The demographic histories, genetic relationships and population structure of sedentary fish Pomatoschistus marmoratus (Risso, 1810), which was sampled from the north-eastern basin of the Mediterranean Sea (including the Turkish coasts of the Black Sea, Aegean Sea, Levantine Sea and Sea of Marmara), were investigated by mitochondrial cytochrome c oxidase subunit I (652 bp) and cytochrome b (526 bp) regions. It was found that the population groups had high haplotype diversity while the nucleotide diversity was quite low for both gene regions. Phylogeographic analyses of the haplotypes indicated that the Levantine population (LEV) were genetically different from other populations. Also, the gene flow between LEV and the other populations was very limited. The results of the analyses of neutrality and mismatch distributions that were applied to the population groups were evaluated as a whole. It was determined that the haplogroup that represents the Black Sea and Sea of Marmara populations (BLAMAR) was stable, but the Levantine population (LEV) was under the sudden demographic expansion model following the population bottleneck. The genetic variance indices indicated sudden demographic expansion following population contraction. This was supported by star-shaped haplotype networks. The reason for this limited gene flow and differentiation between the Levantine population (LEV) and the others was linked with wind-driven offshore transport of the larvae and surface currents in these sub-basins. The timing of the differentiation, demographic histories of populations associated with geological and palaeo-climatic events and current ecological conditions were discussed.

Gyrodactylus arcuatus-like monogeneans collected from the gills of 3 goby species, Pomatoschistus minutus, P. lozanoi and P. pictus, from the Voordelta Area (SW Netherlands) were compared on a morphometric basis with G. arcuatus from its type host, the three-spined stickleback, Gasterosteus aculeatus. Univariate and multivariate analysis was carried out on measurements of 17 characters of the opisthaptoral hard parts of 268 specimens. The contribution of host-dependent and seasonal factors to the observed morphological variation are discussed. Univariate statistics (ANOVA) were used to detect features that were useful in separating the gyrodactylids from the different host groups. Subsequent factor analysis and discriminant analysis, combining all variables, led to the separation of 3 distinct forms (possibly species) dependent on the host species harbouring them. G. arcuatus (sensu stricto) was only present on G. aculeatus, while P. minutus and P. lozanoi carried a common species, Gyrodactylus sp. 1, and P. pictus hosted Gyrodactylus sp. 2. Seasonal variation in size of the sclerities was demonstrated on the Gyrodactylus species from P. minutus and P. lozanoi, and should be taken into consideration when specimens are compared. The findings are discussed in view of the possibility of phylogenetic host specificity of the gyrodactylids occurring on goby species that live sympatrically in the Voordelta area.