Moon jellyfish contain no calories – no carbohydrates, no fat, and no protein – yet they are consumed by a large variety of predators. Sea turtles, sunfish, seabirds, and many other marine species feed on jellyfish. Even animals that do not necessarily prey on jellyfish will eat them when they are available.
Researchers wanted to know why animals consume moon jellies when they seem to be such a low quality food. They discovered that jellyfish provide a variety of fatty acids, and that their nutritional value increases in larger quantities.
Study co-author Jamileh Javidpour is a marine biologist and jellyfish expert at the University of Southern Denmark.
“The jellyfish in our study showed to contain some fatty acids that are very valuable for their predators,” said Javidpour. “Fatty acids are vital components of cell membranes and play a crucial role in processes like growth and reproduction.”
The team collected moon jellyfish in the North German Kiel Fjord every two weeks for two years. The essential fatty acids found in the jellyfish samples included polyunsaturated fatty acids, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid.
The analysis revealed that the fatty acid content changed across different seasons, and the experts linked these variations to developmental stages. For example, mature jellyfish with reproductive tissues had the highest fatty acid content.
“Jellyfish are likely to be more than just an opportunistic prey to many organisms,” said Javidpour. “It is true that a predator does not get much from eating a single jelly fish, but if it eats many, it will make a difference and provide the predator with valuable fatty acids.”
The findings explain why leatherback turtles and ocean sunfish are known to gorge on jellyfish. Researchers have also observed salmon eating jellyfish 20 times faster than shrimp.
Javidpour explained that this strategy makes sense because a predator does not have to spend much energy eating loads of jellyfish.
“Jellyfish often come in shoals and they move slowly through the water. They can’t really swim away when predators start eating them.”
Marine ecosystems are shifting worldwide, and jellyfish populations are expected to grow and replace other types of prey.
“As we see an increase in jellyfish, I suspect that we will also come to see a change in predator populations – especially in areas where the abundances of usual prey items might be endangered by a changing environment,” said Javidpour.
The study is published in the Journal of Plankton Research.