The Atlantic-Mediterranean polychaete Sabellaria alveolata lives in agglutinated tubes adjoined to each other to form discrete reef-like bioconstructions in shallow-water settings characterised by high hydrodynamic energy where sediment particles are constantly resuspended. Tubes are built with sand grains glued by proteinaceous secretions. Analyses of a reef fragment collected near Sampieri (SE Sicily, Sicily Strait) allowed the first detailed description of the tube architecture and biocement of this worms from the Mediterranean. The tube consists of an inner thin organic membrane and three agglutinated layers including: (a) a thin inner layer of flat grains arranged side by side; (b) a thick mid layer with a frame of relatively large sub-rounded grains with cavities partly filled by small grains; and (c) a thin outer layer of large, flat to curved, usually biogenic clasts diverging towards the opening. This particular architecture is distinctive of the family. Morphological and epifluorescence observations revealed that biocement consists of drops at the contact between sub-spherical grains and strips along edges of flat grains. Biocement is a solid foam-like material characterised by high abundance of carbon; the presence of phosphorous and nitrogen confirms its proteinaceous composition. Due to the electrostatic interaction with the proteins, calcium and magnesium are most likely complexed to the cement rather than being trapped in the cells. These elements contribute to the solidification of the glue and stabilisation of the tube structure. However, the organic nature of cement and the high energy of their habitat, make sabellariid reefs dynamic and ephemeral, and the preservation as fossils unlikely, with a confident record only extending back to the Miocene.

An update of the geographical distribution of the sabellariid polychaete Sabellaria alveolata (Linnaeus, 1767) within the Mediterranean Sea is provided after checking the known literature. This shallow-water, reef-forming species is first recorded from new sites in southeastern Sicily, both along the Sicily Straits and the Ionian Sea, from where S. alveolata was so far unknown.These new collections also provided material for a detailed description and SEM documentation of morphological features of the operculum and the body.

The polychaete worm Sabellaria alveolata builds shallow-water aggregates of tubes by agglutinating sands using a secreted glue. Sabellarid bioconstructions represent fragile and dynamic habitats that host numerous associated organisms, playing a key ecological role. A two-year study on bioconstructions from three Sicilian sites (Simeto, Portopalo, and Falconara) investigated the balance between reef status and environmental parameters through a geochemical comparison of biocement tube portions and the surrounding waters. Water pollution by heavy metals, which is monitored in marine waters, is a result of river, domestic, and industrial discharges. The major constituents from the biocements of the three sites showed concentrations comparable to those in the seawater, while trace elements (Cr, Ni, Cu, Zn, and As) showed concentrations significantly higher than the mean seawater composition. These similar trends confirm a close dependence between the presence of trace elements (metals) in the seawater and the subsequent bioaccumulation in the biocement produced by the worm. The results also showed that Ca and Mg are fractionated by biocement independent of their water concentrations, in contrast to the trace elements. Further studies addressing the biomineralization processes and the relative fractionation of trace elements in Sabellaria biocement will allow it to be validated as a valuable proxy for short- and long-term environmental studies.

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