Specimens of the carpet shell Ruditapes decussatus from the Mesolithic shell midden (6th millennium BC) of Beg-an-Dorchenn (Brittany, France) were studied in order to assess their period of collection as well as provide some insight on Paleo-temperature reconstructions. Cross sectioned shells display very clear growth structures allowing an assessment of a growth rhythm of 2 increments per lunar day. From this temporal framework, daily growth increments and winter annual growth breaks were characterized. Our results show the R. decussatus shells were typically collected in the early spring period. Chemical analyses of shell carbonates were also performed to assess a preliminary range of potential sea temperature data from oxygen isotopes values (δ18Oshell). δ18Oshell shows seasonal variations but do not record the whole seasonal temperature range as R. decussatus have a growth break during winter. However, results show predicted SSTs within the range of expected values. Thus, R. decussatus is particularly suited for paleo-environmental reconstructions at high temporal resolution such as seasonal timescales. Additional sclerochronological and chemical research on additional carpet shells and also other mollusk species from Beg-an-Dorchenn or other Mesolithic shell middens along the European Atlantic coast should be considered. Such studies will contribute to the understanding of the seasonal occupation patterns of the last hunter-gatherers of the European Atlantic coast as well as the climate variability at the Mesolithic-Neolithic transition.