Climate-driven spread of the upside-down jellyfish Cassiopea andromeda in a Mediterranean MPA

In October 2025, the first occurrence of C. andromeda was recorded in a semi-enclosed basin within the Marine Protected Area of Porto Cesareo (Italy) representing the northernmost record of the species in the Mediterranean basin to date. The population was monitored from October to December 2025 to assess density, size structure, and environmental conditions. Two sampling methodologies were compared: (i) an invasive approach based on the complete removal of individuals within fixed squares for measurements of bell diameter, and (ii) a non-invasive photographic survey using 50 × 50 cm squares, validated through specimen removal (i). Environmental parameters, including seawater temperature, salinity, dissolved oxygen and chlorophyll-a, were measured during the survey. In addition, the abundance of zooxanthellae in jellyfish issues was quantified across different size classes. Mean jellyfish density in October was 108.8 ± 106.7 ind m⁻² (mean ± SD; n = 41). Individuals ranged from 0.5 to 20 cm in bell diameter (mean 2.82 ± 0.34 cm). Seawater temperature ranged from 17 to 21 °C (mean 19.9 ± 1.7 °C), salinity averaged 33.4 ± 1.2 psu, and dissolved oxygen 7.5 ± 1.3 mg L⁻¹ and chlorophyll-a 4.5 ± 2.3 µg L⁻¹. No statistically significant differences were detected between the two methods. Photographic sampling proved more efficient, allowing wider spa al coverage with reduced effort and minimal disturbance. Zooxanthellae abundance in jellyfish ssues decreased with increasing bell diameter. These results highlight the suitability of shallow, confined Mediterranean habitats for the establishment of Cassiopea and emphasize the importance of early detect on and efficient monitoring strategies under ongoing climate warming.