Invasive comb jellyfish pose no direct threat to Baltic cod eggs and larvae, Danish study finds

The warty comb jellyfish, scientific name Mnemiopsis leidyi, has gained a bad reputation as an invasive species. Introduced into the Black Sea via ballast water in the 1980's, its arrival, coinciding as it did with over-fishing and excessive nutrient loading, was cited as partly to blame for the fact that fish were disappearing and fishermen were losing their livelihoods.

So when Mnemiopsis suddenly appeared in the Baltic Sea, people feared the worst for our ecosystem.

"Very little is known about jellyfish in general and Mnemiopsis in particular other than that the killer jellyfish can reproduce and multiply very fast if there is enough food around. We also know that jellyfish generally eat fish eggs and so it obviously created a lot of worry when scientists from National Institute of Aquatic Resources in Denmark (DTU Aqua) and IFM Geomar, Kiel discovered the invader in exactly the same spots in the Baltic Sea as where cod spawn," says Cornelia Jaspers, PhD student at DTU Aqua, who has just published new and promising results about the feeding habits of the invasive jellyfish Mnemiopsis in the leading American scientific journal "Limnology and Oceanography" with her co-authors Josefin Titelman, Lars Johan Hansson, Matilda Haraldsson and Christine Røllike Ditlefsen.

Hungry Jellies

By studying and filming Mnemiopsis in large containers at salinity levels and water temperatures equivalent to the environment found in the deep water in the Bornholm Basin in the Baltic Sea where cod spawn, she has investigated how Mnemiopsis reacts to cod eggs and larvae. The results are good news for the Baltic's highly prized stocks of cod. Even though the invasive jellyfish feed non-stop, they are apparently not particularly keen on cod eggs.

"Even after having been starved for 24 hours, the jellies did not display any attack response to cod eggs even though they were completely surrounded by eggs. We showed that eggs did not trigger the normal attack reflex in jellies where they close their lobes around their prey like a kind of cage. This might be due to the lack of a hydrodynamic signal, the egg structure or its texture. In comparison, Mnemiopsis reacts to cod larvae and produces the normal attack response," says Cornelia Jaspers.

No to eggs, yes to fish

To be certain that Mnemiopsis was actively feeding under the experimental conditions, they were fed with a mix of cod eggs and a primitive type of water crab, Artemia salina which is widely used as fish food. Mnemiopsis immediately started feeding voraciously on Artemia but not on the cod eggs.

"Each time the jellies came near enough to the Artemia crabs, they ate them. In fact there were no cases where Mnemiopsis did not eat Artemia, while in 15 % of the cases the jellies did not eat cod larvae offered to them and in 40 % of the cases they did not feed at all even though they were surrounded by cod eggs," explains the DTU Aqua scientist.

Larvae swim away

Even though Mnemiopsis eats cod larvae, the larvae have a big advantage over the eggs -- they can swim away.

"We find cod eggs and Mnemiopsis at the same depth in the most important spawning area, but four days after the cod larvae are hatched they swim up to nearer the surface in order to hunt for food. They do this as they have eaten the yolk sack "lunch pack" they get from their mother and which they feed on during the first days after hatching. In this way the cod larvae also move away from the area where the comb jellyfish are," the scientist says.

Vomiting

In order to feed, Mnemiopsis reacts to its prey by closing its body around it like a kind of "cage." In addition the jelly produces a small current in the water, which moves the food within this "cage" towards the mouth, where it is scanned for goodies. Therefore, in a few cases during the experiment, Mnemiopsis accidentally caught an egg in its food current. But surprisingly, these were vomited out again.

"In 8 out of 10 cases, Mnemiopsis vomited the cod eggs out again. Even after being in the jelly stomach for three days, cod eggs were not digested but regurgitated, which indicates that they have problems digesting cod eggs from the Baltic Sea. In comparison cod larvae were digested succesfully after they were caught ," explains Cornelia Jaspers.

The researcher will not however completely write off the comb jellyfish as a problem for cod based purely on this experiment.

"Even though the invasive killer jellies do not eat the cod offspring directly, it might still pose an indirect threat, since they are both competing for the same food resources. In addition Mnemiopsis can reproduce when they are 2 weeks old, combining male and female sexes in one animal (hermaphrodite) and can produce 12,000 eggs per day. Therefore, one jelly can turn into an awful lot of hungry jellies in an incredibly short time," says Cornelia Jaspers.

The food experiments were conducted using Baltic cod provided from the Bornholm Salmon Breeding Plant in Nexø. This work has been conducted in cooperation between the University of Gothenburg, the University of Oslo, DTU Aqua and is part of the Baltic Zooplankton Cascades project (www.BAZOOCA.se).