Hypoxia reduces the predation impact on the non-indigineous jellyfish Mnemiopsis leidyi

The non-indigenous jellyfish Mnemiopsis leidyi thrives in the brackish waters of the Baltic Sea without known predators. Here, it competes with small fish for mesozooplankton prey, particularly copepods. M. leidyi tolerates severe hypoxia, potentially making it more resilient in the face of increasing hypoxic areas in the Baltic Sea. We hypothesize that potential predators do exist but that their predatory impact will be determined by oxygen availability. To explore this hypothesis, we applied video analyses of potential predators, functional response experiments and choice tank setups using oxygen concentrations spanning from anoxic (0%) to normoxic (100%). All experiments were carried out in salinities (20 PSU) and temperatures (18 ˚C) representative for Danish coastal waters during summer and early autumn. Our findings reveal that predators exist throughout the Baltic Sea. However, even moderate hypoxia significantly hampers their predatory impact. Our findings thus support existing claims that M. leidyi may benefit from anthropogenic pressures and emphasizes the importance of further studies on the trophic interactions between fish and jellyfish in the increasingly hypoxic Baltic Sea.