Invasion of Danish and adjacent Waters by the comb jelly Mnemiopsis leidyi - 10 years after

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.


Introduction
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]

The First Observations
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] [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.

Studies in Limfjorden
Mass occurrence of Mnemiopsis leidyi was first observed in Limfjorden in 2007 [9], but it probably arrived the previous year [7]. found in every net sample from 9 locations in Limfjorden, and the population densities were high, up to more than 800 individuals m −3 in Skive Fjord, an inner branch of the fjord-system, but body lengths were small (5 to 15 mm). The specific bio-volumes were very high in the central parts of Limfjorden (i.e., Thisted Bredning: 102 ml·m −3 , Løgstør Bredning: 113 ml·m −3 , Junget Øre: 228 ml·m −3 , Skive Fjord: 312 ml·m −3 ) and were even greater than those from the Black Sea, where the greatest mean bio-volume was approximately 184 ml·m −3 in the autumn of 1989, when the zooplankton and fish stocks collapsed [4].
Mnemiopsis leidyi relies on a holoplanktonic life cycle and is a self-fertilizing hermaphrodite, releasing eggs and sperm in the ambient water where fertilization occurs [22] [23] [24]. High fecundity and rapid generation times during the whole season may explain why M. leidyi occurs in large numbers in Limfjorden.
The incoming water from the North Sea is of decisive importance for the yearly re-invasion by M. leidyi from the warmer southwestern North Sea, which in cold winters seems to serve as a refuge. In this way Limfjorden functions as a nursery for M. leidyi, which subsequently disperse into Kattegat ( Figure 2

Observations in Kattegat and Great Belt
Only  [28] suggested that transport of M. leidyi by advection from "presumably the North Sea" was the main cause for the ctenophore's occurrence in the Baltic Proper where low salinity prevents reproduction and local recruitment [29].
Between mid-December 2011 and mid-January 2012, an unusual mixture of ctenophores was observed in the inlet to Kerteminde Fjord (Great Belt), namely the native species Pleurobrachia pileus and Bolinopsis infundibulum along with their predator Beroe cucumis, and further, Mnemiopsis leidyi along with two other non-native predators on zooplanktivorous ctenophores, B. ovata and B. gracilis [3]. The presence of the mixture of ctenophores in the Great Belt correlated with replacement of Baltic Sea water with Kattegat water. B. ovata is known as the natural predator of M. leidyi and may be expected to follow the spreading of its natural prey. So far, however, the distribution and densities of M. leidyi in Limfjorden [20] [21] and other Danish waters do not seem to have changed since the arrival of its natural predator.

Occurrence in the Central Baltic Sea
The distribution and abundance of Mnemiopsis leidyi in the central Baltic Sea (Bornholm Basin, Figure 2), which is the most important spawning ground for especially Eastern Baltic cod (Gadus morhua), but also sprat (Sprattus sprattus) [30], has been studied rather intensely [30] [31] [32] [33]. In November 2007, Huwer et al. [30] found that individuals of M. leidyi in the central Baltic were small (body length < 2 cm) and patchily distributed. The highest densities occurred at 40 to 60 m water depth around the halocline, and the highest abundances were found north and west of Bornholm, but they were low compared to the densities observed in Limfjorden [9], and the estimated predation impact on zooplankton by M. leidyi was negligible. Four months later the same area was leidyi showed that it was mostly confined to water layers below the permanent halocline, and Schaber et al. [32] suggested that food limitation plays a major role in the decline of M. leidyi in the central Baltic Sea during summer, and while they considered a self-sustaining population unlikely, they stated that M. leidyi is "most likely re-introduced" every year. Schaber et al. [33] investigated the temporal and spatial overlap of M. leidyi and eggs and larvae of cod and sprat in order to assess the potential impact of the new invader on two of the most important Baltic fish stocks. The spatial overlap between M. leidyi and fish eggs and larvae was found to be low for most of the period observed, although situations with high overlaps were detected, for example for sprat larvae and cod eggs in spring, but on a general level it was concluded that M. leidyi "presently does not have a strong impact" [33].

Origin and Spreading
Recent genetic analyses have revealed some of the possible origins and spreading of

Monitoring and Management
More in-depth knowledge about Mnemiopsis leidyi is of importance, not only for a general understanding of how this invasive ctenophore influences the pelagic dynamics in marine ecosystems, often causing strong trophic cascades [38], but also for marine monitoring programs. Existing monitoring programs that routinely use phyto-and zooplankton as parameters to assess the environment  [44]. But from mid-2017 NOVANA will be supplemented with marine

Conclusion
Ten supplemented with marine sampling for investigation of invasive species, among these will be M. leidyi, by detecting environmental DNA (eDNA).