Jellyfish as an alternative source of food for opportunistic fishes

Although scientific interest on jellyfish ecology has substantially increased in the last decades, little is known on the role of potential predators shaping their population dynamics. Jellyfish were long considered as ‘dead ends’ within food webs, and therefore overlooked as potential food source for higher trophic levels, e.g. fishes. Here this question is tackled by using comprehensive laboratory experiments assessing fish predation on jellyfish. The approach included all the life stages (polyps, ephyrae and medusa) of Aurelia sp. versus more traditional aquaculture feeds in an easily farmed opportunistic fish, the gilthead seabream Sparus aurata (L.). Results revealed that all life stages of Aurelia sp. were accepted as a source of food by S. aurata, whose grazing pressure varies depending on the jellyfish life stage. Higher ingestion rates were observed on young stages (i.e. small medusa) indicating their higher vulnerability to fish predation and the potential negative impact this may have on Aurelia sp. population dynamics. These results provide new insights on the so far underestimated role fish predation can have on jellyfish population dynamics. In particular, opportunistic fish species, such as S. aurata may contribute to control jellyfish blooms, through top-down regulations of jellyfish biomass.