INTEGRATING HISTORICAL DATA, FIELDWORK, MORPHOLOGICAL AND MOLECULAR ANALYSES TO MONITOR THE ASCIDIAN BIODIVERSITY IN THE VENICE LAGOON

Biodiversity loss is one of the most pressing environmental challenges of our time, driven by climate change, habitat alteration, and the introduction of non-indigenous species, with particularly strong impacts on transitional ecosystems. The Venice Lagoon, the largest transitional system in the Mediterranean Sea, is characterized by pronounced spatial heterogeneity, steep environmental gradients, and intense human influence, making it especially vulnerable to biological invasions. In this context, ascidians (Tunicata) represent a key component of fouling communities due to their ecological plasticity and invasive potential, while posing major taxonomic challenges because of widespread species crypticity. This thesis aims to update and expand knowledge of ascidian biodiversity in the Venice Lagoon through an integrative taxonomic approach and to evaluate its relevance for ecological and management studies. By critically reviewing literature from the late eighteenth century to the present, the work reconstructs changes in nomenclature and highlights issues of taxonomic discontinuity, emphasizing the complementary roles of morphological and molecular methods. Results demonstrate that, despite the growing use of DNA barcoding, morphological taxonomy remains fundamental, given the limited availability of reference sequences for ascidians and the risk of misidentification when molecular tools are used alone. Seasonal and annual surveys conducted between 2023 and 2025 reveal substantial changes in ascidian assemblages compared to the 1970s, including eleven new species records, four additional NIS, and the first Mediterranean record of Perophora japonica. Increased water temperatures and local ecological knowledge indicate strong links between climate change, aquaculture activities, and ascidian spread. Finally, the integrative study of Didemnum pseudovexillum, a cryptic species of Didemnum vexillum, underscores the importance of accurate species identification for assessing invasion risk and developing effective management strategies.