Variability in phenotypic tolerance to low oxygen in invasive populations of quagga and zebra mussels

Novel biotic or abiotic conditions can cause invasive species to evolve rapidly in their newly invaded habitats and are important factors when predicting species invasions. Zebra mussels (Dreissena polymorpha) have a relatively long invasion history in Western Europe, whereas quagga mussels (Dreissena rostriformis bugensis) began spreading about a decade ago. In a previous invasion to North America, quagga mussels repeatedly colonized stratified lakes at greater depth than zebra mussels. It would be important to know if the same invasion pattern is expected to repeat in Western Europe, as the quagga are expected to reach deep stratified lakes in the near future. This might require quagga mussels to be more tolerant of the low oxygen conditions at depth than zebra mussels. Therefore, using a fully factorial design, we tested survival of different zebra and quagga mussel populations from Western Europe at four oxygen levels (6%, 33%, 66% and 90%) and two temperature regimes (11°C and 18°C). Surprisingly, survival differences among oxygen and temperature treatments depended more on population origin than on species identity. This finding suggests that populations have undergone rapid and convergent adaptation to local conditions after invasion, in particular to low oxygen. We also found that population-by-environment interactions were more variable among quagga than zebra mussel populations. Our results suggest that rapid evolutionary adaptation to low oxygen conditions needs to be considered when predicting the further spread of zebra and quagga mussels.