We describe the first record of the hydromedusan jellyfish Blackfordia virginica from northern Europe, namely Kiel Canal and Kiel Fjord, southwestern Baltic Sea. So far, standardised sampling of gelatinous zooplankton in other parts of the Baltic Sea has not detected the species during the past 10 years. The persistent presence of medusae populations during summer/autumn and seasonal variation in their size distribution in Kiel Canal indicate active recruitment throughout summer since 2016.
Concerning the question of the initial introduction vector to the Kiel Canal, there is no direct evidence. Shipping has been suggested to be the major cause for the primary introduction of non-indigenous species in the marine realm, both due to ballast water release and from fouling communities on ship hulls . No ballast water release takes place in Kiel Canal and the water exchange with the North Sea and Baltic Sea is limited. Hence, it is most likely that B. virginica has been introduced as part of a fouling community on container ships. Kiel Canal is the most navigated, busiest artificial shipping route world-wide with 32,100 commercial ship passages in 2015. In comparison, Panama and Suez Canals had commercial ship passages of 14,000 and 17,500, respectively .
The occurrence of B. virginica in Kiel Canal ranged over a salinity gradient from 7 to 13. In other invaded areas of southwestern Europe, B. virginica has been observed in near fresh water conditions  and up to a salinity of 27.5 . Similarly, in south American estuaries of Brazil, medusae have been documented to occur over a very large salinity range from 2.2 to 36 . This indicates that B. virginica is a truly euryhaline species, which successfully invaded a multitude of estuaries and brackish-water systems around the world (for review see ). It also suggests that the species has a high potential to colonise most of the Baltic Sea.
In other areas of Europe, the peak abundance of B. virginica medusa occurs in summer [27, 32]. For example, in Portugal, the species occurred at temperatures above 12 °C  and similarly, in Gironde Estuary, France between 14.5 to 26.6 °C . In the US, the species has been observed at temperatures above 16.5 °C . In Kiel Canal, animals were recorded at temperatures above 14 °C, matching what has been observed elsewhere. One observation where animals were recorded at a temperature of 8.7 ± 0.3 °C (on 20 April 2016) is exceptional. The observed specimen were mostly large-sized, with an average size being twice as large as observed during the first occurrence in 2017. This suggests that they were produced and grew up in warmer (possibly shallower) sections of the Kiel Canal and subsequently drifted to our sampling station. This is further supported by the observation that B. virginica disappeared from the sampling station for 1.5 months and was first recorded again by mid June, when animals remained in Kiel Canal throughout summer and autumn. During October 2016, an extraordinary shift in the size distribution along with peak abundance of very small medusae were observed, which may be caused by a new cohort, which did not manage to grow up due to temperature effects. This has similarly been observed in the Gironde estuary, France, where 70% of the population consisted of 2 mm sized B. virginica during October before medusae disappeared from the water column .
B. virginica is a hydromedusa, characterised by an alternation between benthic and pelagic life forms. Medusa production from benthic polyps has been suggested to occur throughout the entire summer period in northwestern America [20, 34]. Locations of polyp settling in Kiel Canal remain to be identified. However, recurrent introductions due to ship hull fouling are not likely to have led to the high B. virginica medusa densities observed during recent years. Additionally, the change in size-frequency distribution further indicates an active recruitment at the sampling site in Kiel Canal during summer and autumn, similar to observations in northwestern America .
We further confirm B. virginica from Kiel Bight. This indicates that the high abundances in the Kiel Canal are likely to have seeded animals into the southwestern Baltic Sea, but it remains to be seen if they establish permanent populations in this area. During summer, water temperatures in the Baltic Sea are within the preferred temperature range of B. virginica in Kiel Canal and other invaded areas around the world. This suggests that the Baltic Sea offers ample opportunities for the establishment of self-recruiting populations. Furthermore, rocky coastlines offer plenty of hard substrate for the settlement of polyps, along with the additional availability of hard substrate as a consequence of increasing offshore wind farm structures in the Baltic Sea. We therefore suggest that B. virginica can be regarded as a highly potent non-indigenous species with a large colonisation potential for the Baltic Sea. In other areas of Europe, it has been documented that high abundances of B. virginica (650 to 1700 ind. m−3) have a negative impact on food web structure and functioning [27, 35]. Blackfordia virginica pose a high grazing pressure on local zooplankton standing stocks  as well as fish larvae . The latter is further confirmed by non-quantitative observations from Kiel Canal during summer where a high percentage of B. virginica with ingested gobiid fish larvae were observed (Jaspers, Wendt pers. observation). Therefore, it can be expected that a further spreading and abundance increase of B. virginica may not only result in food competition with local pelagic fish stocks but additionally cause direct predation on fish early life stages. Particularly, this may pose a high risk if B. virginica is spreading further into the Baltic Sea and reaches the deeper areas like the Bornholm, Gdansk and Gotland basins, as these are the major spawning grounds of commercially important fish stocks such as sprat and cod. A similar impact on early life stages of fish had been hypothesised when another non-indigenous comb jelly, Mnemiopsis leidyi, was first recorded in the Baltic Sea. Even though M. leidyi is holoplanktonic, they do not form self-sustaining populations in most areas of the Baltic Sea due to salinity constraints on reproduction rates . Therefore, the spatio-temporal overlap with early life stages of commercially important fish is limited . In contrast, the present study shows that B. virginica is not constrained by low salinity and occurs in the Kiel Canal from early summer throughout autumn. Therefore, negative impacts on fish recruits could be expected via food competition as well as direct predation, especially for coastal areas of the Baltic, where e.g. herring is spawning . This highlights that the new record of this non-indigenous species in northern Europe is a matter of concern for low saline environments.
In addition to the finding of B. virginica medusae, the non-indigenous comb jelly Mnemiopsis leidyi was observed twice during the 8 year investigation period in Kiel Canal. This is the first record of M. leidyi in Kiel Canal. Specifically, animals were recorded during September 2010 and December 2015, but at low densities of 0.1 and 0.01 ind. m−3, when the average salinity was 10.7 ± 0.1 and 10.8 ± 2.7, respectively (pers. comm. Jaspers). This occasional occurrence of M. leidyi in the Kiel Canal indicates that animals do not form self-sustaining populations there. This is likely due to the low salinity, which is below the threshold where M. leidyi do reproduce and multiply [36, 39]. The exceptional high salinity observed during the recordings of M. leidyi in the Kiel Canal further indicates that those animals are likely to be seeded from the Kiel Bight via inflow of higher saline waters. This is further substantiated as M. leidyi was present in Kiel Bight during that time period .
Some invasive jellyfish e.g. Phyllorhiza punctata, Cassiopea andromeda, Rhopilema nomadica, Aurelia spp. , as well as the comb jelly Mnemiopsis leidyi , are well known to reach a global distribution and to have large impact on local ecosystems. Nevertheless, they remain under-represented in ongoing monitoring activities and non-indigenous species databases. This inconsistency is primarily attributed to the overall lack of systematic long term data acquisition for gelatinous macrozooplankton . In this study, we have used a comparable methodology over an investigation period of ca. 10 years covering large areas of the Baltic Sea. We highlight that monitoring activities including less well studied organism groups such as gelatinous zooplankton are important to establish baselines for the detection of non-indigenous species at an early stage of their colonisation. Further, we conclude that not only large gelatinous zooplankton species such as Mnemiopsis leidyi or Phyllorhiza punctata are a matter of concern as non-indigenous species, but also small species such as B. virginica. The Baltic Sea, especially coastal areas, constitutes suitable habitat for B. virginica and the Baltic Sea is thus confronted with a high invasion risk for this non-indigenous jellyfish species.