Investigations into the development and role of a Mediterranean intertidal bioconstruction for coastal conservation: the Vermetid Reef

Vermetid reefs are intertidal biogenic habitats created by a dense aggregation of mollusks, frequently cemented by calcareous algae, and are typical of sub-tropical and warm–temperate rocky shores. These bioconstructions are valuable key-habitats of the coastal zones, increasing their productivity and biological value. In the Mediterranean, the main vermetid reef builders belong to the genus Dendropoma and are associated to encrusting coralline red algae. These organisms are ecosystem engineers protected under international European Legislation, although vermetid reef conservation is limited by a lack of biological and ecological knowledge. The two-way interactions between biota and the physical environment have not been previously studied for these reefs, further limiting understanding of the ecological value and function of this habitat. Phases of the Dendropoma life cycle are also understudied, making the key-processes during the reef development unclear. In an integrated coastal zone management framework, all of this knowledge is fundamental to strengthen vermetid reef conservation and management in the Mediterranean. This thesis aims to further understanding of traits of the Dendropoma reef system, undertaking novel themes of research, and by using an interdisciplinary approach. The research project has two main goals: 1) To describe which contribution the reef provide in preserving rock substrate from physical alteration 2) To understand which cues may promote Dendropoma reef formation A set of laboratory and field experiments and observations has been used to fulfill these aims. The study was conducted in Sicily, central Mediterranean, where the main reef builder species is Dendropoma cristatum (Biondi, 1857), associated with the coralline algae Neogoniolithon brassica-florida (Harvey) Setchell & Mason 1943. In detail, topic 1 aims to describe the bio-protective role of the Dendropoma encrustation on the underneath rock substrate. Bioprotection is described as the contribution that biological layers give to the conservation of the substratum they colonize, by mediating the deteriorative action of other factors, such as physical, chemical and biological weathering. In this study, internal temperature variations and salt weathering have been considered as rock physical stressors that are particularly relevant in the coastal zone, and which may be mediated by the Dendropoma encrustation. Lab experiments and analysis have been employed to describe subsurface variations of temperature and salt content inside rock cores covered by live and dead vermetids vs bare rocks. The experiments have demonstrated that the Dendropoma encrustation exerts a control on the factors responsible for rock weathering. Topic 2 aims to detect which physical and biological factors may promote D. cristatum settlement and recruitment, with implications for the reef development.D. cristatum recruitment and settlement have been measured in the field and in different conditions. A seasonal pattern of recruitment has been described for the Sicilian species D. cristatum, and the hydrodynamic regime has been showed do not affect this pattern within a range of Km. Aside, at a small spatial scale (from mm to cm), biogenic surface have been shown to affect the settlement dynamics of the crawling larvae of Dendropoma cristatum, more than physical complexity of the substratum. In detail, biological cues provided by a layer of crustose coralline algae or by a microbial film, may positively affect settlement dynamic, indicating suitable site for larvae attachment and having an influence on the early stages of Dendropoma cristatum development. In conclusion, this research has contributed to the description of some ecological traits of the reef-builder Dendropomacristatumand provided a wider view of the two-way interactions between this ecosystem engineer and the surrounding physical and biological environment. Within the perspective to improve Dendropoma reef management at Mediterranean scale, the gaining of information about the ecological functioning and role of this coastal systems may provide a valuable contribution.