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 now spreads every summer. This ctenophore comes from North America’s east coast [1] [2] and has so far no effective enemies in Danish waters. Apparently it has come here to stay. Although M. leidyi’s natural predator, another ctenophore, Beroe ovata, was observed as a new species in Danish waters in 2014 [3] , it has not yet, due to a low number of individuals, caused any documented changes in the abundance and distribution of M. leidyi. The biological and environmental factors controlling its sporadic occurrence are still poorly understood. Possible harmful effects of M. leidyi which feeds voraciously on zooplankton, fish eggs and larvae [4] , have so far not been thoroughly studied in Danish waters, nor have possible competition for zooplankton with planktivorous fish (herring and sprat) been studied. 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. Here, I give a brief overview of our current knowledge, as I see it.

In autumn 2006, Mnemiopsis leidyi was observed for the first time in large numbers in the Dutch Wadden Sea, where it had probably been transported by the ballast water in ships [5] . It then spread northwards and up along the west coast of Denmark, into Limfjorden and further north up around the tip of Denmark into the inner Danish waters, where the ctenophore was then coincidently observed in Kiel Bight (western Baltic) in October 2006 [6] . The new comb jelly was first observed in Danish waters in 2007 by Tendal et al. [7] , in the Little Belt in February, and in the Great Belt in mid March. During the summer and autumn of 2007, the new comb jelly was observed everywhere in the inner Danish waters, and the phenomenon was extensively covered by the media. Based on more than 150 e-mails with photos from biologists, fishermen, divers, boaters and beach visitors, Tendal et al. [7] made a map showing that M. leidyi in the late summer of 2007 occurred all over the inner Danish waters (Figure 1), and often in large numbers as in Limfjorden (Figure 2), where it was quantified for

the first time [9] . The situation continued in the following years, but due to the Danish national monitoring program not including jellyfish and other gelatinous zooplankton, we have today no comprehensive overview of the development of the new comb jelly’s presence in the Danish waters, apart from quantitative studies Limfjorden and the western and central Baltic Sea, as it appears from the following sections. However, the many scattered observations that I know of (own observations in Danish waters and harbours, and e-mails with photos from many interested citizens) indicate that M. leidyi still occur every summer and spread ubiquitously in the inner Danish waters.

Mass occurrence of Mnemiopsis leidyi was first observed in Limfjorden in 2007 [9] , but it probably arrived the previous year [7] . Every year since, M. leidyi has apparently re-invaded Limfjorden (which is actually a sound) from the North Sea via Thyborøn Canal. This invader is transported by water masses partly originating from the English Channel [10] [11] , the Belgian part of the North Sea [12] [13] , and the western Dutch Wadden Sea, where it is present year round [14] [15] [16] . The hypothesis of M. leidyi being brought into Limfjorden from the North Sea has not been directly proved, but a number of strong indications support this interpretation. Thus hydrographic observations and model calculations made by Riisgård et al. [17] indicated, along with previous studies [18] [19] , that re-invasion of M. leidyi from the North Sea seeded the autumn population in Limfjorden in mid-September 2011. M. leidyi has frequently exerted a significant predation impact on the zooplankton biomass in Limfjorden during late summer [9] [17] [18] [19] [20] [21] . In most years M. leidyi has thrived extremely well in Limfjorden, with maximum densities and specific bio-volumes in August-September in central parts (Table 1). In late summer 2007, M. leidyi was