Background Spanish marine habitats have been adversely affected recently by increased populations of the predatory jellyfish Carybdea marsupialis. Expansion and mass proliferation of this species can offset the natural balance of biodiversity in marine ecosystems. The reasons behind unusual population growth patterns in C. marsupialis remain unknown, but these may follow a siimilar pattern of behaviour to other invasive species that can take advantage of environmental stresses caused by human activity. Objectives The main objective of this LIFE+ Biodiversity project is to gain sufficient understanding about C. marsupialis blooms to help develop measures capable of mitigating against negative impacts from the jellyfish on Mediterranean marine ecosystems in Europe. Expected results include: Clarifying the cause of exponential growth in C. marsupialis populations; Confirming the consequences of C. marsupialis blooms on marine ecosystem integrity; Preparing and demonstrating techniques for detecting and quantifying C. marsupialis populations, as well as predicting trends in the species throughout Europe’s Mediterranean coast; Restoring and/or improving a minimum of 120 ha of a marine habitat in the Denia coast area that has been invaded by C. marsupialis; Reducing C. marsupialis population numbers in the Denia beach areas by more than 70% compared with the 2008 population size; Raising awareness widely (among more than 2 million people via television) about the impacts on marine ecosystems from invasive species such C. Marsupialis; Creation of a complete database of C. marsupialis and other related marine invasive species in the European Mediterranean. This database will be accessible via the project web page; and Making the full project findings available to all European Mediterranean marine environment authorities and marine research centres, achieving dissemination to more than 70% of target authorities and research centres.