Climate-driven phenological shifts and biogeographical constraints of the hydrozoan Velella velella in Mediterranean coastal waters

The pleustonic hydrozoan Velella velella forms extensive surface swarms worldwide. However, its ecology and biogeography in the Mediterranean Sea remain poorly documented. This study aims to investigate the biogeography and phenology of V. velella in the Mediterranean Sea and assess its spatial distribution, seasonal dynamics, and long-term phenological shifts linked to regional warming, while evaluating the contribution of citizen science to basin-scale monitoring. We compiled the most comprehensive Mediterranean database to date of V. velella (n = 12,015 records, 1841–2024) by integrating scientific literature, citizen science initiatives, social media, and press reports. Temporal, spatial, and environmental patterns between 2009 and 2024 (representing 93% of the data) were analyzed using Bayesian modelling and kernel-based approaches. Sea surface temperature (SST), NAO, and WeMO data were used to characterize the species’ thermal niche and phenological variability. Citizen science accounted for 72% of observations, highlighting its crucial role in documenting widespread but irregular gelatinous plankton events. Occurrence records exhibited a pronounced west–east gradient, with most presences in the Western Mediterranean basin. Seasonal patterns showed a dominant bloom from March to May, with regional variations and occasional secondary peaks in autumn. Bayesian latent peak models revealed an advance of ~32 days in bloom timing between 2009 and 2024 (posterior slope: –2.0 days yr - ¹, 95% CrI: –7.7 to 3.7). This shift was associated with winter SST warming. Bayesian structural equation modelling further revealed a climate-driven cascade linking large-scale atmospheric variability to phenology, with winter NAO influencing bloom timing through its impact on SST (β = –0.073, 95% CrI: –0.20 to 0.02). V. velella thermal niche spanned 13.0–26.7 °C, with an optimum near 14.5 °C, indicating tight coupling between seasonal dynamics and cooler winter-spring temperatures. These findings establish V. velella as a valuable indicator of surface-ocean variability. This study demonstrates the potential of citizen science for identifying biogeographical and phenological responses of pleustonic taxa to climate change.