Benthic Hydroid Assemblages in the South Adriatic: Spatiotemporal Patterns and Life-Cycle Plasticity in Stylactis inermis

We investigated the biodiversity and spatiotemporal dynamics of benthic hydroids at two contrasting eastern South Adriatic sites: exposed, oligotrophic Lokrum Island and sheltered, nutrient-enriched Bistrina Bay. A total of 54 hydroid taxa were recorded, with substantially higher richness at Lokrum (42 taxa) than at Bistrina (24 taxa). Assemblage composition differed markedly between sites, confirming that local environmental conditions are a primary determinant of community structure, while shallow sublittoral assemblages showed the greatest temporal variability due to seasonally short-lived athecate species. The shared seasonal partitioning at both sites suggests that temperature-mediated life-cycle timing is a key structuring mechanism, and the sharp summer decline in richness underscores the need for multi-seasonal sampling. Laboratory observations of Stylactis inermis from Torre del Serpe near Otranto revealed notable life-cycle plasticity, with detached short-lived eumedusoids reverting to a sessile stolonal stage. This trait may promote persistence under fluctuating conditions while reducing field detectability. Together, these results provide the first seasonal, depth-stratified ecological baseline for monitoring eastern South Adriatic benthic communities under environmental and anthropogenic change.