Cold tolerance varies among invasive populations of the Asian clam (Corbicula fluminea)

The distribution of the subtropical Asian clam (Corbicula fluminea (O.F. Müller, 1774)), one of the world’s most invasive freshwater molluscs, is reportedly constrained by a lower thermal tolerance limit of 2 °C. Although its occurrence in north temperate regions is typically restricted to artificially heated waterbodies, the species has been found to overwinter in unheated lakes and rivers. In laboratory experiments, we compared the cold tolerance of populations from several geographically distinct sites spanning 35°N to 46°N in eastern North America. Each population contained individuals that fully recovered from 2 months of continuous exposure to near-freezing (1 °C) conditions, contrary to published accounts of C. fluminea’s thermal ecology. Survivorship increased with body size and was enhanced by prior acclimation to a low temperature (10 °C) compared with a higher one (18 °C). When acclimated to 10 °C, clams from northern populations exhibited greater survivorship (55.0% ± 16.1%) than those from southern populations (26.7% ± 19.2%). However, one southern population demonstrated survivorship as great as that of the most tolerant northern population, suggesting that its clams could overwinter in unheated northern waterbodies. Differences among populations indicate either that contemporary evolution has occurred or that developmental plasticity shapes future acclimation responses.