Our results show that the level of ocean acidification predicted to occur this century and beyond1,25,26 impairs recruitment success, causes shell dissolution and alters the shell mineralogy of the reef-building gastropod Dendropoma petraeum. Post-settlement survival of new recruits did not decrease until very low pH conditions were reached where the results of our experiment were probably artificially enhanced by wide variability in CO 2 levels.

Average surface seawater CO 2 concentrations are expected to reach between 443 and 541 μatm in 2050, up to 936 μatm in 2100 and beyond 1900 μatm in 2300 causing present-day values in average global surface ocean pH to fall by 0.14 units by 2050, of 0.3–0.4 units by 2100 and >0.7 units by 230025,26,27. Our vermetid cores were exposed to near-future pCO 2 and pH levels at the High pH site, to conditions expected by the end of this century at the Mid pH site and to more extreme pCO 2 and pH at the Low pH site. The gradients in carbonate chemistry off Vulcano Island were consistent with previous observations of the area28,29 and of other CO 2 seeps10,11,30 and varied due to changes in wind-driven currents28. Background fluctuations in the seawater carbonate chemistry were the same at our reference site and the reef from which core samples were taken at 81.5 nautical miles distance. Given that the seawater carbonate chemistry of coastal marine ecosystems typically varies widely as a result of diel fluctuations in photosynthesis and respiration10 is useful to incorporate such variability into OA studies31.

The reef-building gastropod D. petraeum has a peculiar reproductive strategy and a highly specialised mode of development. Sperm are encapsulated in spermatophores and held for a couple of months in the female mantel cavity; internal fertilization occurs when the seawater starts to warm in late March-May32. Lecithotrophic larval development occurs in capsules within the female shell (usually each female holds up to 25 capsules each containing 1–6 embryos) and larvae take a month to develop and hatch17. Crawling larvae settle a few hours after hatching17 (Supplementary Video V1 online). We found that although recruits were produced all along the CO 2 gradient, there were 4 to 7 times fewer living young snails in reef cores exposed to CO 2 levels expected by 2100 (Mid pH site) and beyond (Low pH site) than at near-future (High pH) and reference conditions (CTL_Vent and CTL_Core). This trend was not related to the number of adult D. petraeum nor the amount of the coralline alga Neogoniolithon brassica-florida.

As shown for some marine gastropod species33,34 and other invertebrate taxa18,22, the decrease in recruitment we observed may be due to adverse effects of increased pCO 2 levels on D. petraeum early-life history stages (i.e. fertilization, larval development and settlement). To ensure that the female adults of the reef-building gastropod experienced fertilization and brooded fertilised eggs in capsules along the Vulcano Island CO 2 gradient, we transplanted our vermetid cores at the end of the 2010 reproductive season (i.e. late November 2010) and before the next one started (April 2011)17. Under these circumstances all the vermetid females were simultaneously exposed for >1 month (in the short-term experiment) or for >6 months (in the long-term experiment) to the experimental pH conditions. At present it is unknown if vermetid larval development in egg casings within the maternal mantle cavity is affected by external environmental conditions. However, embryonic development within the egg capsules may be robust, since Sepia officinalis larvae are able to develop normally despite very high levels of CO 2 (i.e., 4000 μatm) within the egg capsules35.

Upon hatching, vermetid larvae crawled out of the maternal shell and cemented themselves next to the mother using a flat aragonite disc. Although some molluscs can up-regulate calcification and tolerate acidified waters when a protective organic periostracum prevents shell dissolution12, the vermetid settlement disc lacks this organic protective layer17. Therefore in areas subjected to periodic aragonite sub-saturation we found that the settlement discs were weakened by dissolution and that the new recruits were easily dislodged. We found no sign of shell dissolution of vermetid recruits at pCO 2 levels expected to occur in the next few decades (the High pH site) but their shells did dissolve at pCO 2 levels expected by the end of this century and beyond (in the Mid and Low pH site). Therefore, the lower post-settlement survival we recorded in the cores exposed to the most extreme pH conditions could be even overestimated as dissolution and dislodgement of dead shells under frequent periods of aragonite under-saturation (with average values of pCO 2 above 3000 μatm and Ω ara below 1) can occur. In estuaries, shell dissolution of newly settled juvenile bivalves exposed to under-saturated conditions can be a significant source of mortality that presents a bottleneck that can prevent successful recruitment36. The same is true for foraminifera as the high surface to volume ration of these small organisms means that they dissolve easily at 450 μatm pCO 2 37.

The vermetid shells laid down at elevated pCO 2 had significantly higher Mg/Ca ratios than shells grown in seawater with normal carbonate saturation states (Fig. 4A), as also shown in laboratory studies38,39,40,41. The incorporation of Mg has been used as temperature proxy, yet the role of other factors is poorly understood38,39,40,41. Variation in Mg/Ca ratio due to reduced carbonate saturation has been found in serpulid tubeworms22,23 and some foraminifera41,42,43. The impact of seawater carbonate concentration on Mg/Ca, should therefore be taken into consideration when dealing with Mg/Ca thermometry43. As temperature, salinity and alkalinity were similar across our sampling site, we argue that it was differences in carbonate chemistry that altered the Mg/Ca ratio of the vermetid shells. Mg can substitute for Ca in carbonate and many animals remove Mg from the calcification fluid since suboptimal Mg levels can weaken shells44,45,46. The increased Mg/Ca ratio we found in waters with lowered carbonate ion concentrations may reflect an impaired ability of the vermetids to remove Mg from haemolymph and extrapallial fluids. This inability to remove Mg from the calcification fluid may inhibit crystal nucleation47. In contrast, because no differences were observed on shell Sr/Ca ratios between transplant locations, it is likely that D. petraeum recruits, although growing in seawater with significantly different carbonate concentrations, were calcifying at similar rates (Fig. 4B)48,49,50. Incorporation of the larger cations of Sr plays a role in stabilization of the orthorhombic aragonite lattice51, so an increased incorporation of the smaller Mg may interfere the lattice structure of aragonite. The effects of OA on material behaviour and chemical properties warrants further investigation.