg sizes were higher in P. marmoratus and fecundity was directly related to body size for the two crabs. Size structures differed between studied locations showing size restrictions for specific size-classes. Females from both species remained ovigerous for about 5 months, with P. transversus (May–August) started 1 month after P. marmoratus. Larger sized females displayed higher fecundities. The recruitment of juveniles lasted 5 months for both P. marmoratus (December–May) and P. transversus (November–April). Our results showed that niche partitioning occurred between the two species, where P. marmoratus starts incubating earlier than P. transversus while young of the year of the latter settle earlier on the reefs, taking advantage of convenient seawater temperatures and thus minimizing competition. This study shed some light on the populations of grapsid crabs living in the eastern Mediterranean and provided baseline information on the biology and ecology of the two congeneric shore crabs and their interactions.

A grapsid crab, Percnon gibbesi, is the most invasive decapod species to enter the Mediterranean. The vessel-transported crab spread within a few years from the Balearic Islands to the Tyrrhenian and Ionian coasts of Italy. In the summer of 2005 it was recorded for the first time from Greece. Its presence in the little frequented island of Antikythira possibly points to the role of recreational vessels as vectors of marine alien species.

Specimens of the grapsoid crabs Grapsus granulosus, Plagusia squamosa and Pachygrapsus transversus were recently collected in the Sirte Basin, on the southern rim of the Mediterranean Sea. The Erythrean alien G. granulosus is reported for the first time from the Mediterranean Sea, along with the second record in the Mediterranean of the Indo-Pacific P. squamosa, and the first record of P. transversus along the south-central Mediterranean coast. The relations between introduced grapsoid crabs, vessel traffic and climate change are dicussed.