Massive stranding event revealed the occurrence of an overlooked sponge ecosystem engineer in Sardinia (Italy, Western Mediterranean).

Climate change is increasing the frequency, magnitude, and socio-economic costs of unprecedented extreme weather events and disasters. An exceptional stranding event of benthic species, mainly represented by the ecosystem engineer Porifera (Demospongiae) Geodia cydonium (Linnaeus, 1767) was observed in the Gulf of Oristano (Sardinia, Italy, Western Mediterranean) in 2018 after an extreme meteorological event. The stranding phenomenon was assessed and quantified by means of quadrats sampled along the beach highlighting all the conspicuous species; the benthic sponge community in the area was evaluated through underwater SCUBA transects. Stranding reached density peaks of up to 0.88 ind m−2, and the most abundant species reported was the sponge G. cydonium, followed by molluscs, bryozoans, a rare teleost fish, and many other species. The putative source populations were identified in quite shallow (less than 2.5-m depth) Posidonia oceanica (Linnaeus) Delile, 1813 meadows, where the sponge shows a mean density of 2.0 ind m−2. The present massive stranding event is the first in Sardinia and in the Mediterranean for this species, whose distribution is still poorly known. The living population of the gulf still seems in good status, and probably only a little percentage of specimens has been struck by the exceptionally strong hurricane. This work highlights the relevance of stranding events as opportunities for providing information about local biodiversity; biological traits of G. cydonium and the need of effective monitoring plans to fill the lack of knowledge about the distribution of this protected species are discussed.