Abstract
The invasive ctenophore Mnemiopsis leidyi, which comes from North America’s east coast, was observed in Danish waters for the first time in 2007. Since then, the new invader has every summer spread in Danish and adjacent waters (i.e. North Sea, Limfjorden, Skagerrak, Kattegat, Belt Sea, Baltic Sea). The invasive comb jelly has apparently come to stay, as it has no effective enemies. Possible harmful effects of M. leidyi which feeds voraciously on zooplankton, fish eggs and larvae, have so far not been thoroughly studied in Danish waters, although dedicated attempts have been made in Limfjorden and in the central Baltic Sea. Over the last 10 years, the Danish national environmental monitoring program did not include gelatinous zooplankton, but new initiatives have been recently taken. A brief overview of our current knowledge on the impact of M. leidyi in Danish waters is given here.
Highlights
In the last 10 years, we have witnessed a full-scale experiment in Danish and adjacent waters, where the alien invasive comb jelly Mnemiopsis leidyi spreads every summer
It is unknown to what degree M. leidyi in this way may be brought into the inner Danish waters [28], and likewise, it remains uncertain if M. leidyi may be brought into the Baltic Sea via the Kiel-Canal which connects the North Sea with the Kiel Bight [6] [42] [43], or if M. leidyi is occasionally released with ballast water from various ports or native habitats, resulting in mixed populations [11]
Few sporadic observations of M. leidyi have been made in Kattegat and Great Belt
Summary
In the last 10 years, we have witnessed a full-scale experiment in Danish and adjacent waters, where the alien invasive comb jelly Mnemiopsis leidyi spreads every summer. This ctenophore comes from North America’s east coast [1] [2] and has so far no effective enemies in Danish waters. The Danish national environmental monitoring program does not yet include occurrences and possible environmental damages caused by jellyfish and other gelatinous zooplankton. During the last 10 years and after the arrival of M. leidyi to the Danish waters, its possible adverse impacts have largely been ignored by the environmental authorities. I give a brief overview of our current knowledge, as I see it
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