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Since its appearance in 2006 in a freshwater section of the Rhine-Meuse estuary (Hollandsch Diep, The Netherlands), the non-indigenous quagga mussel has displayed a rapid range expansion in Western Europe. However, an overview characterising the spread and impacts of the quagga mussel in this area is currently lacking. A literature study, supplemented with field data, was performed to gather all available data and information relating to quagga mussel dispersal. Dispersal characteristics were analysed for rate and direction and in relation to hydrological connectivity and dispersal vectors. To determine ranges of conditions suitable for quagga mussel colonisation, physico-chemical characteristics of their habitats were analysed. After its initial arrival in the freshwater section of the Rhine-Meuse estuary and River Danube, the quagga mussel demonstrated a rapid and continued range expansion in Western Europe. Quagga mussels have extended their nonnative range to the network of major waterways in The Netherlands and in an upstream direction in the River Rhine (Germany), its tributaries (rivers Main and Moselle) and the River Meuse (Belgium and France). The calculated average quagga mussel dispersal rate

Summary Laboratory data on desiccation tolerance of native and non‐native mollusc species were used to derive species sensitivity distributions (SSDs), to predict effects of desiccation on mollusc assemblages in rivers during low discharge events and to prioritise various environmental stressors (i.e. desiccation, temperature and salinity). The predicted absence and observed absence of mollusc species by desiccation at a specific site were expressed as potentially not occurring fraction (PNOF) and not occurring fraction (NOF) of their regional species pool in the River Rhine. Calculations of PNOFs for desiccation explained 57% (LT99) and 65% (LT50) of the NOFs of the mollusc species. Sensitivity to desiccation did not differ between native and non‐native mollusc species pools. Due to differences in frequency of low river discharge events and water level fluctuations, mollusc species in an impounded reach of the River Meuse were less affected by desiccation than in a free‐flowing distributary of the River Rhine. Earlier calculations of the PNOFs for combined effects of temperature and salinity in the River Rhine explained 22 and 3% of the NOFs of native and non‐native species, respectively, for the period 1988–2003. Accounting for the effects of desiccation, 62 and 80% of the NOFs of the native and non‐native species pool, respectively, were explained, indicating that desiccation during low discharge events was an important stressor in comparison with water temperature or salinity. Since a relatively high percentage of the NOFs of native species in the River Rhine still remain unexplained, an effort to assess potential effects of other stressors is recommended (e.g. toxic substances or water turbulence caused by commercial shipping).

Insufficient oxygen delivery to tissues is hypothesised to limit thermal tolerance, but evidence in ectotherms is mixed. We assessed heat tolerance under hypoxia, normoxia and hyperoxia to test whether the extent in which oxygen can lower or increase heat tolerance differed with mode of respiration, comparing gill-breathing caenogastropods and lung-breathing pulmonates with or without an accessory gill. Hypoxia lowered heat tolerance in three of the four pulmonates (Physa fontinalis, Physa acuta and Planorbis carinatus) by 1.2-2.1°C. Hyperoxia, however, did not increase the heat tolerance in any of the pulmonate species. Thus, heat tolerance limits of these pulmonates does not appear to be oxygen limited under normoxia, possibly because of their high capacity to regulate oxygen consumption associated with aerial gas exchange. Instead, other processes may become limiting at thermal extremes such as loss of protein function, loss of membrane stability or neuronal dysfunction. The caenogastropod species tested (Potamopyrgus antipodarum, Bithynia tentaculata) closed their operculum during the warming experiments. This behavioural response prevented us from obtaining clear results. Nevertheless, our results suggested hyperoxia may increase heat tolerance in B. tentaculata. This could be related to its lower capacity to regulate oxygen, owing to its fully aquatic gas exchange mechanism.

Abstract. Invasive alien species (IAS) are species whose introduction or spread outside their native range threatens biological diversity, ecosystem functioning, economy, and/or public health. The recent European Union (EU) regulation on the management of IAS emphasizes the need for a consistent approach to alien species assessment that will underpin international measures for the early identification of newly introduced IAS followed by rapid action aimed at the prevention of introduction, spread, and negative impacts. The goals of the present study were (1) to present the risk classifications of 18 aquatic alien species for The Netherlands using the Invasive Species Environmental Impact Assessment protocol, (2) to compare these with available risk classifications made for countries spanning similar climatic and biogeographical regions to the EU, and (3) to provide explanations for inconsistencies between different risk classifications. Five species were classified as high risk: Carassius gibelio (Prussian carp), Cyprinus carpio (common carp), Sander lucioperca (pike-perch), Cabomba caroliniana (fanwort), and Dreissena rostriformis bugensis (quagga mussel). Of the 14 species with existing risk classifications for countries spanning similar climatic and biogeographical regions to the EU, all but two of the assessed species (C. gibelio and D. rostriformis bugensis) were classified inconsistently. Reasons for these inconsistencies are the application of different risk assessment schemes, application on a national rather than biogeographical scale, differences in the definition and application of criteria, differences in habitat availability, and uncertainties that are intrinsic to risk assessment methodologies. Approaches that increase transparency by highlighting these methodological aspects, normative choices, and uncertainties are vital to the legitimacy of any risk assessment method and will increase acceptance among decision makers, nature managers, and stakeholders.