A study on Pomatoschistus tortonesei Miller 1968 (Perciformes, Gobiidae) reveals the Siculo-Tunisian Strait (STS) as a breakpoint to gene flow in the Mediterranean basin.

The current genetic structure of P. tortonesei, exclusively inhabiting lagoons, may reflect a Plio-Pleistocene colonisation of an ancestor line which has undergone phases of population decline and expansion, following alternate cooling phases. Regarding the calculation of divergence time, it has been estimated that P. tortonesei seems to be the most recent species within the Pomatoschistus genus. It appears also that the discrepancy of P. tortonesei into two distinct phyletic lineages occurred 0.1 Mya. Recurring shifts in sea level and sea surface temperatures of Mediterranean Sea caused the desiccation of shallower lagoons and the consequent bottleneck phenomena of the brackish populations. Genetic differentiation in P. tortonesei seems to have been affected by its historical demography. Colonization and re-colonization processes could have had strong effects on mitochondrial genealogy lineages. The SPN showed two haplogroups for each gene, each consistent with the two main clades, W-MED and E-MED, and resulting in a star-like shape network in agreement with the expansion model. These results correlated with those obtained through neutrality statistic tests, where significantly negative values for Tajima’s D and Fu’s F statistics were most likely the result of recent population growth, since the mismatch distribution of both genes fitted the predicted distribution under a model of sudden population expansion.