Understanding the climate of the past and past seasonal temperature amplitudes is essential to evaluate the effects of future climate change on marine ecosystems. The Mediterranean Sea is of great importance due to its crucial role in modern atmospheric phenomena such as the North Atlantic Oscillation (NAO). Fossil shells of the bivalve Arctica islandica were collected from three different Pleistocene successions in Italy. The seasonal water temperature amplitude was reconstructed using stable oxygen isotope (δ18Oshell) analysis. Samples were derived by the micro-milling approach and measured by isotope ratio mass spectrometer. Results show a low seasonality scenario (~3°C). This is in sharp contrast to the assumption that the simultaneous occurrence of boreal and warm-water species in the middle Calabrian Mediterranean Sea can be explained by high seasonality (~10°C). A prominent 6-year cyclicity was identified in the shell growth time-series by means of spectral analysis. This signal might be linked to the NAO whose periodicity ranges between 5-9 years. However, a connection to the Mediterranean Oscillation cannot be excluded. The low seasonality (~3°C) and the relatively low mean water temperature (9-10°C) indicate that the middle Calabrian Mediterranean Sea was characterized by colder climatic conditions compared to nowadays, indicating a maximum glacial phase.