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This paper concerns the changes occurred over both decennial and millennial spans of time in a sponge assemblage present in coralligenous biogenic build-ups growing at 15 m depth in the Ligurian Sea (Western Mediterranean). The comparison of the sponge diversity after a time interval of about 40 years (1973-2014) showed a significant reduction in species richness (about 45%). This decrease affected mainly the massive/erect sponges, and in particular the subclass Keratosa, with a species loss of 67%, while the encrusting and cavity dwelling sponges lost the 36% and 50%, respectively. The boring sponges lost only one species (25%). This changing pattern suggested that the inner habitat of the bioconstructions was less affected by the variations of the environmental conditions or by the human pressures which, on the contrary, strongly affected the species living on the surface of the biogenic build-ups. Five cores extracted from the bioherms, dating back to 3500 YBP, allowed to analyse the siliceous spicules remained trapped in them in order to obtain taxonomic information. Changes at generic level in diversity and abundance were observed at 500/250-years intervals, ranging between 19 and 33 genera. The number of genera showed a sharp decrease since 3500-3000 to 3000-2500 YBP. After this period, the genera regularly increased until 1500-1250 YBP, from when they progressively decreased until 1000-500 YBP. Tentatively, these changes could be related to the different climatic periods that followed one another in the Mediterranean area within the considered time span. The recent depletion in sponge richness recorded in the Ligurian coralligenous can be considered relevant. In fact, the analysis of the spicules indicated that the sponges living in these coralligenous habitats remained enough stable during 3000 years, but could have lost a significant part of their biodiversity in the last decades, coinciding with a series of warming episodes.

Several sponge species incorporate a wide range of foreign material. Whether such material is actively selected by the sponge is controversial. Here we compare the available suspended matter and the sediment incorporated in the tissue of the demosponge Chondrosia reniformis. Field observations and laboratory experiments indicate that this species selects and incorporates only siliceous materials, in particular quartz particles and opal sponge spicules, avoiding carbonate particles. The reaction of ectosome cells of Chondrosia depends on the forms of silica: after settlement of crystalline quartz particles on the sponge surface, the pinacocytes contract uniformly, giving rise to a ruffled surface that remains throughout the incorporation of foreign material. In contrast, the opal spicules elicit a motile response in pinacocytes, which cover the spicules as a result. After incorporation, while the opal spicules remain unaltered within sponge tissue, the engulfed quartz particles are quickly etched, reduced in size, and released from the sponge. The etching of quartz particles by C. reniformis is produced by ascorbic acid, and is the first evidence of such activity from the animal kingdom. Ascorbic acid has been found to change the quartz surface features, which leads to an increased radical production and a consequent dissolution of quartz. This process does not occur on opal spicules., (Copyright 2003 Wiley-Liss, Inc.)

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. [ABSTRACT FROM AUTHOR]

During the last decades, the Mediterranean shallow-water benthic communities have experienced significant changes in taxa composition and distribution. These variations were related to a complex set of anthropogenic stressors as well as to mass mortality events starting from 1999 and related to the current climatic changes. To evaluate the effect of these changes on long-living species with a limited larval dispersal capacity, a quantitative and qualitative monitoring of the shallow-water hard bottom community structure was performed over a pluri-decennial interval of time. The aim of this work was the comparison of the quali-quantitative occurrence and seasonal cycles of several benthic taxa living on a rocky cliff of the Portofino Promontory (eastern Liguria, Mediterranean Sea), from 15 to 20 m depth. The studied community, within a Lithophyllo–Halimedetum tunae association, was analysed by sets of photographs repeated, exactly in the same site, at 25 years of distance (1987/88 and 2012/13). The results suggest that the macroalgal coverage of the surveyed cliff suffered, during the monitored span of time, a significant depletion in quali-quantitative terms, while the overall sponge coverage remained almost unvaried. Nevertheless, a discrete analysis of the most common sponge species present in the study area showed significant variations, with some taxa now more abundant (Axinella spp. and Agelas oroides), some virtually stable (Cliona viridis and the complex of red encrusting sponges), and others drastically depleted (Chondrosia reniformis, Phorbas tenacior, Acanthella acuta, Ircinia spp., Dysidea avara and Petrosia ficiformis). Finally, some sponge species showed clear seasonal cycles indicating recruitment periods and post-recruit mortalities. [ABSTRACT FROM PUBLISHER]

The Mediterranean coralligenous substratum is a hard bottom of biogenic origin, mainly composed of calcareous algae, growing in dim light conditions. Sponges are among of the most representative taxa of the coralligenous assemblages, with more than 300 recorded species of different habits: massive, erect, boring and insinuating. When sponges die, their siliceous spicules remain trapped in the biogenic concretion, offering the opportunity to describe the coralligenous spongofauna over a very long span of time, virtually dating back to a large part of the Holocene period. The data reported here were obtained from core samples collected from four coralligenous concretions. Each block was collected in a different locality of the Ligurian Sea: Santo Stefano Shoals, Bogliasco, Punta del Faro ( Portofino Promontory) and Punta Manara. Radiocarbon age determinations indicate for these conglomerates a maximal age between 1600 and 3100 years. The spicules trapped in the cores show deep dissolution marks in the form of circular holes on their surface or present an enlargement of the axial canal. However, their original shape, generally intact, suggests the absence of mechanical injuries and allows a tentative identification at the species level. The analysis of these old spicules reveals an ancient sponge assemblage composed of 30 recognisable species. This indicates that almost one half of the sponge community today settled on coralligenous substrata has been present in the conglomerates for their entire existence. [ABSTRACT FROM AUTHOR]