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.
Reusch, T.B.H., Bolte, S., Sparwel, M., Moss, A.G. and Javidpour, J. (2010) Microsatellites Reveal Origin and Genetic Diversity of Eurasian Invasions by One of the World’s Most Notorious Marine Invader, Mnemiopsis leidyi (Ctenophora). Molecular Ecology, 19, 2690-2699. https://doi.org/10.1111/j.1365-294X.2010.04701.x
Costello, J.H., Bayha, K.M., Mianzan, H.W., Shiganova, T.A. and Purcell, J.E. (2012) Transitions of Mnemiopsis leidyi (Ctenophora: Lobata) from a Native to an Exotic Species: A Review. Hydrobiologia, 690, 21-46. https://doi.org/10.1007/s10750-012-1037-9
Shiganova, T., Riisgard, H.U., Ghabooli, S. and Tendal, O.S. (2014) First Report on Beroe ovata in an Unusual Mixture of Ctenophores in Great Belt (Denmark). Aquatic Invasions, 9, 111-116. https://doi.org/10.3391/ai.2014.9.1.10
Purcell, J.E., Shiganova, T.A., Decker, M.B. and Houde, E.D. (2001) The Ctenophore Mnemiopsis in Native and Exotic Habitats: U.S. Estuaries versus the Black Sea Basin. Hydrobiologia, 451, 145-176. https://doi.org/10.1023/A:1011826618539
Faasse, M.A. and Bayha, K.M. (2006) The Ctenophore Mnemiopsis leidyi A. Agassiz 1865 in Coastal Waters of the Netherlands: An Unrecognized Invasion? Aquatic Invasions, 1, 270-277. https://doi.org/10.3391/ai.2006.1.4.9
Javidpour, J., Sommer, U. and Shiganova, T. (2006) First Record of Mnemiopsis leidyi A. Agassiz 1865 in the Baltic Sea. Aquatic Invasions, 1, 299-302. https://doi.org/10.3391/ai.2006.1.4.17
Caveen, A.J., Fitzsimmons, C., Pieraccini, M., Dunn, E., Sweeting, C.J., Johnsen, M.L., Boomfield, H., Jones, E.V., Lightfoot, P., Gray, T.S., Stead, S.M. and Polunin, N.V.C. (2014) Diverging Strategies to Planning an Ecologically Coherent Network of MPAs in the North Sea: The Roles of Advocacy, Evidence and Pragmatism in the Face of Uncertainty. Advances in Marine Biology, 69, 325-364. https://doi.org/10.1016/B978-0-12-800214-8.00009-8
Riisgard, H.U., Bottiger, L., Madsen, C.V. and Purcell, J.E. (2007) Invasive Ctenophore Mnemiopsis leidyi in Limfjorden (Denmark) in Late Summer 2007—Assessment of Abundance and Predation Effects. Aquatic Invasions, 2, 395-401. https://doi.org/10.3391/ai.2007.2.4.8
Boersma, M., Malzahn, A.M., Greve, W. and Javidpour, J. (2007) The First Occurrence of the Ctenophore Mnemiopsis leidyi in the North Sea. Helgoland Marine Research, 61, 153-155. https://doi.org/10.1007/s10152-006-0055-2
Antajan, E., Bastian, T., Raud, T., Brylinski, J.-M., Hoffman, S., Breton, G., Cornille, V., Delegrange, A. and Vincent, D. (2014) The Invasive Ctenophore Mnemiopsis leidyi A. Agassiz, 1865 along the English Channel and the North Sea French Coasts: Another Introduction Pathway in Northern European Waters? Aquatic Invasions, 9, 167-173. https://doi.org/10.3391/ai.2014.9.2.05
Van Ginderdeuren, K., Hostens, K., Hoffman, S., Vansteenbrugge, L., Soenen, K., De Blauwe, H., Robbens, J. and Vincx, M. (2012) Distribution of the Invasive Ctenophore Mnemiopsis leidyi in the Belgian Part of the North Sea. Aquatic Invasions, 7, 163-169. https://doi.org/10.3391/ai.2012.7.2.002
Vansteenbrugge, L., Regenmortel, T.V., Roch, M.D., Vincx, M. and Hostens, K. (2015) Gelatinous Zooplankton in the Belgian Part of the North Sea and the Adjacent Schelde Estuary: Spatio-Temporal Distribution Patterns and Population Dynamics. Journal of Sea Research, 97, 28-39.
Van Walraven, L., Langenberg, V.T. and van der Veer, H.W. (2013) Seasonal Occurrence of the Invasive Ctenophore Mnemiopsis leidyi in the Western Dutch Wadden Sea. Journal of Sea Research, 82, 86-92.
Van Walraven, L., Daan, R., Langenberg, V. and van der Veer, H.W. (2017) Species Composition and Predation Pressure of the Gelatinous Zooplankton Community in the Western Dutch Wadden Sea before and after the Invasion of the Ctenophore Mnemiopsis leidyi A. Agassiz, 1865. Aquatic Invasions, 12, 5-21. https://doi.org/10.3391/ai.2017.12.1.02
Van der Molen, J., van Beek, J., Augustine, S., Vansteenbrugge, L., van Walraven, L., Langenberg, V., van der Veer, H.W., Hostens, K., Pitois, S. and Robbens, J. (2015) Modelling Survival and Connectivity of Mnemiopsis leidyi in the Southern North Sea and Scheldt Estuaries. Ocean Science, 11, 405-424. https://doi.org/10.5194/os-11-405-2015
Riisgard, H.U., Jaspers, C., Serre, S. and Lundgreen, K. (2012b) Occurrence, Inter-Annual Variability and Zooplankton-Predation Impact of the Invasive Ctenophore Mnemiopsis leidyi and the Native Jellyfish Aurelia aurita in Limfjorden (Denmark) in 2010 and 2011. BioInvasions Records, 1, 145-159. https://doi.org/10.3391/bir.2012.1.3.01
Riisgard, H.U., Madsen, C.V., Barth-Jensen, C. and Purcell, J.E. (2012a) Population Dynamics and Zooplankton-Predation Impact of the Indigenous Scyphozoan Aurelia aurita and the Invasive Ctenophore Mnemiopsis leidyi in Limfjorden (Denmark). Aquatic Invasions, 7, 147-162. https://doi.org/10.3391/ai.2012.7.2.001
Riisgard, H.U., Andersen, P. and Hoffmann, E. (2012c) From Fish to Jellyfish in the Eutrophicated Limfjorden (Denmark). Estuaries and Coasts, 35, 701-713. https://doi.org/10.1007/s12237-012-9480-4
Riisgard, H.U., Goldstein, J., Lundgreen, K. and Lüskow, F. (2015) Jellyfish and Ctenophores in the Environmentally Degraded Limfjorden (Denmark) during 2014 Species Composition, Population Densities and Predation Impact. Fisheries and Aquaculture Journal, 6, 137. https://doi.org/10.4172/2150-3508.1000137
Riisgard, H.U. and Goldstein, J. (2014) Jellyfish and Ctenophores in Limfjorden (Denmark) Mini-Review, with Recent New Observations. Journal of Marine Science and Engineering, 2, 593-615. https://doi.org/10.3390/jmse2040593
Baker, L.D. and Reeve, M.R. (1974) Laboratory Culture of the Lobate Ctenophore Mnemiopsis mccradyi with Notes on Feeding and Fecundity. Marine Biology, 26, 57-62. https://doi.org/10.1007/BF00389086
Jaspers, C., Haraldsson, M., Lombard, F., Bolte, S. and Kiorboe, T. (2013) Seasonal Dynamics of Early Life Stages of Invasive and Native Ctenophores Give Clues to Invasion and Bloom Potential in the Baltic Sea. Journal of Plankton Research, 35, 582-594. https://doi.org/10.1093/plankt/fbt001
Jaspers, C., Costello, J.H. and Colin, S.P. (2015) Carbon Content of Mnemiopsis leidyi Eggs and Specific Egg Production Rates in Northern Europe. Journal of Plankton Research, 37, 11-15. https://doi.org/10.1093/plankt/fbu102
Kellnreitner, F., Pockberger, M., Asmus, R. and Asmus, H. (2013) Feeding Interactions between the Introduced Ctenophore Mnemiopsis leidyi and Juvenile Herring Clupea harengus in the Wadden Sea. Biological Invasions, 15, 871-884. https://doi.org/10.1007/s10530-012-0336-4
Tendal, O.S., Riisgard, H.U. and Jensen, K.R. (2007b) Dræber-Ribbegoplen i danske farvande en folkelig undersogelse. Dyr i natur og museum. Nr. 2, side 2-5. Statens Naturhistoriske Museum, Zoologisk Museum, Kobenhavns Universitet. (In Danish) http://www.e-pages.dk/ku/421/
Riisgard, H.U., Barth-Jensen, C. and Madsen, C.V. (2010) High Abundance of the Jellyfish Aurelia aurita Excludes the Invasive Ctenophore Mnemiopsis leidyi to Establish in a Shallow Cove (Kertinge Nor, Denmark). Aquatic Invasions, 5, 347-356. https://doi.org/10.3391/ai.2010.5.4.03
Haraldsson, M., Jaspers, C., Tiselius, P., Aksnes, D.L., Andersen, T. and Titelman, J. (2013) Environmental Constraints of the Invasive Mnemiopsis leidyi in Scandinavian Waters. Limnology and Oceanography, 58, 37-48. https://doi.org/10.4319/lo.2013.58.1.0037
Jaspers, C., Moller, L.F. and Kiorboe, T. (2011) Salinity Gradient of the Baltic Sea Limits the Reproduction and Population Expansion of the Newly Invaded Comb Jelly Mnemiopsis leidyi. PLoS ONE, 6, e24065. https://doi.org/10.1371/journal.pone.0024065
Huwer, B., Stoor-Poulsen, M., Riisgard, H.U. and Haslob, H. (2008) Abundance, Horizontal and Vertical Distribution of the Invasive Ctenophore Mnemiopsis leidyi in the Central Baltic, November 2007. Aquatic Invasions, 3, 113-124. https://doi.org/10.3391/ai.2008.3.2.1
Storr-Paulsen, M. and Huwer, B. (2008) Changes in Distribution and Lengths of Mnemiopsis leidyi in the Central Baltic Sea between Fall and Spring. Aquatic Invasions, 4, 429-434. https://doi.org/10.3391/ai.2008.3.4.9
Schaber, M., Haslob, H., Huwer, B., Harjes, A., Hinrichsen, H.-H., Koster, F.W., Storr-Paulsen, M., Schmidt, J.O. and Voss, R. (2011a) The Invasive Ctenophore Mnemiopsis leidyi in the Central Baltic Sea: Seasonal Phenology and Hydrographic Influence on Spatio-Temporal Distribution Patterns. Journal of Plankton Research, 33, 1053-1065. https://doi.org/10.1093/plankt/fbq167
Schaber, M., Haslob, H., Huwer, B., Harjes, A., Hinrichsen, H.-H., Storr-Paulsen, M., Schmidt, J.O., Voss, R., Neumann, V. and K?ster, W. (2011b) Spatio-Temporal Overlap of the Alien Invasive Ctenophore Mnemiopsis leidyi and Ichthyoplankton in the Bornholm Basin (Baltic Sea). Biological Invasions, 13, 2647-2660. https://doi.org/10.1007/s10530-011-9936-7
Ghabooli, S., Shiganova, T.A., Zhan, A., Cristescu, M.E., Eghtesadi-Araghi, P. and MacIsaac, H.J. (2011) Multiple Introductions and Invasion Pathways for the Invasive Ctenophore Mnemiopsis leidyi in Eurasia. Biological Invasions, 13, 679-690. https://doi.org/10.1007/s10530-010-9859-8
Ghabooli, S., Tamara, T.A., Briski, E., Piraino, S., Fuentes, V., Thibault-Botha, D., Angel, D.L., Cristescu, M.E. and MacIsaac, H.J. (2013) Invasion Pathway of the Ctenophore Mnemiopsis leidyi in the Mediterranean Sea. PLoS ONE, 8, e81067. https://doi.org/10.1371/journal.pone.0081067
Bolte, S., Fuentes, V., Haslob, H., Huwer, B., Thibault-Botha, D., et al. (2013) Population Genetics of the Invasive Ctenophore Mnemiopsis leidyi in Europe Reveal Source-Sink Dynamics and Secondary Dispersal to the Mediterranean Sea. Marine Ecology Progress Series, 485, 25-36. https://doi.org/10.3354/meps10321
Hosia, A. and Falkenhaug, T. (2015) Invasive Ctenophore Mnemiopsis leidyi in Norway. Marine Biodiversity Records. https://doi.org/10.1017/S1755267215000044
Tiselius, P. and Møller, L.F. (2017) Community Cascades in a Marine Pelagic Food Web Controlled by the Non-Visual Apex Predator Mnemiopsis leidyi. Journal of Plankton Research, 39, 271-279.
Hansson, H.G. (2006) Ctenophores of the Baltic and Adjacent Seas—The Invader Mnemiopsis Is Here! Aquatic Invasions, 1, 295-298. https://doi.org/10.3391/ai.2006.1.4.16
Oliveira, O.M.P. (2007) The Presence of the Ctenophore Mnemiopsis leidyi in the Oslofjorden and Considerations on the Initial Invasion Pathways to the North and Baltic Seas. Aquatic Invasions, 2, 185-189. https://doi.org/10.3391/ai.2007.2.3.5
Møller, J.S. (1996) Water Masses, Stratification and Circulation. In: Jørgensen, B.B. and Richardson, K., Eds., Eutrophication in a Coastal Marine Ecosystem. American Geophysical Union, Coastal and Estuarine Studies, Vol. 52, 51-66. https://doi.org/10.1029/CE052p0051
Javidpour, J., Molinero, J.C., Peschutter, J. and Sommer, U. (2009a) Seasonal Changes and Population Dynamics of the Ctenophore Mnemiopsis leidyi after Its First Year of Invasion in the Kiel Fjord, Western Baltic Sea. Biological Invasions, 11, 873-888. https://doi.org/10.1007/s10530-008-9300-8
Javidpour, J., Molinero, J.C., Lehmann, A., Hansen, T. and Sommer, U. (2009) Annual Assessment of the Predation of Mnemiopsis leidyi in a New Invaded Environment, the Kiel Fjord (Western Baltic Sea): A Matter of Concern? Journal of Plankton Research, 31, 729-738. https://doi.org/10.1093/plankt/fbp021
Stæhr, P.A., Jakobsen, H.H., Hansen, J.L.S., Andersen, P., Storr-Paulsen, M., Christensen, J., Lundsteen, S., Göke, C. and Carausu, M.-C. (2016) Trends in Records and Contribution of Non-Indigenous Species (NIS) to Biotic Communities in Danish Marine Waters. Scientific Report from DCE Danish Centre for Environment and Energy, No. 179, 44. http://dce2.au.dk/pub/SR179.pdf
Thomsen, P.F. and Willerslev, E. (2015) Environmental DNA—An Emerging Tool in Conservation for Monitoring Past and Present Biodiversity. Biological Conservation, 183, 4-18.
Thomsen, P., Kielgast, J., Iversen, L.L., Wiuf, C., Rasmussen, M., Gilbert, M.T.P., Orlando, L. and Willerslev, E. (2012) Monitoring Endangered Freshwater Biodiversity using Environmental DNA. Molecular Ecology, 21, 2565-2573. https://doi.org/10.1111/j.1365-294X.2011.05418.x
Sigsgaard, E.E., Nielsen, I.B., Bach, S.S., Lorenzen, E.D., Robinson, D.P., Knudsen, S.W., Pedersen, M.W., Jaidah, M.A., Orlando, L., Willerslev, E., Møller, P.R. and Thomsen, P.F. (2016) Population Characteristics of a Large Whale Shark Aggregation Inferred from Seawater Environmental DNA. Nature Ecology & Evolution, 1, 4.