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Mollusc assemblages of three concrete artificial reefs (ARs) in the Gulf of Castellammare were compared with those of two nearby natural reefs (NRs). The reefs were located in areas characterized by different degrees of water transparency, with average annual Secchi disk visibility ranging from clear (>20 m) to turbid waters (<6 m). In spring 1995, 28 samples of 400 cm2 were scraped off the reefs at depths of 16–22 m (20 from ARs; 8 from NRs), which yielded a total of 116 species and 1084 specimens of molluscs. The assemblage derived from a NR in clear water showed the highest diversity, species richness, and evenness, while the highest number of specimens and dominance values were observed at an AR in turbid water. The lowest values of diversity and species richness were found at ARs in turbid water and the lowest number of specimens at a NR in turbid water. Comparison of the five reefs showed significant differences in average number of species and in diversity values, while differences in the number of specimens were not significant. Factorial correspondence analysis showed a pattern that was strongly polarized along the first axis by the NR in clear water and by the AR in turbid water. The most distinctive feature was the dominance of Bittium latreillii at AR sites. Three years after deployment, mollusc assemblages of ARs in the Gulf remain entities that are distinct from those of nearby natural reefs. However, the potential of the assemblages in terms of density of individuals is comparable to that of natural reefs. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved. [ABSTRACT FROM PUBLISHER]

Expected temperature rise and seawater pH decrease may affect marine organism fitness. By a transplant experiment involving air-temperature manipulation along a natural CO 2 gradient, we investigated the effects of high pCO 2 (~1100 μatm) and elevated temperature (up to +2 °C than ambient conditions) on the reproductive success, recruitment, growth, shell chemical composition and oxygen consumption of the early life stages of the intertidal reef-building vermetid Dendropoma cristatum. Reproductive success was predominantly affected by temperature increase, with encapsulated embryos exhibiting higher survival in control than elevated temperature conditions, which were in turn unaffected by altered seawater pH levels. Decreasing pH (alone or in combination with temperature) significantly affected the shell growth and shell chemical composition of both embryos and recruits. Elevated temperatures along with lower pH led to decreases of ~30% oxygen consumption and ~60% recruitment. Our results suggest that the early life stages of the reef-builder D. cristatum are highly sensitive to expected environmental change, with major consequences on the intertidal vermetid reefs they build and indirectly on the high biodiversity levels they support., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Experiments have shown that increasing dissolved CO 2 concentrations (i.e. Ocean Acidification, OA) in marine ecosystems may act as nutrient for primary producers (e.g. fleshy algae) or a stressor for calcifying species (e.g., coralline algae, corals, molluscs). For the first time, rapid habitat dominance shifts and altered competitive replacement from a reef-forming to a non-reef-forming biogenic habitat were documented over one-year exposure to low pH/high CO 2 through a transplant experiment off Vulcano Island CO 2 seeps (NE Sicily, Italy). Ocean acidification decreased vermetid reefs complexity via a reduction in the reef-building species density, boosted canopy macroalgae and led to changes in composition, structure and functional diversity of the associated benthic assemblages. OA effects on invertebrate richness and abundance were nonlinear, being maximal at intermediate complexity levels of vermetid reefs and canopy forming algae. Abundance of higher order consumers (e.g. carnivores, suspension feeders) decreased under elevated CO 2 levels. Herbivores were non-linearly related to OA conditions, with increasing competitive release only of minor intertidal grazers (e.g. amphipods) under elevated CO 2 levels. Our results support the dual role of CO 2 (as a stressor and as a resource) in disrupting the state of rocky shore communities, and raise specific concerns about the future of intertidal reef ecosystem under increasing CO 2 emissions. We contribute to inform predictions of the complex and nonlinear community effects of OA on biogenic habitats, but at the same time encourage the use of multiple natural CO 2 gradients in providing quantitative data on changing community responses to long-term CO 2 exposure., (Copyright © 2019 Elsevier B.V. All rights reserved.)