In recent years, jellyfish blooms have attracted considerable scientific interest for their potential impacts on human activities and ecosystem functioning, with much attention paid to jellyfish as predators and to gelatinous biomass as a carbon sink. The mauve stinger Pelagia noctiluca (Forskål, 1775) (Scyphozoa) can be considered as one of the dominant jellyfish species in the Mediterranean Sea. Due to its high abundance and painful stings P. noctiluca outbreaks are generally described in negative terms, because the medusae may cause damage to economic activities, such as fisheries, mariculture and tourism, and to the ecosystem by affecting the structure and organisation of plankton communities. Several studies on P. noctiluca have been carried out in the Mediterranean Sea, but many questions related to jellyfish population ecology and biology still remain unsolved, preventing a mechanistic understanding of jellyfish blooms dynamics and of their impacts on the marine pelagic ecosystem.
P. noctiluca can reproduce throughout the year, although the ability to form a bloom is limited to two main events of reproduction, one in autumn and another in spring.
The work carried out in VECTORS aimed to increase knowledge of the ecophysiology, reproductive biology, trophic ecology and bioenergetics of P. noctiluca from the Strait of Messina1, an important proliferation and reproduction area for this species, due to the favourable temperature range and high productivity. The reproduction was investigated by histological analysis of gonads (size distribution of oocytes), the calculation of gonadosomatic and fecundity indexes, and the biochemical composition of gonads. Trophic ecology was studied by isotopic and fatty acid analyses, and by studies of gut contents. A bionergetic model based on respiration, consumption, assimilation, and growth was developed for the ephyra, juvenile and adult stages in P. noctiluca’s life cycle. Finally, the effect of predation by a coastal fish (Boops boops) on P. noctiluca was studied over a year cycle.
The results showed that P. noctiluca can reproduce throughout the year, although the ability to form a bloom is limited to two main events of reproduction, one in autumn and another in spring. Females of P. noctiluca produce great amounts of small-sized mature eggs (diameter < 200 μm) during high food availability, whereas a reduced number of larger eggs (diameter > 200 μm) are produced when prey availability is low. Protein contents stored within the gonads seasonally change, with the highest concentrations during the pre-spawning periods. Pelagia noctiluca is a generalist predator, with a wide diet. Stable isotope and fatty acids analysis highlighted that its diet varied over the year, according to prey availability in the environment, and then varied its trophic level, too. Analysis of fatty acids trophic tracers suggested the transfer of key molecules from somatic tissue to gonads to enhance reproductive success. Biochemical, reproduction and observational data are in agreement with the hypothesis of seasonal vertical migratory patterns. An Individual-Based Growth Model has been developed in VECTORS, representing the first modelling approach on jellyfish, aiming to predict daily growth rate and daily carbon ingestion of P. noctiluca under natural conditions. This model will represent an essential tool to measure the impact of P. noctiluca outbreaks on planktonic community.
Finally, VECTORS work has increased general awareness that P. noctiluca should not be considered as an energy dead-end in the ecosystem. The demersal fish Boops boops foraged on jellyfish all year long, with different intensity among jellyfish body parts, depending on their relative energy content2.