In recent decades, an increase in the abundance and frequency of bloom events has been reported for the scyphozoan Rhizostoma pulmo in the Mediterranean Sea. Understanding such events requires a thorough assessment of the species' population dynamics through environmental windows allowing species development. The semi-enclosed coastal lagoon of Bages Sigean, France (43 degrees 05 ' 12.72 '' N; 3 degrees 00 ' 35.3 '' E) offers an exceptional framework for investigating the population dynamics of the species, and how its growth rates and environmental niches vary over time. Three cohorts starting in April, May and June 2019 were identified, while the overall population growth reached the maximum biomass (10.2 g m(-3)) in July. Bell diameter and total length were identified as the best morphological proxies of biomass estimation. The abundances of the two most abundant copepods' species appear to drive R. pulmo's dynamics in the lagoon. Based on multinomial analysis and using the von Bertalanffy model, different growth rates for juveniles (4.7 and 2.4 mm day(-1)) and adults (1.8 and 0.9 mm day(-1)) were determined for the first two cohorts. Thermohaline niches varied during ontogeny, but also among populations in three coastal Mediterranean lagoons: Bages Sigean (France), Mar Menor (Spain) and Bizerte (Tunisia), shedding light on the metapopulation dynamics of R. pulmo inhabiting the Mediterranean Sea. The pressing need for understanding the dynamics of jellyfish abundances and their impacts on ecosystems, calls for increased efforts on monitoring these populations and their life history traits to parametrize and build reliable ecosystem models.