Marine jellyfish are recognized as subject to proliferations in many coastal areas, where their populations experience seasonal and inter-annual large fluctuations, characterized by sudden outbreaks alternate with rarity periods. When they are exceedingly abundant, jellyfish cause substantial ecological impacts on marine biodiversity, interfere with economic and recreational human activities, and may be harmful to public health. For these reasons, jellyfish “blooms” are regarded as a multi-billion Euro problem for human activities in the sea and coastal zones. Understanding of jellyfish biology and ecology is therefore mandatory to prevent or mitigate critical ecological and economic drawbacks related to massive proliferations of gelatinous organisms. Under a more positive perspective, the large amount of jellyfish biomass could be considered as an untapped source of bioactive compounds including peptides, collagen and gelatin, sugars, fatty acids, enzymes, calcium, water-soluble minerals, and biopolymers making them a potentially valuable material for industrial uses in cosmetic, pharmaceutical and biomedical industry as well as food or feed. PULMO aimed to investigate mechanisms of massive proliferations of one of the most common jellyfish in the Mediterranean Sea, Rhizostoma pulmo, and to assess its potential utilization as exploitable biomass for the development of a new sea-based bioeconomy. Overall objectives included: [I] gathering novel information about the biology, trophic ecology, biochemical and molecular composition of the jellyfish; [II] investigating key parameters of population dynamics and life history traits; [III] discovering the jellyfish-associated microbial diversity; [IV] assessing the potential for the exploitation of jellyfish-related biomaterials.