The processes that lead to a successful invasion are complex. Here, we investigated life history characteristics potentially explaining the invasion success of Potamopyrgus antipodarum, a small parthenogenetic and ovoviviparous freshwater snail that was recently added to the top “hundred worst” alien species in Europe. We monitored monthly, over the course of 1 year, shell size, number of brooded embryos, and the presence of castrating parasites at three Northeast German sites: a lake (the Kiessee), a stream (Hohen Sprenz), and a small spring brook (Rügen) (N = 1165). We found that (1) despite sharing the same clonal lineage, drastic differences in space and time for size and fecundity were observed, and these differences were linked to specific environmental variables (water temperature, salinity, and current); (2) P. antipodarum reproduces all year around, except at one of our sites, the spring Rügen, where the reproduction was seasonal; (3) none of our dissected specimens was infected by parasites. Together with ovoviviparity and the ability to reproduce parthenogenetically while being released from parasite pressure, the ability to adapt readily to a wide range of habitat conditions is likely paramount for the invasive success of P. antipodarum.