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The European seas are known to be the recipient of several hundreds of Non-Native Species (NNS) due two main origins: shipping and aquaculture. More than 160 NNS had been recorded along the Normandy coast, which remains low in comparison with the number of marine invertebrate species known in the English Channel (> 3,000). The main sites of introduction are the harbours, especially Le Havre and the oyster cultivation zones. Among these NNS, 56 are present in Normandy and considered invasive along the European coasts, but only ten (one macro-algae and nine invertebrates) are invasive in Normandy. Six examples are illustrate, the molluscs Magallana gigas, Crepidula fornicata, Ensis leei, and Ruditapes philippinarum, the polychaete Ficopomatus enigmaticus and the crab Hemigrapsus sanguineus. The two first species have changed the dynamics and functioning of the coastal ecosystems, while the others have lower impact. In fact, the oyster and the manila clam are now key species for the French economy, while the rapid expansion of the other species change biodiversity in the more colonised sites. The ALEX (ALien Biotic IndEX) proposed by Cinar and Bakir in 2014 for the Mediterranean Sea is used for the first time in Atlantic waters.
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