Climate-driven shifts in a Mediterranean hydrozoan assemblage over 44 years

Abstract Temporal changes in marine assemblages are well documented for large habitat-formers, while data on inconspicuous groups like hydrozoans are scarce. Long-term shifts in hydrozoan communities were assessed by replicating in 2024 an annual survey originally conducted in 1980 and repeated in 2004 along a depth gradient along the rocky cliff of Portofino Promontory (Ligurian Sea). The species richness declined significantly from 83 (1980) to 72 (2004) and 43 (2024), with the greatest losses in cold-affinity species. Several winter-dominant species, such as Eudendrium glomeratum Picard, 1952, Sertularella crassicaulis (Heller, 1868), and Ectopleura larynx (Ellis and Sollander, 1786), have strongly reduced or disappeared. In contrast, summer species like Eudendrium racemosum (Cavolini, 1785) are now present year-round. The appearance of southern species like Corydendrium parasiticum (Linnaeus, 1767) and Pennaria disticha Goldfuss, 1820, along with reduction in sexual reproduction reflect a community-level response to ocean warming, leading to seasonal and bathymetric homogenization. This 44-year dataset provides a rare long-term benchmark and supports the importance of accurate taxonomic analysis for understanding climate-driven transformations in Mediterranean benthic ecosystems.