The recent aquatic invasive species American comb jelly Mnemiopsis leidyi in the Baltic Sea
The American comb jelly (Mnemiopsis leidyi) invaded the southern Baltic Sea in October-November 2006, and was found from the western part of the Gulf of Finland, the northern Baltic Proper and the Bothnian Sea in August-September 2007. It was very abundant in the water layers around halocline (max. 694 ind m-2), and was observed to reproduce effectively.
Figure 1. The occurrence and abundance of the American comb jelly (Mnemiopsis leidyi) in the northern Baltic Sea in August-September 2007.
Results and assessment
Relevance of the indicator for describing developments in the environment
Introduction of alien invasive species (AIS) 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.
The most recent invader in the Baltic Sea is the American comb jelly Mnemiopsis leidyi A. Agassiz 1865, which has earlier been transferred unintentionally via ballast waters to the Black and Caspian Seas from the eastern coast of North, Central and South America. It was found for the first time in the southern Baltic in autumn 2006. It has by now spread further north and was found abundant in August and September 2007 in the deeps of the Bothnian Sea, the northern Baltic Proper and the Gulf of Finland, but not in the Bothnian Bay.
Comb jelly abundances ranged from 100 to 700 ind m-2 in the northern Baltic Sea. In September the maximum abundances were found in the northeastern parts of the Baltic Proper. In that area also numerous eggs and larvae were found indicating effective reproduction. The densest layers of M. leidyi were observed near halocline, in 50-80 m depths (max 24 ind m-3), larvae occurring higher in the water column than adults. Presently only relatively small individuals have been found; the size ranging between 0.3-15 mm.
Figure 2. Mnemiopsis leidyi adult (left) and egg ready to hatch (right).
Hansson HG 2006. Ctenophores of the Baltic and adjacent Seas – the invader Mnemiopsis leidyi is here! Aquat Invasions 4:295-298.
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.
Vinogradov ME, Shushkina EA, Musayeva EI, Sorokin PY 1989: A new exotic species in the Black Sea: the ctenophore Mnemiopsis leidyi (Ctenophora: Lobata). Oceanology 29: 220-224
1. Data source: The collection and microscopic counting of Mnemiopsis leidyi were made during FIMR research and monitoring cruises in August and September 2007. The data produced by FIMR is kept at the FIMR database.
2. Description of data: M. leidyi was collected by vertical net tows with a 500 µm WP-2 plankton net mainly from the bottom to the halocline and from the halocline to the surface. At some stations also 100 µm WP-2 plankton net tows were taken in order to estimate the abundance of eggs. The specimens were counted right after sampling under research microscope to prevent misidentification due to lost body parts and fragmentation.
3. Geographical coverage: Sampling covered the northern part of the Baltic Proper, the western part of the Gulf of Finland, the Bothnian Sea and Bay.
4. Temporal coverage: August and September 2007.
5. Methodology and frequency of data collection: Samplings were done on a monitoring cruise and on a research cruise 2007 during four weeks.
6. Strength and weakness (at data level): Data covers well the different parts of the northern Baltic Sea. The eastern parts of the Gulf of Finland have not been sampled so far. The future monitoring of M. leidyi, which would be important in terms of value as an indicator, is not clear yet due to difficulties in proper preservation of the animals.
7. Reliability, accuracy, robustness, uncertainty (at data level): Data gives reliable estimates of abundances of both larvae and adults of M. leidyi, but underestimates the densities of eggs because of the large mesh sized net used.
8. Further work required (for data level and indicator level): In the future it has to be decided if both 100 and 500 µm nets are used in order to quantify also the abundances of eggs. However, if both nets are used together, only the eggs should be counted from the 100 µm net as otherwise the abundances of larvae and adults will be overestimated. The fragility of M. leidyi in the most common preservatives makes the frequent monitoring of the species difficult because the specimens have to be counted fresh immediately after sampling.
Global Invasive Species database: Mnemiopsis leidyi. http://www.issg.org/database/welcome/
Mnemiopsis video: http://www.itameriportaali.fi/en/galleria/en_GB/videoklipit/
For reference purposes, please cite this indicator fact sheet as follows:
[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2007. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
Last updated: 1.10.2007