Abstract:In this study, analyses of 87 biometric parameters and of genetic variation of the cytochrome b gene show that A. boyeri populations from shallow waters of Tunisian Islands (Kerkennah) belong, in spite their marine habitat, to the lagoonal group. Moreover, in all the phylogenetic analyses, the sequences of these marine atherines constitute with the lagoon ones a clade strongly statistically supported. Moreover, in accordance with ecological rules, vertebral numbers of wild individuals increased with latitude for all the Atherina species, but for A. boyeri a low decrease of vertebral number in its more septentrional distribution area is suggested. In addition, the lower values are found for species living in particular habitats, rocky sea beds or lagoons for respectively punctated and lagoonal fish, and a rapid decrease of mean vertebral number by latitude has been found for lagoon fish.