The westernmost record of the scyphomedusa Cassiopea andromeda in the Mediterranean: marine citizen science contributions to invasive species detection and monitoring

The Mediterranean Sea, although a biodiversity hotspot, is one of the most affected seas by non-indigenous species (NIS). This problem is worsened by rising sea temperatures due to climate change, which promotes the spread of thermophilic species. Among the NIS scyphozoan jellyfish species recorded in the Mediterranean, Cassiopea andromeda – commonly known as the "upside-down jellyfish"– is a notable example. Observadores del Mar ( OdM ) is the leading platform for marine citizen science in Spain and works towards ocean conservation and health. It is a well-established tool for generating knowledge in marine research and has successfully provided early warning of NIS reports in the Mediterranean, while also serving as an effective network for the monitoring of NIS and other indicators. Three reports of C. andromeda from Almeria, southern Spain have been reported in OdM and thanks to the involvement of its community, 12 samples were collected for phylogenetic analysis and monitoring was done for 15 months in the study area. The results confirmed the first record of C. andromeda in Spanish Mediterranean waters representing the westernmost record in the basin. Monitoring also suggest the species establishment in the area. This study contributes to the knowledge of C. andromeda invasiveness and highlights the importance of marine citizen science in the detection and monitoring of NIS. It also underscores the collaboration and commitment already established between scientists and citizens, which will allow further progress in the fields of biological invasions, management, and policy.