The increase of anthropogenic carbon dioxide emissions and the subsequent uptake of CO2 by the sea, is leading to a decrease in the pH of the oceans, a process known as Ocean Acidification. One of the main challenges of the current research on climate change is to determine how marine species respond to low pH/elevated pCO2 conditions. This thesis has investigated the effects of natural OA on the polychaete species Platynereis dumerilii and its sibling P. massiliensis (Annelida, Nereididae) as driver of genetic differentiation and phenotype/genotype selection. Platynereis spp. populations were sampled in five geographical areas situated along a thermo-latitudinal gradient along the Italian coasts, characterized by different pH conditions (acid vs normal). A multidisciplinary approach, focused on different aspects of the target species biology, was chosen and the following analyses were performed: (a) morphological and morphometric analyses of different populations/genotypes; (b) laboratory rearing of different populations to study the reproductive biology and gamete morphology; (c) population genetics by the amplification of a mitochondrial DNA marker (COI); (d) population genomics by a next-generation sequencing approach (RAD-seq); (e) background analyses and a long term laboratory experiment on selected genotypes/populations to study physiological responses to different pH conditions. This work has confirmed that Platynereis dumerilii and P. massiliensis represent two complexes of sibling species characterized by contrasting life history traits, reproductive biology and gamete morphology. The overall Platynereis massiliensis predominance in the CO2 vent systems is not a direct consequence of elevated pCO2, but it seems to derive from a winning reproductive strategy (brooding habit) in low pH conditions. Unlike Platynereis dumerilii, P. massiliensis is potentially able to thrive in the CO2 vents thanks to the higher stability of its antioxidant defence systems over temporal scale and its greater responsiveness to extreme hypercapnia conditions.