The brackish water mussel, Mytilopsis leucophaeata, is a rapidly expanding invasive bivalve in Europe with great biofouling capacities. Being a typical brackish water species with very broad habitat preferences and environmental limits, adults are extremely tolerant to fluctuations in temperature and salinity. The life cycle of mussels however, consists of two phases: (1) from fertilization until larval settlement they are pelagic, only protected by a larval soft shell and (2) after settlement, the individuals become benthic and develop a hard mytiliform shell. The fact that adult mussels can close their protective valves is the major reason why they are important fouling species and are difficult to remove once settled. Therefore, vulnerability of different larval life stages of M. leucophaeata to temperature and salinity was investigated during standardized acute 48 h experimental tests. In addition, the survival limits of the most vulnerable larval life stage were determined at different temperature-salinity combinations. Results indicated that larval stages show a differential vulnerability: 4 h old embryos were more vulnerable to changes in temperature and salinity than 2 day old larvae. Maximal survival of 4 h old embryos was found at 22°C at salinity 15. Surrounding this optimum, conditions stayed good for survival in a rather wide range: only salinities of 0 and 25 and temperatures below 10°C or above 30°C caused high embryonic mortality. Thus, even the most vulnerable larval stage in the life cycle of M. leucophaeata can be considered highly resistant to environmental conditions. Considering the broad environmental limits of adult as well as larval M. leucophaeata, we can expect this species to appear many brackish water bodies worldwide, with only colder regions potentially limiting its invasion success
