Sabellaria from larvae to reff: growth, ecological role and habitat mapping

Sabellaria (Annelida, Polychaeta) is a genus of marine polychaete worms belonging to the family Sabellariidae. The member of this taxon are gregarious engineer species called "honeycomb worms" because they are able to build bioconstructions (sheets, hummocks and reefs) composed by tubes made of sand and bioclasts, held together with an organic mucus. Under opportune environmental conditions, Sabellaria can form dense and solid aggregates, resulting in massive structures. The sabellariid worm Sabellaria alveolata is known to form large intertidal colonies on the North-western coasts of Europe which can reach several km² and it can be considered as the most important building organism in temperate shallow environments. Sabellaria bioconstructions contribute to physical and biological processes by stabilizing the sandy substrate, trapping sediment, shaping the seascape, and therefore mitigating coastal erosion. Moreover, such bioconstructions produce secondary substrate that could increase substratum heterogeneity and coastal biodiversity by providing an array of microhabitats suitable for the settlement of benthic organisms. As a consequence, Sabellaria reefs are considered as coastal biodiversity hotspots, supporting a rich associated fauna and playing a key role in biodiversity conservation, by acting as nursery for some coastal species and gaining an important functional role. Two species belonging to the genus Sabellaria are widespread in the Mediterranean Sea: S. alveolata (Linnaeus, 1767) and S. spinulosa (Leukhart, 1849). In the Mediterranean area, S. alveolata can usually form true reefs, while S. spinulosa rarely forms extended bioconstructions and often it only encrusts the hard-substrate with isolated tubes. Recently, along the northern Gargano coast (Adriatic Sea, southern Italy), a large S. spinulosa reef has been discovered for the first time in the Mediterranean Sea. As regards the Mediterranean basin, no quantitative analyses to assess their current health status and to define their dynamics, such as larval stages, development phases, ecological role and their occurrence along Italian coasts, let alone evaluate the most important threats and impacts, are currently available. In fact, despite such organogenic structures are included in the European Red List of Habitats and evaluated for protection by a lot of European Union legislative instruments in the Atlantic area, in the Mediterranean Sea they are currently listed as "Data Deficient", thus needing more specific investigation aimed to fill the gap in the empirical and quantitative data. The present work is the starting point for appropriate studies of Sabellaria reefs, providing useful supplies for the assessment of protection measures in the Mediterranean Sea, as requested by the International Union for Conservation of Nature (IUCN). During this PhD project, some interesting results emerged, remarkably increasing the knowledge about Sabellaria reefs along Italian coasts: the main phases of the life cycle of Sabellaria were investigated, starting from the larvae, whose larval peaks was detected for the first time in the Mediterranean Sea, to the adult stage. During the developmental phases, the reef growth and the seasonal variations were investigated for a period of one year in two different stations, reporting the largest S. alveolata reef in the Mediterranean Sea and the first S. spinulosa true reef in this basin. Moreover, the ecological role of Sabellaria as biodiversity hotspot and as reservoir of preferential species was investigated, by considering the close relation of its associated fauna and the benthic species living in adjacent substrates, both sandy and rocky bottoms. Finally, the Sabellaria bioconstructions widespread along Latium coast (Central Tyrrhenian Sea) were mapped by using in situ observations and innovative technologies, such us an unmanned aerial vehicle ("drone"), in order to achieve extremely detailed habitat maps, filling the gaps about their occurrence and area of occupancy. The great amount of data collected during this research allow us to achieve useful information to comprehend the ecology of Sabellaria bioconstructions in the Mediterranean Sea, so contributing to reduce the information gap on Mediterranean reefs.