Sabellaria alveolata sandcastle worm from the Mediterranean Sea: new insights on tube architecture and biocement

Abstract

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.

<p class="ZootaxaTitle"><strong>Distribution of<em> Sabellaria alveolata</em> (Polychaeta Sabellariidae) in the Mediterranean Sea: update and new findings</strong></p>

“…The present paper and Sanfilippo et al (2019) contribute three new findings, still from the western Mediterranean, all from the Sicily Straits, in the sector H, which add to the known literature ones. Finally, bioconstruction found in the Ionian Sea represent the first ever record from the sector G (Tab.1, Fig.…”

“…The structure of individual tubes, the particle sizes and shapes used in the tube construction, as well as the nature of the biocement of the bioconstruction have been analysed by Sanfilippo et al (2019).…”

“…S. alveolata bioconstructions consist of tube aggregates forming pillows or hummocks often coalescing into reefs, up to one meter in height and several tens square meters in extension (Delbono et al 2003;. Reef accretion happens on both hard and soft bottoms and relays on the worm's ability to catch sand grains and cement them to build the walls of the aggregated tubes (Vovelle 1965;Gruet et al 1987;Dubois et al 2005;Sanfilippo et al 2019;Lisco et al 2020).…”

Sabellaria alveolata versus Sabellaria spinulosa Reefs along the Italian Coasts: A New Methodological Proposal to Compare Different Growth Models

“…At the microscale, the general framework of the reef is defined by two main elementary structural units [15,26,27]: (1) the sands cemented directly by the worms (the tube area that is ordered in concentrical layers-t) and (2) the sands that remain trapped between a tube and the adjacent one (the intertube area with elements irregularly arranged-i). According to the literature, the tube has a known three-layered structure with different sizes and shapes of cemented grains [27][28][29]. The grains of sand, the inner layer of the tube, cemented directly by the worm are mainly elongated, flattened and oriented tangentially to the development of the tubes.…”

Devonian agglutinated polychaete tubes: all in all it's just another grain in the wall

“…Finally, and possibly most importantly, none of the above-mentioned groups show flanged tubular morphologies, besides annelids. Indeed, many extant polychaete families are capable of building tubular constructions with agglutinated material (some with high capacity of particle selectivity), including Sabellariidae [38], Pectinariidae [39], Oweniidae [10], Terebellidae [40], Maldanidae [41], Sabellidae [42], Capitellidae [43], Cirratulidae [43], Spionidae [44], Ampharetidae [45], Eunicidae [46], Fabricidae [47], Onuphidae [48], Nereididae [49], Trichobranchidae [50] and possibly even Phyllodocidae [51]. However, most of these groups produce agglutinated tubes that are simple cylindrical forms, without complex morphologies.…”