Distribution and abundance of the American comb jelly ( Mnemiopsis leidyi )-a rapid invasion to the northern Baltic Sea during 2007

The invasive ctenophore Mnemiopsis leidyi is expanding its range in the Baltic Sea. It was observed in the Landsort Deep in March 2007 and in the western part of the Gulf of Finland, Åland and Bothnian Seas in August 2007. In September the abundances were several times higher in the Gulf of Finland and the Åland Sea than in August, maximum abundances being 694 ind m, and densities 24 ind m. In December M. leidyi was observed also in the eastern Gulf of Finland at low abundances. Eggs and newly hatched larvae as well as juvenile stages were observed during autumn, indicating efficient reproduction and establishment of the species in the northern Baltic Sea.

Aquatic invasive species (AIS) have been identified globally as one of the major threats to marine ecosystems, causing biodiversity loss and adverse environmental, economic and social impacts (Leppäkoski et al. 2002b).The ecosystem of the Baltic Sea is young and simple, hence vulnerable to ecological changes.For invasive species this provides an opportunity to find a vacant ecological niche and consequently establish permanent populations (Leppäkoski et al. 2002a).
A recent invader to the Baltic Sea is the American comb jelly, Mnemiopsis leidyi A. Agassiz 1865.This species originates from the estuaries and coastal regions of the east coast of North, Central and South America (Purcell et al. 2001).It is known to have wide tolerance for environmental conditions, including 2-38 ‰ salinities, 2-32 ºC temperatures and low oxygen, which enables effective spreading to new areas (Purcell et al. 2001).M. leidyi has been unintentionally introduced with ballast water to the Black Sea in 1982 (Vinogradov et al. 1989) and later, in 1999, to the Caspian Sea (Ivanov et al. 2000).The first records of M. leidyi in northern European waters were made in 2005 from Oslofjorden, Norway (Oliveira 2007), then later in the North Sea, off the western coast of the Netherlands (Faasse and Bayha 2006), and from the Swedish west coast, in the Kattegat region and the southern Baltic Sea in autumn 2006 (Javidpour et al. 2006, Hansson 2006).In 2006-2007 it was widely distributed in the Danish waters (Tendal et al. 2007).
M. leidyi is a simultaneous hermaphrodite capable of self-fertilization.Thus, under suitable conditions its abundance may increase rapidly, forming mass occurrences usually in late summer and autumn (reviewed by Purcell et al. 2001, Shiganova et al. 2001).M. leidyi was able to overwinter in low abundances (1-4 ind m -3 ) in the southern Baltic at temperatures below 10 ºC (Kube et al. 2007).It has extended its distribution range from the south-western to the central Baltic Sea (the Gotland Basin) between summer 2006 and spring 2007 (Kube et al. 2007).
Here we report on the further spread of M. leidyi from the central to the northern Baltic Sea during spring, summer and autumn 2007.In March 2007 samples were taken using a WP-2 net (mesh size 90 μm) in vertical hauls from 100 to 60 m, 60 to 30 m and 30 to the surface in the Landsort Deep (Swedish National Monitoring Program).Further sampling was undertaken on a regular HELCOM monitoring cruises at R/V Aranda (Finnish Institute of Marine Research, FIMR) in August, September and December 2007.The samples were taken by vertical hauls using a WP-2 net (mesh size 500 μm) from the bottom to the halocline and from the halocline to the surface, according to the temperature and salinity profiles measured by CTD casts at each sampling site.At the stations where oxygen was depleted in deeper waters, the vertical tows were taken only from well-oxygenated water, from 50 m to the surface.In December the tows were taken from the bottom to the surface.At stations where M. leidyi was most abundant in September, additional vertical tows (WP-2 net, mesh size 100 μm) were taken in order to assess the abundance of eggs.1a).The increase in M. leidyi abundance was observed one week later (at the beginning of September).The maximum abundances of 508 ind m -2 and 694 ind m -2 were then observed in the western part of the Gulf of Finland and the Åland Sea, respectively (Figure 1b).In December M. leidyi was observed at low abundances in the eastern Gulf of Finland (Figure 1c, see Annex for sampling and abundance details).
M. leidyi was most abundant in deep waters near the halocline.Densities ranged from 4 to 24 ind m -3 in 50-80 m depths at stations where the abundances were highest.The bulk of the population remained below the thermocline (~below 30 m) with densities above it always being <1 ind m -3 .The most abundant stages were cydippid larvae (Figure 2), which occurred higher in the water column than adults.The largest individuals recovered were 15 mm long indicating early reproduction.In December only larvae (1-2 mm long) were found.Eggs were found in the Åland Sea in September from depths of 155 to 50 m, with the highest abundances (90 egg m -3 ) occurring around the halocline at 80-60 m (temperature 4.5-5ºC).Eggs at different developmental stages were present and hatching of the final stage, i.e. embryo with tentacles still within the egg envelope, was observed (Figure 2).The distribution of M. leidyi in the northern Baltic Sea seems to be limited either by low salinity (no observations below 5 PSU) or low deep water oxygen levels (no observations at stations where oxygen was depleted below 60-70 m).Based on our data, it seems that M. leidyi has established populations in the northern Baltic Sea and will survive winter in water layers around the halocline.This assumption is based on observations that the species reproduced at low salinities (6.5-9 PSU) and temperatures (4-7 ºC) in the northern Baltic Sea, and was collected from the thermally stable part of the water column (i.e. the main part of the population was at 4-5 ºC), where temperature does not decrease much even during winter.The December observations of small individuals at low abundances also indicate overwintering as shown in the southern Baltic Sea (Kube et al. 2007).
Earlier studies in other areas show that M. leidyi requires temperatures over 12 ºC to reproduce (Purcell et al. 2001), which are much higher than in this study.In its native areas and in the Black Sea and adjacent waters, population density seems not to be limited by salinity, but does decrease towards winter due to temperature decline (Purcell et al. 2001).In the northern Baltic, the most important factor reducing its abundance is probably decreasing food availability due to cooling of surface waters and decreasing plankton productivity towards late autumn and winter (Viitasalo 1992, Johansson et al. 2004).
The Black Sea was the first example of the damaging impact of Mnemiopsis leidyi on an ecosystem in general, especially on the pelagic community.The accidental introduction and subsequent outbreak of M. leidyi in late 1980s resulted in a dramatic decrease in abundance of almost all prey species of pelagic fish, and also in the disappearance of some zooplankton species (e.g.Vinogradov et al. 1989, Purcell et al. 2001).In the Caspian Sea, where individuals were mostly small, indicating early reproduction, the species invaded almost the entire sea in less than a year (Ivanov et al. 2000, Finenko et al. 2006).The rapid expansion of this invader in the Baltic Sea is similar to the invasion of the Caspian Sea.The high abundances observed and, especially, the high numbers of larval stages indicate establishment of the species.The magnitude of the effect of this species on the Baltic ecosystem has yet to be established but in similar eutrophic systems where similar prey taxa (e.g.Centropages spp., Acartia spp., Eurytemora affinis and cladocerans) are present, invasion by this ctenophore has profoundly changed the ecosystem (reviewed by Purcell et al. 2001).

Figure 1 .
Figure 1.Occurrence and abundance of Mnemiopsis leidyi in a) August, b) September and c) December 2007 in the northern Baltic Sea during cruises of R/V Aranda (Finnish Institute of Marine Research).* no observations in net tows taken from 50-0 m.In September no data were collected from the Bothnian Sea.December cruise covered only the eastern part of the Gulf of Finland