A long-term time series of plankton records collected by the continuous plankton recorder (CPR) Survey in the northeast Atlantic indicates an increased occurrence of Cnidaria since 2002. In the years 2007 and 2008, outbreaks of the warm-temperate scyphomedusa, Pelagia noctiluca, appeared in CPR samples between 45° N to 58° N and 1° W to 26° W. Knowing the biology of this species and its occurrence in the adjacent Mediterranean Sea, we suggest that P. noctiluca may be exploiting recent hydroclimatic changes in the northeast Atlantic to increase its extent and intensity of outbreaks. In pelagic ecosystems, Cnidaria can affect fish recruitment negatively. Since P. noctiluca is a highly venomous species, outbreaks can also be detrimental to aquaculture and make bathing waters unusable, thus having profound ecological and socio-economic consequences.

Les méduses du golfe de Tunis représentées par 25 espèces constituent du point de vue taxonomique le troisième groupe zooplanctonique après les Tintinnides et les Copépodes. Parmi les 20 Hydroméduses recensées au cours du cycle d'étude (décembre 1993-novembre 1995), les anthoméduses (huit espèces), les trachyméduses (cinq espèces) et les leptoméduses (quatre espèces) sont les plus diversifiées. Les Limnoméduses (deux espèces) et les Narcoméduses (une espèce) étant les sous-classes les moins représentées. Ce sont les espèces du genre Obelia et Clytia qui commandent la dynamique quantitative des hydroméduses dans la baie avec des poussées susceptibles d'atteindre des valeurs exceptionnelles de l'ordre de 25 800 ind/m 3 pour Obelia spp. Au sein des scyphoméduses dont la richesse spécifique s'élève à cinq espèces, Pelagia noctiluca est l'espèce la plus commune, dont les invasions s'observent en saison automnale et surtout hivernale. Le golfe de Tunis apparaît comme formé par deux types de communautés de Méduses ; la première endogène constituée d'espèces constantes dans leur apparition tel que Olindias, Cladonema, Aurelia, Cotylorhiza et Rhizostoma et parfois pérennes comme Obelia spp., Clytia spp. ; la deuxième exogène à répartition spatiotemporelle irrégulière et représentée par des espèces visiteuses comme Velella, Sminthea ou encore Pelagia dont la présence sur la côte résulte de la présence de vents et de courants locaux particuliers.

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Gelatinous zooplankton species composition, distribution and abundance were investigated in the bay and the lagoon of Bizerte (North Tunisian coast) from January 2004 to December 2005. In total, 48 species were identified in the study area: 24 medusae, 11 siphonophores, four appendicularians, six chaetognaths, two pteropods and one doliolid. The hydromedusa Eutima mira was recorded for the first time in the Western Mediterranean Sea. The diversity in the Bay of Bizerte was greater than the diversity in the Bizerte lagoon. There was a loss of species diversity. Siphonophores were the most sensitive group to anthropogenic discharge. Only two species of siphonophores were recorded in the lagoon. Four species of gelatinous zooplankton, Muggiaea kochi, Lensia conoidea, Oikopleura longicauda and Podocorynoides minima were the most frequent species in the lagoon and may benefit from eutrophication. Appendicularians were numerically the most dominant group in the bay and the lagoon of Bizerte. Mesozooplankton density declined significantly in autumn and winter of 2004 and 2005 in relation to the dense aggregations of the scyphomedusae Pelagia noctiluca. Statistical analysis divided the study area into three zones: lagoon zone, bay zone and channel zone.

Science is addressing global societal challenges, and due to limitations in research financing, scientists are turning to the public at large to jointly tackle specific environmental issues. Citizens are therefore increasingly involved in monitoring programs, appointed as citizen scientists with potential to delivering key data at near to no cost to address environmental challenges, therein fostering scientific knowledge and advising policy- and decision-makers. One of the first and most successful examples of marine citizen science in the Mediterranean is represented by the integrative and collaborative implementation of several jellyfish-spotting campaigns in Italy, Spain, Malta, and Tunisia starting in 2009. Altogether, in terms of time coverage, geographic extent, and number of citizen records, these represent the most effective marine citizen science campaigns thus far implemented in the Mediterranean Sea. Here, we analyzed a collective database merging records over the above four countries, featuring more than 100,000 records containing almost 25,000 observations of jellyfish specimens collected over a period of 3 to 7 years (from 2009 to 2015) by citizen scientists participating in any of the national citizen science programs included in this analysis. Such a wide citizen science exercise demonstrates a valuable and cost-effective tool to understanding ecological drivers of jellyfish proliferation over the Western and Central Mediterranean basins, as well as a powerful contribution to developing tailored adaptation and management strategies; mitigating jellyfish impacts on human activities in coastal zones; and supporting implementation of marine spatial planning, Blue Growth, and conservation strategies.