The abundance of comb jellies in the northern Baltic Sea
The American comb jelly (Mnemiopsis leidyi) invaded the southern Baltic Sea in autumn 2006, and was thought to spread to the northern Baltic in August 2007. However, in nucleotide sequence analysis it was shown that all specimens collected from August 2008 to August 2009 were Mertensia ovum, which is an arctic comb jelly species never before reported from the Baltic Sea. These results emphasize the utility of applying molecular tools to biological surveys and the importance of rigorous species identification.
Figure 1. (Click here to enlarge) The abundances of the ctenophores at three monitoring stations in the northern Baltic Sea 2007-2009. ND denotes no data.
Results and assessment
Relevance of the indicator for describing developments in the environment
Introduction of alien invasive species has been identified as one of the major threats to marine ecosystems causing biodiversity loss and adverse environmental, economic and social impacts from the local level upwards. The Baltic Sea is a young and simple ecosystem, hence vulnerable to ecological changes. For invasive species this gives an opportunity to find a free ecological niche and consequently establish permanent populations. The present eutrophic conditions and rapid expansion in shipping increases the risk of new AIS establishments, increasing the threat to the ecosystem. Recently it has been shown that eutrophication, overfishing as well as climate change may increase the abundance of gelatinous zooplankton and cause large ecosystem changes.
The American comb jelly (Mnemiopsis leidyi) invaded the southern Baltic Sea in autumn 2006, and was thought to spread to the northern Baltic in August 2007. However, in nucleotide sequence analysis of 18S ribosomal RNA gene (rRNA), internal transcribed spacer, and 5.8S rRNA it was shown that all specimens collected from August 2008 to August 2009 were Mertensia ovum, which is an arctic comb jelly species never before reported from the Baltic Sea. These results emphasize the utility of applying molecular tools to biological surveys and the importance of rigorous species identification.
It is very probable that the species has all the time been M. ovum, which has been misidentified to M. leidyi in the northern Baltic Sea. The ctenophore abundances have been intensively studied now for 3 years. It is evident that the peak abundances are observed during winter in the northern Baltic Sea. The abundances increased from 2007 to 2008 but during 2009 the increasing trend was observed only in the Gulf of Finland. The observed abundances are much higher compared with the abundances in the Arctic Sea where this species forms a permanent part of the plankton community.
Figure 2. Mertensia ovum in the northern Baltic Sea. Photo: Maiju Lehtiniemi (SYKE).
Gorokhova, E, Lehtiniemi, M, Viitasalo-Frösen, S, Haddock, SHD 2009. Molecular evidence for the occurrence of ctenophore Mertensia ovum in the Northern Baltic Sea and implications for the status of Mnemiopsis leidyi invasion. Limnol. Oceanogr. 54 (6): 2025-2033.
Javidpour J, Sommer U, Shiganova T 2006. First record of Mnemiopsis leidyi A. Agassiz 1865 in the Baltic Sea. Aquat Invasions 4:299-302.
Leppäkoski E, Gollasch S, Gruszka P, Ojaveer H, Olenin S, Panov V 2002. The Baltic—a sea of invaders. Can J Fish Aquat Sci 59:1175-1188.
Richardson, AJ, Bakun, A, Hays, GC, Gibbons, MJ 2009. The jellyfish joyride: causes, consequences and management responses to a more gelatinous future. Trends in Ecology and Evolution 24(6): 312-322.
1. Data source: The collection and microscopic counting of the ctenophores were made during FIMR/SYKE research and monitoring cruises in 2007-2009. The data produced by FIMR/SYKE is kept at the Institute's database.
2. Description of data: Ctenophores were collected by vertical net tows with a 500 µm WP-2 plankton net mainly from the bottom to the surface. The specimens were counted directly after sampling under research microscope and processed for genetic species identification analyses.
3. Geographical coverage: Samplings cover the Gulf of Finland, the Åland Sea and the Bothnian Sea.
4. Temporal coverage: January, May-June and August-September 2007-2009.
5. Methodology and frequency of data collection: Samplings were done on several monitoring and research cruises during three years, mostly three times per year.
6. Strength and weakness (at data level): Data covers well the different parts of the northern Baltic Sea although station grid is scarce. The future monitoring of ctenophores, which would be important in terms of value as an indicator, is not clear yet due to difficulties in proper preservation of the animals as well as due to the need of genetic analyses in order to properly identify the small individuals. Wintertime sampling is difficult due to often stormy and or icy conditions, which prevent sampling with a large zooplankton net. This causes lack of data, which decreases the value of estimates on population dynamics. Wintertime samples would be especially important because the population peak of these arctic comb jellies seems to occur during winter.
7. Reliability, accuracy, robustness, uncertainty (at data level): Data gives reliable estimates of abundances of both larvae and adults of comb jellies, but underestimates the densities of eggs because of the large mesh sized net used.
8. Further work required (for data level and indicator level): The fragility of ctenophores in the most common preservatives makes the frequent monitoring of the species difficult because the specimens have to be counted fresh immediately after sampling. The difficulty in species identification based on morphological characteristics emphasizes the need for development of fast and reliable genetic identification methods.
The Baltic Sea portal: Arctic comb jelly http://www.fimr.fi/en/tietoa/tulokaslajit/en_GB/arktinen-maneetti/ The Baltic Sea portal: American comb jelly http://www.fimr.fi/en/tietoa/tulokaslajit/en_GB/kampamaneetti/
Global Invasive Species database: Mnemiopsis leidyi. http://www.issg.org/database/welcome/
For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2009. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated: 29 September 2009