For many jellyfish, the magnitude and timing of medusae blooms are recognized to result from the benthic stage dynamics. However, information on the scyphistomae of jellyfish populations in the wild remains scarce. Here, bi-mensual underwater photoquadrat surveys were combined with scyphistomae sampling and observation to describe the annual (February 2017-January 2018) benthic stage dynamics and asexual reproduction strategy of Aurelia coerulea in the Thau lagoon (43°25′31.1″N; 03°42′0.9″E). Our results revealed unexpected seasonal patterns of variation: scyphistoma coverage peaked in the spring (11.6 ± 3.7% on 21st April) and was minimal in the summer and autumn (1.4 ± 1.3% on 10th October). The increase in scyphistoma coverage mainly resulted from an intense production of buds between February and April during the spring rise in water temperature (peak of 12,800 buds m−2 on 21st April), but scyphistoma coverage appeared to be negatively influenced by the interaction of high summer temperatures and salinities. Strobilation was observed from November to April. It peaked on 17th November, with 33.1% of the scyphistomae strobilating and an average production of 19,100 strobila disks m−2. However, the low scyphistoma coverage at this time of the year (< 2%) likely limited the intensity of ephyrae liberation and the subsequent medusae bloom. The final population size of A. coerulea thus results from a complex interaction of abiotic and biotic factors. Our results bring into question how the different populations of Aurelia spp. will respond to the predicted global warming scenarios.

The pelagic dynamics of the cosmopolitan scyphozoan Aurelia sp. was investigated in three French Mediterranean lagoons, Thau, Berre and Bages-Sigean, which harbour resident populations. The annual cycles showed a common univoltine pattern in all lagoons where the presence of pelagic stages in the water column lasted 8 months. Field observations showed a release of ephyrae in winter time followed by pronounced growth between April and July, when individuals reached the largest sizes, before disappearing from the water column. Maximum abundance of ephyrae and medusae were registered in Thau. Medusae abundance attained a maximum of 331 ind 100 m 23 in Thau, 18 ind 100 m 23 in Berre and 7 ind 100 m 23 in Bages-Sigean lagoons. Temperature and zooplankton abundance appeared as leading factors of growth, where Bages-Sigean showed the population with higher growth rates (2.66 mm day 21) and maximum size (32 cm), followed by Thau (0.57-2.56 mm day 21 ; 22.4 cm) and Berre (1.57-2.22 mm day 21 ; 17 cm). The quantification of environmental windows used by the species showed wider ranges than previously reported in the Mediterranean Sea, which suggests a wide ecological plasticity of Aurelia spp. populations in northwestern Mediterranean lagoons.

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. Highlights ► During bloom events, jellyfish might be a source of food for opportunistic fishes. ► All life stages of Aurelia sp. are used as a source of food by Sparus aurata. ► Higher ingestion rates of fish predation were observed on Aurelia sp. young stages. ► Opportunistic fish species might contribute to control jellyfish blooms.