Ontogenetic dietary shifts of the medusa Rhizostoma pulmo (Cnidaria: Scyphozoa)

Identifying ontogenetic changes in jellyfish diet is fundamental to understand trophic interactions during their life cycle. Scyphomedusae blooms exert major predation pressure on plankton communities, although their role in ecosystems has long been misrepresented. This study assesses seasonal and ontogenetic changes in the diet of the scyphomedusa Rhizostoma pulmo , one of the largest yet overlooked Mediterranean jellyfish. Medusae gut contents ( n = 127) were collected during one year in Bages Sigean lagoon, southern France. Results show that the diet composition differs from the availability of prey in the environment with contrasting preferences along ontogeny. Calanoid (70%) and harpacticoid (45.8%) copepods were the most frequent prey and the major carbon contributors for small medusae (bell diameter 15 cm), which obtain most of their carbon intake from ciliates and fish eggs (20.9%). The overall impact on micro and mesozooplankton showed that small medusae consume 5% of the copepods daily standing stock, while large medusae consumed 8% of ciliates daily standing stock. Our results stress that R. pulmo display different trophic pathways along its life cycle, firstly interacting with the classical food web, and shifting afterwards to a greater interaction with the microbial loop.