Invasive species have far-reaching consequences for whole ecosystems. In particular, filter feeders such as zebra and quagga mussels have a huge impact on lake ecosystems by filtering out resources that are otherwise used by native filter feeders. Quagga mussels can colonise deeper parts of a lake while zebra mussels prefer shallow areas. Both species show large variability in shell morphology, but only quagga mussels show a shallow and deep morphotype. The aim of this study is to investigate the depth adaptation of both dreissenid species. We hypothesize potential advantages of the quagga mussel over the zebra mussel with regard to feeding. We conducted common garden experiments, where we measured filtration rates at different temperatures (4, 12, and 20 °C) of quagga mussels collected from different water depths (1, 30, and 60 m) and compared them to zebra mussel from 1m depth. We found filtration rates to be strongly temperature dependent, and in particular, lower at 4 °C. Overall, zebra mussels fed less than quagga mussels taken from the same water depth. Quagga mussels collected from 1 m depth fed more at higher temperature than quaggas from 30 and 60 m depth. We hypothesize that deep-water mussels are less sensitive to lower temperature and have a higher temperature threshold to increase their filtration. Otherwise, filtration rates were independent of the depth of origin of the mussels. Our results indicate that zebra mussels are less competitive filter feeders compared to quagga mussels and quagga mussels are not locally adapted with regard to their feeding. Rather, quagga mussels are better accustomed to a wider range of temperatures in shallow water.
