1. Over the past few decades, Asiatic clams (Corbicula spp.) have spread spectacularly in several large European rivers. In the River Meuse, a transnational lowland river, a substantial chlorophyll a decline has been recorded since the mid-2000s, which seems to be related to the invasion by these exotic bivalves. This study aimed at verifying this hypothesis, using data on clam density from field surveys, water quality monitoring data and a simulation model. 2. Corbicula density was estimated at between 50 and 900 m À2 , depending on the site. Assuming a maximum filtration rate per clam body mass of 0.086 m 3 g C À1 day À1 at 20°C, derived from the literature, we ran simulations with a non-stationary model to estimate the impact of the bivalve on the river plankton and water quality. 3. In the stretches where the invasive clams were most abundant, we estimated a 70% loss of phytoplankton biomass, due to their filtration, and a 61% decline in annual primary production compared with a situation without clams. Model simulations also showed that zooplankton may have suffered as much as a 75% loss of biomass. 4. The simulations also point to substantial effects of Corbicula on the river oxygen budget and on nutrient cycling. We suggest that, in the heavily regulated sectors of the river, the loss of planktonic production due to these invasive filter-feeders negatively affects other suspension feeders and alters ecosystem processes and productivity.

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DNA methylation, a well-studied epigenetic mark, is important for gene regulation in adulthood and for development. Using genetic and epigenetic approaches, the present study aimed at evaluating the effects of heat stress and copper exposure during zebrafish early embryogenesis when patterns of DNA methylation are being established, a process called reprogramming. Embryos were exposed to 325 μg Cu/L from fertilization (<1 h post fertilization - hpf) to 4 hpf at either 26.5 °C or 34 °C, followed by incubation in clean water at 26.5 °C till 96 hpf. Significant increased mortality rates and delayed hatching were observed following exposure to combined high temperature and Cu. Secondly, both stressors, alone or in combination, significantly upregulated the expression of de novo DNA methyltransferase genes (dnmt3) along with no differences in global cytosine methylation level. Finally, Cu exposure significantly increased the expression of metallothionein (mt2) and heat shock protein (hsp70), the latter being also increased following exposure to high temperature. These results highlighted the sensitivity of early embryogenesis and more precisely of the reprogramming period to environmental challenges, in a realistic situation of combined stressors.

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Due to their high filtration rates, exotic freshwater bivalves remove suspended organic matter from the water column, transferring resources to the sediment and increasing water clarity, which alters ecosystems. While there is a considerable amount of data on filtration rate of exotic bivalves, comparison between species is often invalid due to the utilization of different protocols for assessing filtration in experimental conditions. In this study, we quantified and compared for the first time the filtration rates of the zebra and quagga mussels and of two invasive Corbicula lineages (forms R and S) as a function of chlorophyll a concentration and season. The highest filtration rate observed was for the zebra mussel in fall and at high algal biomass. The incipient limiting level (ILL), defined as the chlorophyll a concentration at which the maximum filtration rate is observed, was observed in spring and summer in Corbicula species, and in summer and fall in Dreissena species. Overall, filtration rates presented a large range of variation, depending on chlorophyll a and season. Overall, Corbicula form S was observed as the best adapted to low food concentration. Moreover, Corbicula can switch to pedal feeding which gives them a competitive advantage at low chlorophyll a concentrations.