Snapshot of the Distribution and Biology of Alien Jellyfish Cassiopea andromeda (Forsskål, 1775) in a Mediterranean Touristic Harbour

SIMPLE SUMMARY: Alien species are an important cause of biodiversity loss and changes to ecosystems. Harbors are hotspots for the introduction of these species, and, usually, the impacts and pathways of invasion of the host populations are poorly known. Since 2014, an alien jellyfish, Cassiopea andromeda, coming from the Red Sea, has invaded a Mediterranean touristic harbor and established a population there. In this study, the distribution and trophic behavior of C. andromeda were investigated to improve knowledge on this species within the Mediterranean. The preliminary results highlight and confirm that C. andromeda is a perfect invader thanks to its nutritional strategy and capacity to adapt to heavily anthropized areas. Therefore, its potential impact on the local biodiversity and thus on the ecosystem’s structure and functioning is worth considering. ABSTRACT: Harbors are hotspots for the introduction of alien species, and, usually, investigations on their host populations help fill the knowledge gap in their pathways of invasion and in their impacts on marine biodiversity and ecosystems. In 2014, the upside-down alien jellyfish Cassiopea andromeda invaded a Mediterranean touristic harbor (“Cala”), and its abundance has since increased over time. In the present study, the distribution and trophic behavior of C. andromeda in Cala were investigated for the years 2017–2018 through visual sampling, and GIS-based statistical and stable isotope analyses. Since Cala is a hard-to-reach area (with many anchor cables and boats), Megabenthos Underwater Video was used to count the number and estimate the size of jellyfishes. The variations in size throughout the study period suggest that the population of C. andromeda is quite established in Cala at depths lower than 7.5 m. The ranges of the environmental parameters recorded (temperature, salinity, and transparency) were consistent with the ideal conditions for maintaining a Cassiopea population, but they did not seem to influence aggregation. Additionally, the carbon and nitrogen isotopic signatures studied highlight the mixotrophic behavior of this species. These preliminary results confirm the capacity of C. andromeda to live and reproduce in heavily anthropized areas.