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.

The present study was carried out to evaluate the biological characteristics of Pachygrapsus marmoratus in the southern Black Sea, Ordu, Turkey. A total of 1503 crabs, ranging from 0.9 to 4.3 cm in carapace length (CL), were collected monthly from November 2012 to October 2013. Female:male ratio was found to be 1:1. Measured male and female crabs were grouped in 0.49 cm CL class intervals. The mean CL, carapace width (CW) and weight (W) were determined as 2.55 ± 0.02 cm, 2.90 ± 0.02 cm and 14.21 ± 0.27 g, respectively, for all individuals. Correlation analysis was also conducted to reveal relationships between size parameters for males and females as well as between egg number and size. The b values of CW and W for male, female and ovigerous crabs were found to be 3.1, 3.06 and 2.95, respectively. Number of large-sized individuals and growth rate was higher in males compared to females.The highest correlation was found between CW and CL in both sexes. The mean condition index values were calculated as 49.03 for males and 45.79 for females. Ovigerous females were observed only from May to July 2013. The fecundity estimates ranged from 8989 to 151,578 eggs/crab, with an average value of 57,814.53 ± 3349 eggs/crab. The egg diameter ranged between 328.40 μm and 423.38 μm, with a mean size of 365.83 ± 1.97 μm. There was a positive relationship between egg number and female size.

Since its first sighting in 2008 on the central part of the western coast of northern Cotentin, the marbled crab Pachygrapsus marmoratus has been regularly sampled in the sites where it was first reported. The presence of ovigerous females and juveniles confirmed the presence of a reproductive population in the English Channel. However, the absence of this species in other sites around the Cotentin, prospected for the survey of the progression of two invasive Hemigrapsus decapods is questionable. It is suggested that the species may have good abilities to colonize new areas, considering the larval dispersal with a long planktonic larval phase, but failed in this particular case, probably due to non-favourable conditions.

The sighting of the marbled crab Pachygrapsus marmoratus on the western coast of northern Cotentin confirms the presence of this species in the English Channel. This new sighting may be related to the northern progression of the species probably due to increased sea surface temperatures.

Landmark-based geometric morphometric methods were used to test which chela is most suitable to use in population differentiation of brachyuran crabs. Two species were analysed: Pachygrapsus marmoratus, a homochelous species, and Carcinus maenas, a heterochelous species. Variance analysis of shape variables and of each chela was calculated to determine which claw presents less variation within populations. The results showed that the right chela of P. marmoratus is more appropriate to use when the goal is to differentiate populations. In C. maenas, the males major chela and females right chela are more suitable to use, probably due to its heterochely. To test these results, specimens from different geographical locations were compared. Pachygrapsus marmoratus populations presented significant differences in manus shape, and C. maenas populations had differences in polex size and shape. Differences in carapace shape were also encountered between populations of both species. This phenotypic variation was not corroborated with genetic data, which were obtained using the mitochondrial DNA gene cytochrome oxidase I as a marker. These opposite results suggest that environmental factors, such as diet and parasitism, can be determining shape differentiation.

The occurrence in British coastal waters of the marbled crab Pachygrapsus marmoratus is confirmed for the first time. Features are given to enable this crab to be distinguished from native species. Its spread along the French Atlantic coast and introduction into British waters is discussed.