Have climate changes driven the diversity of a Mediterranean coralligenous sponge assemblage on a millennial timescale?

Changes occurring over a pluri-millennial span of time (6207–1767 yr B.P.) within the sponge assemblages present in two coralligenous pillars, about 60 cm high, collected at 20 m depth, inside the Marine Protected Area of Porto Cesareo (Ionian Sea, Mediterranean Sea) have been studied. The sponge diversity has been analyzed at generic level, using as a proxy the siliceous spicules trapped into the sediment present into the crevices of these bioherms. In total, 41 genera of ancient demosponges were identified. The modern sponge fauna of these coralligenous pillars totals 31 genera, 15 of them shared with the ancient assemblages. The sponge biodiversity during the considered span of time showed critical changes, with remarkably similar trends in both the considered pillars. Tentatively, these changes were attributed to different climatic events that affected the Mediterranean area during the studied time span. The number of genera, 6000 yr B.P., was low and increased between 5500 and 4000 yr B.P. During the Late Bronze Age (4000–3000 yr B.P.), a new decrease was recorded, followed by a further increase around 2000 yr B.P. Afterwards, the bioherms stopped growing and no more siliceous spicules were trapped. From a biogeographic point of view, the wide presence of spiny microrhabdose microscleres belonging to the genus Alveospongia (Heteroxyidae), recently described from the tropical Atlantic and still unknown in the Mediterranean Sea, has to be underlined. In addition the trends recorded over time in the Ionian Sea match with those already observed in the Ligurian Sea. The comparison between the past periods and the modern one shows evident changes in biodiversity in both areas. The sharp variations of the seawater temperature - occurred in both the Mediterranean regions - could have driven the changes in genus richness. Using sponges to interpret the effects of climate changes can be a profitable strategy to unravel long term changes in biodiversity, supporting future predictions.