Salt gradient limits the invasive comb jelly

Originating from the waters off the east coast of America, the comb jelly has been carried to other parts of the world in ballast water of cargo ships. In recent years, the invasive comb jelly has found its way to the Baltic Sea.

The comb jelly earned its bad reputation after it invaded the Black Sea in the 1980'es. Each individual of this hermaphrodite is capable of producing up to 14,000 eggs a day, and in the Black Sea, having left its natural enemies behind, the comb jelly thrived and bloomed to massive numbers eating the native fish' food as well as their larvae and eggs.

Due to the comb jelly's bad reputation, it caused concern when it was first spotted in the Baltic Sea. The Bornholm Basin is the most important cod spawning ground in the central Baltic and it was feared that the jelly would pose a threat to the valued cod by eating the eggs and larvae or by gulping down the food that the larvae are dependent on for survival.

In order to try to determine, how the comb jelly will expand into the Baltic Sea and whether they might affect the cod, PhD-student Cornelia Jaspers from the National Institute of Aquatic Resources (DTU Aqua) in Denmark has investigated how the salinity gradient in the Baltic Sea might affect the comb jelly's egg production.

"Our results show that due to the low salinity, the comb jelly cannot reproduce enough to get a population going in the most important cod spawning ground of the Baltic Sea," says PhD-student Cornelia Jaspers, who has recently published her results in the peer-reviewed scientific journal, PLoS ONE, with co-authors Professor Thomas Kiørboe, DTU Aqua, and Assistant Professor Lene Friis Møller, University of Gothenburg.

Drifting in from other areas

After the first sighting of the comb jelly in 2005, it has rapidly become abundant in northern European waters with the highest densities in the Kattegat and south western Baltic, especially during summer. So far, the comb jelly has generally been absent from the central Baltic Sea during summer, but it does appear in small numbers in these low salinity areas during the colder months from autumn to spring.

The researchers conclude that due to the small amounts of eggs being produced in the central Baltic Sea, the appearance of comb jelly in this area from autumn to spring must be due to comb jelly being drifted in from higher salinity source areas like the Kattegat and the south western Baltic Sea into the central Baltic Sea. This means, that the comb jelly is not around to eat the cod eggs and larvae in the central Baltic Sea during the summer or to compete with the larvae for food.

No threat to the cod

In a previous study, Cornelia Jaspers and her colleagues showed that comb jelly do not eat cod egg or larvae and therefore pose no predatory threat to the cod population.

"Overall our work demonstrates that the predatory threat of the invasive comb jelly on cod eggs and larvae can be ruled out and that the indirect threat due to food competition is unlikely since they cannot sustain a population in the central Baltic Sea," says Cornelia Jaspers.

Comb jellies in the lab

In order to find out, whether salinity could limit the comb jelly's reproduction, the researchers measured how many eggs the comb jelly produced per day in areas with low salinity in the Bornholm Basin and Arkona Basin in the central Baltic as well as in the high salinity Kattegat. Furthermore, in the lab, they studied how many eggs the comb jelly produced under different salinities corresponding to the salinities that the comb jelly might encounter in the Baltic Sea.

The lab results showed that the comb jelly produced almost no eggs at salinities corresponding to the low salinity in the central Baltic, intermediate amounts of eggs at salinities corresponding to the intermediate salinity levels found in the south western Baltic Sea, and the highest amounts of eggs at salinities corresponding to the high salinity found in the Kattegat.