Implementation of artificial substrates for Dendropoma cristatum (Biondi 1859) reef restoration: Testing different materials and topographic designs.

Vermetid reefs are intertidal key habitats that strongly increase the biological and ecological value of the coastal ecosystems, providing ecological goods and services. In some parts of the Mediterranean Sea, this habitat is experiencing an increasing decline, due to environmental and anthropogenic pressures and the lack of officially coordinated strategies promoting its conservation at the basin scale. Moreover, experimental data testing for vermetid reef restoration is presently scant. Here we tested the potential to achieve new vermetid colonies on engineered substrates, aiming for their further transplantation to depleted reefs for restoration purposes. We investigated in the field the settlement rate of the central-Mediterranean vermetid species Dendropoma cristatum on artificial tiles made of different concrete mixtures (geopolymer concrete, pozzolanic concrete mixed with fine sand, magnesian concrete, pozzolanic concrete mixed with magnesium and coarse sand) and one plastic material (expanded polyvinyl chloride), engineered with a range of interstice shapes and sizes (i.e., holes and crevices of 1 and 2 mm), obtaining on each tile four different topographic designs. Also, the benthic fauna colonization on these engineered tiles has been monitored. While the material typology had a relative effect on the settlement of D. cristatum, the presence of specific topographic designs played an important role, not only for the reef-builder species but also for the benthic fauna. A significantly higher vermetid density and invertebrate taxonomic richness and individual abundance were associated with the hole presence on the tiles, confirming to be the preferred interstice type among those texted. Furthermore, geopolymer concrete was the material that showed higher resistance to the intertidal conditions and durability after the experiment. The production of geopolymer tiles engineered with hole interstices may be a valid option for future studies aiming at the restoration of this intertidal key habitat. However, while our achievements correspond to a higher vermetid and benthic invertebrate settlement within a few weeks of field experiment, post-settlement processes need to be considered in the perspective to restore the functionality of a degraded reef.