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The invasive ctenophore Mnemiopsis leidyi A. Agassiz, 1865 was recorded for the first time in Northern Europe in summer 2005, while the first records in the North Sea date back to summer 2006. The first sightings in the Belgian part of the North Sea were made in August 2007 in the port of Zeebrugge, but most probably M. leidyi had already been present for a longer period in this area. The high densities in the port of Zeebrugge suggest that M. leidyi entered the Belgian marine waters via ballast water transport, comparable with the invasion in the Black and Caspian Seas and the Dutch part of the North Sea. In the period 2009-2011, M. leidyi was found in all ports and all along the Belgian coastline, up to 27 km offshore. Further offshore, no M. leidyi were found in zooplankton samples and small meshed otter trawl samples. Sightings of adult individuals in the coldest winter months imply that the species can survive Belgian winters. Highest densities (17 ind.m(-3)) were found in the Sluice dock in the port of Oostende. Along the coastline, average densities of 0.4 ind.m(-3) were recorded. As M. leidyi might previously have been misidentified on the basis of morphological features alone, we also determined the species with genetic identification tools. Taking into account the notorious impact of this species in its native and in other invaded waters, it is recommended to continue the monitoring of M. leidyi populations in the Belgian part of the North Sea.
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