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The presence of the ctenophore Mnemiopsis leidyi in the Oslofjorden and considerations on the initial invasion pathways to the North and Baltic Seas


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The ctenophore Mnemiopsis leidyi is one of the most cited invasive species due to its ecological and economical impact that occurred in the Black and Caspian Seas in previous years. Recent reports on the species presence in the Baltic and North Seas alarmed the scientific community. While the earliest report of M. leidyi in the Baltic was spring 2006, this paper indicates of the presence of M. leidyi in the Oslofjorden as early as autumn 2005, and discusses the possible invasion routes from its native range along the eastern coast of the Americas.
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Aquatic Invasions (2007) Volume 2, Issue 3: 185-189
DOI 10.3391/ai.2007.2.3.5
© 2007 The Author(s)
Journal compilation © 2007 REABIC (
This is an Open Access article
Research article
The presence of the ctenophore Mnemiopsis leidyi in the Oslofjorden and
considerations on the initial invasion pathways to the North and Baltic Seas
Otto M. P. Oliveira
Departamento de Zoologia, Instituto de Biociências, Universidade de São Paulo, R. Matão, Trav. 14, 101, 05508-900, São Paulo,
SP, Brazil and Centro de Biologia Marinha, Universidade de São Paulo, Rodovia Manoel H. do Rego km 131,5, 11600-000, São
Sebastião, SP, Brazil
Received 22 May 2007; accepted in revised form 8 August 2007
The ctenophore Mnemiopsis leidyi is one of the most cited invasive species due to its ecological and economical impact that
occurred in the Black and Caspian Seas in previous years. Recent reports on the species presence in the Baltic and North Seas
alarmed the scientific community. While the earliest report of M. leidyi in the Baltic was spring 2006, this paper indicates of the
presence of M. leidyi in the Oslofjorden as early as autumn 2005, and discusses the possible invasion routes from its native range
along the eastern coast of the Americas.
Key words: Ctenophora, invasive species, ballast water, North Atlantic Current, transoceanic spread
Mnemiopsis leidyi A. Agassiz, 1865 is a lobate
ctenophore, originally distributed along the
American coast of the Atlantic, from
Narragansett Bay, USA (41°N) to the Valdez
Peninsula, Argentina (42°S) (GESAMP 1997).
The species is a simultaneous hermaphrodite,
able to perform self-fecundation (Oliveira and
Migotto 2006) and can live and reproduce in a
wide salinity and temperature range (Baker and
Reeve 1974, GESAMP 1997), inhabiting both
coastal and estuarine waters (GESAMP 1997).
Feeding mainly on copepods, fish eggs and
larvae (Burrell and Van Engel 1976), M. leidyi
populations rapidly grow when food is abundant
(cf. Purcell et al. 2001).
In the early 1980’s, M. leidyi was introduced
in the Black Sea, possibly transported from its
native area in ships ballast water (cf. GESAMP
1997). Due to suitable living conditions in the
receiving environment (food abundance and a
physical environment similar to the estuaries in
the native region), populations established and
grew in the Black Sea (Purcell et al. 2001,
Shiganova et al. 2001). The absence of potential
predators and the favorable environmental
conditions allowed M. leidyi to spread along the
Black Sea and adjacent areas, as the Azov,
Marmara and northeastern Mediterranean Seas
(GESAMP 1997, Shiganova et al. 2001, Kideys
2002). The invasion of the Caspian Sea occurred
in the mid of the 1990’s. The species was likely
transported in ballast water through the Volga-
Don Channel (Kideys 2002, Bilio and Niermann
Otto M. P. Oliveira
2004). Population explosions in the Black Sea
were observed in 1989 and 1995 (Shiganova et
al. 2001), a time period when fisheries of
anchovies drastically decreased in the area, appa-
rently due to trophic competition and feeding on
eggs and larvae by M. leidyi (GESAMP 1997).
Overfishing and increased water pollution have
also contributed to the decline of the Black Sea
In the late 1990’s, another ctenophore, Beroe
ovata Chamisso & Eysenhardt, 1821 (a natural
predator of M. leidyi), was accidentally intro-
duced to the Black Sea. Its predatory behavior
resulted in a biological control of the M. leidyi
population and an environmental equilibrium
(Shiganova et al. 2001, Kideys 2002). Nowa-
days, M. leidyi populations are still affecting the
environmental equilibrium in the Caspian Sea,
due to the absence of the predator B. ovata there
(Stone 2005).
The recent reports on the M. leidyi presence in
the Baltic (Javidpour 2006, Hansson 2006, Kube
et al. 2007) and North Sea (Faasse and Bayha
2006, Boersma et al. 2007) alarmed the scientific
community, since those are some of the most
productive fishery areas of the world. The region
presents large rates of catch on planktotrophic
fishes as herrings (Jansson 2003, Sea Around Us
Project 2007), which were probably the most
affected organisms by M. leidyi populations
explosions in the Black Sea. Along the Dutch
coast, sole and plaice fisheries are also
potentially threatened during high abundances of
M. leidyi (Faasse and Bayha 2006).
This study presents new records of M. leidyi
for the Oslofjorden (Norway) and discusses
possible invasion routes from its native range
along the eastern coast of the Americas.
Materials and Methods
Mnemiopsis leidyi specimens were photographed
underwater by two divers in the Oslofjorden
between November 2005 and March 2007 (see
Annex). Photos were taken in dives up to 10 m
depth. Most of the register locations are on the
east side of the fjord (except for the Pumpehuset,
Drøbak register), areas constantly affected by
inflow streams with higher temperatures (see
Meteorologisk Institutt 2007).
Water surface temperatures at the register
locations were obtained from Bundesamt für
Seeschifffahrt und Hydrographie (2007) and
Meteorologisk Institutt (2007).
All M. leidyi specimens photographed were
adults, with a least 40 mm in length, biggest up
to 80 mm. The position of the lobes insertion,
near the aboral end of the body, attests the
identification of the species (see Figure 1).
One of the divers (Vidar Aas) goes into water
weekly, from March to November, every year.
He observed more specimens in each location
and in other occasions than that when they were
photographed. However, dense agglomerations
of adult comb-jellies were never observed in
those locations.
At the register moments, water surface
temperatures ranged from 4–11°C (Annex).
Faasse and Bayha (2006) reported the presence
of M. leidyi in the Netherlands estuarine regions
and suggested that this species may have
occurred in the area for several years, being
misidentified as Bolinopsis infundibula (O.F.
Müller, 1776), another lobate ctenophore. The
authors also suggested that the resident
population from Dutch estuaries could serve as a
yearly supply of M. leidyi to the Baltic Sea,
through Skagerrak and Kattegat or through the
Kiel Canal, if the species was unable to
overwinter in the Baltic (Faasse and Bayha
2006). Hansson (2006) agreed with the
hypothesis of an annual reintroduction in the
Baltic Sea originating from the North Sea.
However, Kube et al. (2007) data suggested that
M. leidyi survived the last winter in the southern
Baltic Sea.
The specimens observed in the Oslofjorden in
different localities and occasions (see Figure 1,
Annex), are well developed adults, suggesting
that reproduction could have been occurring in
the Oslofjorden. However, I cannot infer that the
Oslofjorden population of M. leidyi survived the
winters, once temperature reached about 1°C or
lower in these periods (Bundesamt für
Seeschifffahrt und Hydrographie 2007). A low
temperature of that magnitude kills the Azov Sea
population of M. leidyi every winter and the sea
is reinvaded by Black Sea specimens in spring
(Studenikina et al. 1991). It is therefore possible
that the Faasse and Bayha (2006) hypothesis of
annual reintroduction is correct for the popu-
lation in the Oslofjorden. However, it is unclear,
Mnemiopsis leidyi in the Oslofjorden
Figure 1. Mnemiopsis leidyi from the Oslofjorden, Norway. A-D, specimens observed in November, 2005; E, specimen observed
in May, 2006; F, specimen observed in November, 2006; G, specimen observed in March, 2007. Arrows indicate the position of
the lobes insertion, near the aboral end of the body in M. leidyi, a distinctive characteristic from Bolinopsis infundibula (other
lobate ctenophore that also occurs in the Oslofjorden). Photos by Vidar Aas (A–B, E–G) and Asbjørn Hansen (C–D)
Otto M. P. Oliveira
for instance, if the annual reinvasion of the
Oslofjorden population originates from Baltic or
the North Sea comb jelly populations.
Regarding the possible transport of M. leidyi
from the native areas to the North and Baltic
Seas, Faasse and Bayha (2006) suggest the
possibility of the ballast water transfer once two
of the largest European ports (Antwerp and
Rotterdam), are near to the Dutch estuaries
where they found M. leidyi. However, I believe
that a discussion on natural oceanic transport, by
the North Atlantic Current (NAC) should be
considered. A recent study shows that surface
water temperature of the North Atlantic,
including the NAC, was more than 1°C warmer
in the last five years than the historical means
(cf. Hughes and Holliday 2006). The superficial
waters of NAC arrive in the UK coast and in the
North Sea with mean temperatures above 9°C
(Hughes and Holliday 2006). The North Sea also
presented an increase in mean water tempera-
tures of more than 2°C in the last five years
(Hughes and Holliday 2006). Possibly the low
temperatures, a major factor against the trans-
oceanic spread of M. leidyi, has not been the
problem in recent years due to water temperature
increase in NAC and in the recipient regions.
Mnemiopsis leidyi is known as a coastal
ctenophore (GESAMP 1997, Mianzan 1999).
However, there are some specimen records in
oceanic waters (Harbison et al. 1978), including
localities inside the inflow of the Gulf Stream
(see collecting stations 509, 510 and 584 in
Harbison et al. 1978). This certainly indicates
the potential of transoceanic spreading of M.
leidyi. The drifting from the northern Gulf
Stream to the North Sea, through NAC, can least
15 to 60 days, depending on the season and wind
oscillations (cf. Siedler et al. 2001). In a recent
study, adult specimens were able to live up to 17
days under starvation in the laboratory (Oliveira
2007). In natural conditions of a transoceanic
current, it is not expected that coastal species
have an adequate food supply as natural coastal
waters prey organisms are either absent or occur
in much lower abundance. However, considering
the M. leidyi predatory potential and trophic
plasticity, I expect this ctenophore to be capable
of such a way of dispersal. Furthermore, this
transoceanic spread hypothesis needs to be
The absence of M. leidyi records on the U.K.
coast, as well as in the Atlantic coasts of France
and Spain, are negative arguments for such
hypothesis. However, the species could just have
been misidentified as Bolinopsis infundibula in
the area, as previously occurred in other
European seas (Faasse and Bayha 2006, Boersma
et al. 2007). Samplings on the NAC, associated
with molecular identification of M. leidyi
lineages should be of great value to elucidate this
The presence of M. leidyi have been recorded
for several points along the North and Baltic seas
(see Kube et al. 2007, Figure 3), proving once
more the species great spreading potential. Now,
studies on its trophic interaction with native
plankton organisms of such regions are strongly
needed to evaluate its possible ecological and
economical impacts
I gratefully acknowledge the two anonymous
referees for their important suggestions and
contributions, and two divers Asbjørn Hansen
and Vidar Aas for allowing the use of their
photos. This study was supported by FAPESP
(Proc. 2004/15300-0).
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Records of the ctenophore Mnemiopsis leidyi in the Oslofjorden waters
Location coordinates
Latitude,°N Longitude,°E
Date of record
Figure Photographer
59º50'40" 10º38'40" November 14, 2005 ~10°C
Vidar Aas
59º48'18" 10º36'24" November 14, 2005 ~10°C
Vidar Aas
59º39'00" 10º36'00" November 17, 2005 ~10°C
Asbjørn Hansen
59º35'00" 10º39'00" November 20, 2005 ~7°C
Asbjørn Hansen
59º46'50" 10º35'38" May 5, 2006 ~11°C
Vidar Aas
Alvern, Akershus 59º49'06" 10º36'04" November 16, 2006 ~9°C
Vidar Aas
Alvern, Akershus 59º49'06" 10º36'04" March 20, 2007 ~4°C
Vidar Aas
* Approximated values for water surface temperature, based on Bundesamt für Seeschifffahrt und Hydrographie (2007)
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... The invasive success of these groups has been attributed to their generalist (eurytolerant and omnivorous) and r-selected life-history characteristics (Hänfling et al., 2011 and references therein). Shipping has been highlighted as one of the most important pathways of introduction, both via ballast water and biofouling, for all these taxonomic groups (Oliveira, 2007;Molnar et al., 2008;Hänfling et al., 2011;Ruiz et al., 2015). ...
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The Canadian Arctic is receiving increased ship traffic, largely related to non-renewable resource exploitation and facilitated by climate change. This traffic, much of which arrives in ballast, increases opportunities for the spread of aquatic invasive species (AIS). One of the regions at greatest risk is the Hudson Bay Complex. A horizon scanning exercise was conducted using the semi-quantitative Canadian Marine Invasive Screening Tool (CMIST) to identify AIS of potential concern to the region. This screening-level risk assessment tool, uses documented information to answer questions related to the likelihood and impact of invasion. Species were analyzed by ecological categories (zoobenthos, zooplankton, phytobenthos) and taxonomic groups, with 14 species (out of 31) identified as being of highest relative risk. Crabs, mollusks, macrozooplankton and macroalgae were the taxonomic groups with the highest overall risk scores, through a combination of higher likelihood of invasion and impact scores relative to other taxa. Species that may pose the highest AIS risk are currently mainly distributed on the east and west coasts of the North Atlantic Ocean. Their distributions coincide with source ports and shipping pathways that are well connected to the Hudson Bay Complex. This first horizon scan to identify potential high-risk AIS for the Canadian Arctic incorporated two novel approaches into the CMIST analysis: i) use of the tool to assess two new ecological categories (phytobenthos and zooplankton), and ii) use of averaged CMIST results to interpret general risk patterns of ecological categories. This study is also the first to use CMIST scores to highlight common source regions and connected ports for the highest risk species. In a scenario of climate change and increasing ship traffic, this information can be used to support management actions such as the creation of watch lists to inform adaptive management for preventing AIS establishment, and mitigating associated environmental and economic impacts.
Technical Report
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Norway has the second longest coastline in the world, and it is challenging to monitor non-indigenous marine species (NIMS) along the entire shore including the Norwegian areas in the Barents Sea and along Svalbard. There is currently no national program for such monitoring, however some activity is taking place on specific species and organism groups which is presented here. Historically transport of NIMS is ballast water have been the main pathway into the Norwegian coast, but with the implementation of the Ballast Water Convention this risk is minimized. Biofouling on vessels coming into the Norwegian coast is thus considered to be the most important vector for marine introduction of new species. An analysis of the frequency and origin (last port call) for 158 000 vessel arrivals into Norwegian ports in the period 2020-2021 is presented. The results show that the Oslofjord area and the west coast is the areas with highest risk for marine introductions by vessels. Other vectors for such introductions into Norway are evaluated like the increasing amount of floating debris which can carry fouling organisms, larvae and eggs to new areas. An analysis of historical data for the established NIMS in Norway show that the southern area of Norway is most susceptible to new species. This pattern is not only dependent on the vector pressure but also reflects the temperature gradient northwards along the coast. Measures for prevention of new species to arrive and management of problematic species is also discussed.
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The ctenophore Mnemiopsis leidyi, ranking among the 100 most damaging bioinvaders in the world, is a major predator of zooplankton, known to alter the biodiversity and functioning of the ecosystems in which it has been introduced. This first survey on the trophic relationships of M. leidyi in a Mediterranean lagoon (Berre, South of France) was performed through stable isotope analyses. Carbon and nitrogen isotope composition was used (1) to determine the types of prey ingested by this ctenophore and (2) the influence of individual size on its isotope composition, and (3) to make assumptions to explore its trophic relationships with the native jellyfish Aurelia sp.. The two gelatinous species ingested mainly (79% to 97% of the diet) planktonic prey (copepods, cirriped nauplii, gastropod larvae and cladocerans), but also preyed upon benthic organisms (mainly harpacticoid copepods and the amphipod Monocorophium insidiosum) in lower proportions (2.5% to 21%). Size-related changes in M. leidyi diet were evidenced with an increase in trophic level and benthic prey consumption in the larger individuals. These two gelatinous organisms probably play an important and underestimated role in the benthic–pelagic coupling in coastal lagoons by transferring benthic organic matter to the pelagic food webs.
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The ctenophore fauna of Brazilian coast is poorly known. Only one planktonic species, Mnemiopsis leidyi, was previously recorded for the southeastern coast. The present study describes and gives some biological notes of this and four other species (Beroe ovata, Bolinopsis vitrea, Leucothea multicornis, and Ocyropsis crystallina) that occur in the area, two of which (B. vitrea and L. multicornis) are new records for the subtropical southwestern Atlantic.
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* Corresponding author Received 24 May 2007; accepted in revised form 4 June 2007 Abstract In autumn 2006 the first observations of the West Atlantic comb jelly Mnemiopsis leidyi in Northern Europe were reported from the North Sea, the Skagerrak and the south-western Baltic Sea. Here we report on the further spread of this invasive ctenophore from the south-western towards the central Baltic Sea up to the south eastern Gotland Basin during autumn/winter 2006 and spring 2007. The abundances were generally low (max. 4 ind. m -3 ). While M. leidyi was located in the entire water column in Kiel Bight, it was found exceptionally below the halocline in the deep stratified central Baltic basins. Data of a weekly sampling program at a near shore sampling location in Mecklenburg Bight between January and May 2007 showed that up to 80 % of the individuals were juveniles, smaller than 1 mm total body length and that M. leidyi survived the winter in the Southern Baltic Sea, even if abundances dropped down to
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The appearance in 1995 of the alien ctenophore Mnemiopsis leidyi in the Caspian Sea attracted considerable attention. In 1989, enormous mass development of the comb jelly in the Black Sea coincided with a breakdown of the commercially highly important anchovy Engraulis encrasicholus fishery in this area. This collapse probably resulted from multiple factors, among which overfishing and food competition from the ctenophore played a prominent role, enhanced by predation of M. leidyi on the early life stages of the anchovy. The abundance of food due to the depletion of the anchovy stock permitted a population explosion of M. leidyi. Subsequently, reduced fishing pressure allowed recovery of the anchovy stock and led to a new peak in anchovy landings in the Black Sea in 1995. Moreover, changes in the atmospheric and oceanic patterns in the northern hemisphere in the second half of the 1980s could have altered the composition of the phyto- and zooplankton communities and thus the food base of the small pelagic fish species. In the second half of the 1990s, the invasion of the Black Sea by another alien ctenophore, Beroe ovata, preying on ctenophores, raised hopes that this species could control M. leidyi. However, it is uncertain whether the preying capacity of B. ovata is sufficient to control M. leidyi in other than locally favourable conditions. At the present time, judging by the recent development in the Black Sea, the invasion of the Caspian Sea by M. leidyi does not yet seem to have reached maximum intensity. More consistent and comparable investigations in the Caspian and Black Seas are necessary to improve the protection, management and exploitation of the stocks of small pelagic fishes, in order to increase their capacity to resist and survive unforeseeable disturbances of their ecosystem by invaders such as M, leidyi. In view of the new invasion by this ctenophore, research into the exploitation of the Caspian kilka Clupeonella spp. stocks and the influence of atmospheric teleconnection patterns on ecological conditions in the Caspian Sea is of particular importance.
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The introduction of the American ctenophore Mnemiopsis leidyi to the Black Sea was one of the most dramatic of all marine bioinvasions and, in combination with eutrophication and overfishing, resulted in a total reorganization of the pelagic food web and significant economic losses. Given the impacts this animal has exhibited in its invaded habitats, the spread of this ctenophore to additional regions has been a topic of much consternation. Here, we show the presence of this invader in estuaries along the Netherlands coast, based both on morphological observation and molecular evidence (nuclear internal transcribed spacer region 1 [ITS-1] sequence). Furthermore, we suggest the possibility that this ctenophore may have been present in Dutch waters for several years, having been misidentified as the morphologically similar Bolinopsis infundibulum. Given the level of shipping activity in nearby ports (e.g. Antwerp and Rotterdam), we find it likely that M. leidyi found its way to the Dutch coast in the ballast water of cargo ships, as is thought for Mnemiopsis in the Black and Caspian Seas. Given the magnitudes of the impacts this ctenophore has shown in its native and introduced ranges, the animal's potential effects on the North Sea pelagic ecosystem and fisheries warrant close observation in the coming years. The development of large ctenophore aggregations during recent years was probably driven by the higher than average sea surface temperatures in the North Sea, and we hypothesize that populations from the southern North Sea may have been the source of the invaders reported in the Baltic Sea and the Skagerrak. If these northern populations (Baltic, etc.) are not able to over-winter, it is possible that established populations in southern North Sea estuaries may serve as a yearly supply of invaders to the colder waters to the north.
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The invasive ctenophore Mnemiopsis leidyi was first recorded in the Kiel Bight (western Baltic Sea) on 17 October 2006 during a regular weekly sampling program. The M. leidyi abundance gradually increased from 29.5±12.7 ind.m -3 in mid-October to 92.3±22.4 ind.m -3 in late November 2006. The occurrence of M. leidyi in the Baltic Sea is of great concern as this invader has caused negative impacts in the southern seas of Europe.
The tentaculate ctenophore, Mnemiopsis leidyi, occurred in the York River estuary, Virginia, throughout a 22-month study, August 1965–May 1967. It was found in salinities above 15 ‰ in winter, but in less than 6 ‰ in late summer. Numbers of small plankters, such as copepods and the larvae of annelids, mollusks and barnacles, varied inversely with the volume of ctenophores present. Stomodaeum analyses confirmed M. leidyi as a predator of these plankters. Plankters greater than 6 mm in length were not preyed upon.Most of the fishes using this estuary as a nursery ground were large enough on entering ctenophore-infested waters to avoid predation. Young fishes in the area subsisted chiefly on organisms not preyed on by M. leidyi.Another ctenophore, Beroe ovata, preyed on M. leidyi during summer and fall to such an extent that the tentaculate species was restricted to areas outside the range of the beroid. The medusa Chrysaora quinquecirrha also preyed on M. leidyi but did not noticeably reduce its numbers.
The invasive ctenophore Mnemiopsis leidyi has been detected during late summer and autumn 2006 in northern Europe. So far it has been found in the southern North Sea area, along the Swedish west coast, in the southwestern Baltic and along the south and southwestern Norwegian coasts. From the large populations observed, it is obvious that it must have been introduced before 2006, but remained unrecorded until this autumn.