On 8 May 2025, a significant aggregation of mauve stinger jellyfish Pelagia noctiluca was observed in the Pagasetic Gulf, central Greece, near Prasouda Island. This observation raises concerns about a potential new localized bloom of this species, which is known for its irregular but mass appearances throughout the Mediterranean. The bloom occurred only 2 years after the conclusion of the major 2020–2023 jellyfish outbreak that affected most of Greece and extended to other parts of the Mediterranean Sea.

The jellyfish were recorded from a small boat, with video footage showing high densities of medusae in surface waters. Notably, both adult specimens, characterized by their violet pigmentation and stinging capability, and juvenile forms, exhibiting a distinct brownish coloration, were present. Water temperatures in the area were seasonally average (c. 17.7 °C), suggesting that typical spring conditions could be facilitating bloom formation.

Although P. noctiluca is a pelagic species with wide-ranging dispersal capabilities, this early-season appearance in a semi-enclosed gulf may suggest either (1) the emergence of a new localized bloom, (2) retention of residual populations from the previous widespread event, or (3) altered oceanographic or trophic conditions favouring population growth. The Pagasetic Gulf’s limited water exchange, and nutrient input from land, may provide favourable conditions for bloom persistence.

Given the ecological and socio-economic impacts of P. noctiluca, particularly for tourism and fisheries, we urge monitoring and documentation of this potential bloom. Local marine users, including fishers, divers and recreational boaters, are encouraged to report sightings, which may aid in understanding the scale and progression of the phenomenon. The use of citizen science tools such as the iNaturalist platform and social media may provide effective early-warning signals for such events.

Long-term monitoring of jellyfish blooms in the Aegean Sea, integrated with temperature, salinity and planktonic data, will be vital to determine if P. noctiluca blooms are becoming more frequent or shifting in phenology as a result of climate change or other anthropogenic pressures. The recent observation from the Pagasetic Gulf serves as a reminder of the dynamic nature of Mediterranean marine ecosystems and the need for coordinated responses to emergent ecological patterns.

The videos are available at doi.org/10.6084/m9.figshare.28961093.v2.