Invasive comb jellies now found throughout Baltic Sea

Experts fear the non-indigenous tiny predators could survive the Baltic Sea winter

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The destructive comb jelly (Mnemiopsis leidyi) has spread throughout the Baltic Sea. Indigenous to the waters off the east coasts of North and South America, the tiny sea creature is believed to have been introduced into the sea by ballast water from ships.
      The research vessel Aranda returned from a voyage in the Gulf of Finland east of Helsinki on Monday. Comb jellies were found even in the furthest reaches of the gulf.
      "The largest numbers were off Loviisa", says researcher Maiju Lehtiniemi.
      Other areas of the Baltic had been examined before. Only the northern part of the Gulf of Bothnia has not been invaded by the species.
      The comb jellies were larvae between one and two millimetres in length. They were found at shallower depths than before.
     
Comb jellies were found in the east of the Gulf of Finland in varying densities: 5-150 individuals per square metre.
      The numbers were significantly smaller than what was seen in August. The reason is the colder water temperatures of the winter.
      In August there were more than 500 individuals per square metre.
      "At this point it seems that they can survive the winter. That is bad from the point of view of the coming summer", Lehtiniemi says.
     
The Gulf of Finland has not yet frozen over, so the survival assessment can change as the winter progresses.
      The survival of just a single comb jelly is enough. They are able to multiply on their own as each individual has both male and female reproductive organs.
      Next summer is decisive. If the comb jellies grow and multiply, it could affect fish populations. The first to suffer will be stocks of Baltic herring and sprat.
      "If comb jelly populations do not grow, and if the individuals remain relatively small in size, they might not be too harmful. The numbers seen in the autumn should increase tenfold for the comb jellies to be a real threat", Lehtiniemi points out.
      "Fishermen don't need to sell their boats just yet."
     
The Finnish Institute of Marine Research says that the comb jelly is the first arrival in the history of the Baltic Sea that really could endanger the whole ecosystem.
      Lehtiniemi says that the rapid spread of the comb jellies is the most worrying.
      However, the low salinity of the Baltic Sea could limit the appearance of the comb jelly in shallow waters.
     
At present, lack of oxygen and eutrophication are greater threats to the Baltic than the comb jelly, says Professor Markku Viitasalo of the Marine Research Institute.
      "The more eutrophication, the better the conditions are for the comb jelly", he notes.
      The oxygen situation in the Baltic Sea has weakened since last winter. The situation at the depths is poor in some places, but no hydrogen sulphide was found.
      The opening of new waste water treatment plants in St. Petersburg has not yet led to a reduction in phosphorous levels.


Links:
  Comb jelly poses serious threat to Baltic Sea ecosystem (27.8.2007)
  Global Invasive Species Database: Mnemiopsis leidyi (comb jelly)

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