Neave, E. F.; Dimitriadis, C.; Shum, P.; Riesgo, A.; Mariani, S.

Tropicalization, the process by which tropical species expand their ranges poleward due to global ocean warming, is a prominent threat to Mediterranean marine ecosystems, challenging their effective management and conservation. The arrival of non-indigenous fish is exacerbated by the Suez Canal that allows Indo-Pacific species to enter the region from the Red Sea. While some non-indigenous species (NIS) have already changed the composition and function of native Mediterranean communities, many others continue to arrive but often go unnoticed without regular surveys or until they are strongly established. We conducted biodiversity surveys using underwater visual census (UVC), aqueous environmental DNA (eDNA) and sponge-derived eDNA (i.e., eDNA accumulated in sea sponge tissue) at two protected locations in Zakynthos, Greece. Seven NIS were detected by eDNA, but only one (dusky spinefoot, Siganus luridus) was detected with all three methods. The fish assemblages of the two locations could be distinguished based on both UVC and eDNA but not from sponge-derived eDNA data, perhaps given low filtration rates. Of the three methods, aqueous eDNA metabarcoding provided the most comprehensive species list including new NIS detected (with the red-toothed triggerfish, Odonus niger, and the houndfish, Tylosurus crocodilus, being the first and third record in the Mediterranean Sea, respectively). Our findings highlight the potential value of incorporating molecular methods into regular monitoring as early warning tools for detecting NIS in marine protected areas threatened by ocean warming.