Coastal regions represent environments with heterogenic ecological conditions and with potential barriers to dispersal, making them ideal systems to study the variation of morphology and genetics of the organisms inhabiting them. Using individuals of the damselfish Abudefduf saxatilis from the three main reef systems along the Mexican Atlantic coast, we examined genetic and environmental variation to evaluate their relative contribution to body and head shape, and body size. We determined if there was a relationship between morphological variation and environmental factors and whether morphological, mitochondrial DNA (mtDNA) and microsatellite variation were concordant. We found morphological differences among A. saxatilis individuals from the Tuxpan, Veracruz and Mexican Caribbean reef systems. Differences in body and head shape involved positions of pectoral fins and mouth. Individuals from Tuxpan and Veracruz were larger than those from the Mexican Caribbean. Furthermore, we found a significant correlation between shape (body and head) and environmental variables (chlorophyll-a, geostrophic velocity and temperature) as well as between body size and chlorophyll-a concentration. Both mtDNA and microsatellite markers detected high levels of genetic diversity and lack of population genetic differentiation among reefs, reef systems and between marine regions. We did not find a correlation between morphological and genetic differentiation. Our results suggest that the phenotypic differences of A. saxatilis along the Mexican Atlantic coast may be determined by environmental factors in the presence of gene flow.

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