Ancient and recent sponge assemblages from the Tyrrhenian coralligenous over millennia (Mediterranean Sea)

The demosponge siliceous spicules, entrapped in the Mediterranean coralligenous bioherms, are a good tool for reconstructing the sponge ancient communities across time, also allowing a comparison with the current biodiversity. The aim of this paper has been the description of the ancient sponge assemblages of the coralligenous bioherms grown up in the Tavolara-Capo Coda Cavallo Marine Protected Area (northeastern Sardinia, Tyrrhenian Sea) between 5800 and 3700 YBP and its comparison with the living sponge community. On the base of radiocarbon age determinations, it was possible to estimate that these bioherms cover a span of time ranging from the Holocene Climatic Optimum until the Bronze Age Crisis. Their growth rates, 0.21–0.24 mm year −1 , were of the same order of estimations conducted around the Mediterranean. The Tavolara ancient sponge diversity was compared with those found in ancient accretions from Ligurian Sea and Ionian Sea. Our results supported the already-described general trend, supporting the hypothesis that the genus richness in the coralligenous bioherms was driven by climatic vicissitudes. Nevertheless, our data strongly indicated that the sponge diversity is also related to the conditions of the coralligenous structures: in the accretions characterized by an active growth of coralline algae the number of sponge genera is similar to those recorded in the ancient assemblages while in coralligenous where the accretion is stopped the number of sponge genera strongly decreases probably due to the competition with frondose macroalgae.