Coexistence and Fouling of Asian Clam <i>Corbicula fluminea</i> by Dreissenid Mussels at Two Transects in Upper Lake Constance

ABSTRACT The abundance and vertical distribution of Dreissena polymorpha (zebra mussel), Dreissena rostriformis bugensis (quagga mussel), and Corbicula fluminea (Asian clam) at two sites characterized by different dominant sediment types, and the fouling rate of Corbicula by dreissenids were studied. Sediment samples were taken, using a sediment grab (base area 22 × 25 cm = 550 cm 2 ), at depths of 3, 6, 10, 15, 20, 50, and 100 m. The abundance of Corbicula was significantly higher at the sandy substrate site (Schussen Estuary) than at the site where coarse, gravelly substrate predominated (Nonnenhorn), whereas no difference in the abundance of quagga mussels was observed between the two sites. No living zebra mussels were found at either station. However, the quantification of dead zebra mussel, that is, their shells, revealed a much higher former abundance at Nonnenhorn than at the Schussen Estuary. Depth distribution revealed that Corbicula and quagga vertical distributions overlapped at the Schussen Estuary. Fouling rates on Corbicula were low when all individuals were considered: only 44 (5%) of the overall 871 living Corbicula individuals examined were fouled by quagga mussels. However, Corbicula larger than 1 cm showed a considerably higher fouling rate of 31%. Comparison of Corbicula abundances in the years 2016, 2021, and 2023 indicated that no overall decrease in abundance had occurred but a decrease of the proportion of large individuals relative to small ones had. Consequently, we recommend further investigations on the topic of Corbicula fouling by quagga mussels.