Specimens of carpet shell Ruditapes decussatus from the Mesolithic shell midden (6th millennium BC) of Beg-an-Dorchenn (Brittany, France) were studied in order to access their period of collect as well as some insight on Paleo-temperature reconstructions. Cross sectioned shells display very clear growth structures free from diageneses allowing to assess a growth rhythm of 2 increments per lunar day. From this temporal framework, daily growth increments, fortnightly ridges, winter annual growth break and spawning events were characterized. The R. decussatus studied shells collected in late autumn and/or early spring period. Chemical analyses of shell carbonates were also conducted to access a preliminary range of potential sea temperature data from oxygen isotopes values (δ18Oshell). δ18Oshell shows seasonal variations but do not displays the whole range of temperature as R. decussatus have a growth break during winter. However, the range of reconstructed SST did not show nonsensical values and suggest sea temperatures slightly higher than the current ones. Thus, R. decussatus is particularly suited to address paleo-environmental issue at high temporal resolution such as the seasonal periodicity. To conclude additional sclerochronological and chemical researches on other mollusk species from Beg-an-Dorchenn or other Mesolithic shell middens along the European Atlantic façade will contribute to the problematics of seasonal occupation as well as climate variability at the Mesolithic-Neolithic transition.