In this study we analysed mitochondrial DNA variation in Penaeus kerathurus prawns collected from seven locations along a transect across the Siculo-Tunisian region in order to verify if any population structuring exists over a limited geographical scale and to delineate the putative transition zone with sufficient accuracy. Partial DNA sequences of COI and 16S genes were analysed. In contrast to the highly conservative 16S gene, the COI sequences exhibited sufficient diversity for population analysis. The COI gene revealed low levels of haplotype and nucleotide diversities. The size of the annual landings of this commercial species suggests large population sizes. Hence, the low genetic diversity detected in this study could indicate a possible reduction in effective population sizes in the past. We detected significant genetic differentiation between eastern and western populations likely due to restricted gene flow across the Siculo-Tunisian boundary. We discuss the different evolutionary forces that may have shaped the genetic variation and suggest that the genetic divide is probably maintained by present-day dispersal limitation.

Ten populations of Penaeus kerathurus (ForskÌl), a prawn of high commercial value, were sampled from the eastern and western Mediterranean coastal waters of Tunisia and screened electrophoretically for genetic variation at 13 allozyme loci. Four among the six polymorphic loci were out of Hardy^Weinberg equilibrium (H^WE) in at least one population. In the same way, the multilocus test showed deviation from H^WE in six populations. These populations showed heterozygote de¢ciency whereas the average heterozygosity for the four remaining ones is quite similar to the expected levels. Genetic variability was low. The number of alleles per locus ranged from 1.2 to 1.5 (average 51.3), and the observed heterozygosity varied between 0.010 and 0.048 (average 5 0.021). Signi¢cant population di¡erentiation (F ST 5 0.076, Po0.05) in the total data set re£ected the di¡erentiation of the two populations, which were at the margins of the range sampled, from all the others (Pairwise F ST values ranged from 0.035 to 0.208). Although there was no signi¢cant di¡erentiation among the other populations (pairwise F ST values ranged from À 0.006 to 0.201, P40.05). Our data suggest a population structure consistent with separation by Mediterranean Sea basins that might re£ect di¡erent local biogeographical zones.

This study reports on the polymorphism of 15 allozyme loci in Ruditapes decussatus clams collected from 11 locations along the Tunisian coasts. We concentrated our sampling effort around the Siculo-Tunisian region to verify if any population structuring exists in this region and to identify the factors that have shaped this structure. Measurements of genetic diversity were quantified both within and between populations, and the geographic variability of gene frequencies was analyzed. Our study shows that the Siculo-Tunisian Strait is an important genetic boundary between eastern and western regions, which agrees with findings for a variety of other species. We suggest that vicariance is a predominant factor shaping the current distribution of genetic diversity of R. decussatus, and the mixing of divergent gene pools from the eastern and western regions still seems to be limited by some physical and/or biological factors.

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