This work reports 60 marine alien molluscs, collected by two Belgian citizen scientists, from nine countries across the Mediterranean Sea and the Sea of Marmara. Some of their published observations concerning the collecting dates are compared with the year of first publication for these alien molluscs reported in the literature, which enabled to backdate some of these introductions. This underlines the importance of collaboration between volunteers and institutional scientists in tracing variations and changes in the environment and biodiversity.