In 2006, a new kind of ctenophore was observed in the Baltic Sea. It was to be feared that these ctenophores belonged to the American comb jelly Mnemiopsis leidyi which has become a plague in the Black Sea. Last autumn it turned out that at least some of the observed individuals must belong to some other species. New ctenophore samples were taken in January and March during the tour of the sea on research vessel Aranda. The samples were analyzed by accurate genetic methods. The results confirmed that the ctenophores collected in the beginning of 2009 from the Gulf of Finland and the Bothnian Sea belonged exclusively to the Arctic comb jelly Mertensia ovum.
The Mertensia ctenophores are very common in Arctic ocean waters, and the species is a permanent part of open sea plankton communities. In their region of origin, the size of these ctenophores can be as large as 6 to 8 cm. In the early stages of their life they eat microplankton, in the later stages zooplankton as well as fry and roe. It is not known that this species would have caused major ecosystem changes like those caused by the above mentioned American comb jelly. In the northern Baltic Sea, the size of the largest observed Arctic comb jelly individuals has been about 0.5 cm.
Research on the ctenophores of the northern Baltic Sea was started by Finnish and Swedish scientists in 2007. After more than two years of observations we know more about the ctenophores. In addition to the species question there is new information about the seasonal patterns of occurrence. According to the monitoring results, the ctenophores are most abundant in winter and least abundant in summer. In 2009 the seasonal numbers of ctenophores have been higher than in the previous years.
The research will continue. The monitoring of the numbers of ctenophores goes on this summer, and the ctenophore fauna of the entire Baltic Sea is surveyed in cooperation with international research partners. Laboratory experiments are expected to show what the ctenophores in the Baltic Sea eat and how their reproduction can succeed in the Baltic Sea where the salinity is much lower than in ocean water. These studies will contribute to our knowledge about the impacts of this species on the ecosystem of the Baltic Sea.
More information
Ms Maiju Lehtiniemi, research specialist, Finnish Environment Institute, phone +358 40 725 5085
Ms Sirpa Pellinen, chief information officer, Finnish Environment Institute, phone +358 40 740 2754
