Global Warming and the Spread of the Introduced Jellyfish Cassiopea andromeda: Thermal Niche and Habitat Suitability in the Mediterranean Sea.

Climate change affects marine ecosystems in multiple ways, including sea warming and changes in biological community structure and diversity. The Mediterranean Sea has emerged as one of the most vulnerable regions, also because of the diverse patterns of introduction of non-native species. First recorded in the coastal waters of Cyprus in 1903, the Red Sea jellyfish Cassiopea andromeda (Forskål, 1775) is spreading its distribution and local abundance, posing questions on its potential ecological implications. Here we identified the thermal tolerance, habitat suitability, and potential distribution range of the C. andromeda polyps, a key life cycle stage responsible for asexual reproduction and population persistence. By laboratory-controlled respirometric measurements, we assessed that the polyps of C. andromeda exhibit their optimal metabolic performances at high water temperatures, but they are tolerant to winter conditions across the Mediterranean basin. Combining experimental respiration measurements with modelling approaches enabled the definition of the species' fundamental thermal niche, with an optimal seawater temperature at 35.7°C and critical limits at 6.4°C (minimum) and 39°C (maximum). Trait-based thermal habitat suitability maps indicated a future increase of favourable habitats for the species under warming conditions according to the Representative Concentration Pathways (RCP 4.5 and 8.5 for 2050) in Mediterranean coastal areas. In the context of climate change scenarios, the rise of seawater temperature may enable polyps to thrive across a wider geographic range, predicting a westward and northward enlargement of C. andromeda populations in the Mediterranean Sea.