SUMMARYAnthropogenic CO 2 is reducing the pH and altering the carbonate chemistry of seawater, with repercussions for marine organisms and ecosystems. Current research suggests that calcification will decrease in many species, but compelling evidence of impaired functional performance of calcium carbonate structures is sparse, particularly in key species. Here we demonstrate that ocean acidification markedly degrades the mechanical integrity of larval shells in the mussel Mytilus californianus, a critical community member on rocky shores throughout the northeastern Pacific. Larvae cultured in seawater containing CO 2 concentrations expected by the year 2100 (540 or 970ppm) precipitated weaker, thinner and smaller shells than individuals raised under presentday seawater conditions (380ppm), and also exhibited lower tissue mass. Under a scenario where mussel larvae exposed to different CO 2 levels develop at similar rates, these trends suggest a suite of potential consequences, including an exacerbated vulnerability of new settlers to crushing and drilling attacks by predators; poorer larval condition, causing increased energetic stress during metamorphosis; and greater risks from desiccation at low tide due to shifts in shell area to body mass ratios. Under an alternative scenario where responses derive exclusively from slowed development, with impacted individuals reaching identical milestones in shell strength and size by settlement, a lengthened larval phase could increase exposure to high planktonic mortality rates. In either case, because early life stages operate as population bottlenecks, driving general patterns of distribution and abundance, the ecological success of this vital species may be tied to how ocean acidification proceeds in coming decades.Key words: biomineralization, early survivorship, environmental change, form and function, shell properties. THE JOURNAL OF EXPERIMENTAL BIOLOGY Impaired larval shell integrityPotential tradeoffs among calcification and other physiological responses are likewise poorly understood (Hofmann and Todgham, 2010). Most marine calcifiers can increase fluid pH and carbonate ion concentration at the site of crystal nucleation, which enables synthesis of shells and/or skeletons even when external seawater parameters are thermodynamically unfavorable for the formation of CaCO 3 (Cohen and Holcomb, 2009). Maintenance of local conditions that differ from those of surrounding waters, however, often depends on active ion transport that is energetically costly (Palmer, 1992; Cohen and Holcomb, 2009). Whether OA-induced energetic expenditures require that organisms differentially prioritize certain physiological processes is largely unknown (Widdicomb and Spicer, 2008). In shell-forming species, for instance, it is unclear whether decreased growth arises from somatic resources being redirected to fortify the shell or as a direct consequence of acidified seawater. If OA reduces both growth and shell integrity, it is uncertain which might be degraded more strongly.Such que...

Predicting impacts of global environmental change is challenging due to the complex life cycles that characterize many terrestrial and aquatic taxa. Different life stages often interact with the physical environment in distinct ways, and a growing body of work suggests that stresses experienced during one life stage can "carry over" to influence subsequent stages. Assessments of population responses to environmental perturbation must therefore consider how effects might propagate across life-history transitions. We investigated consequences of ocean acidification (decreased pH and carbonate saturation) for early life stages of the Olympia oyster (Ostrea lurida), a foundation species in estuaries along the Pacific coast of North America. We reared oysters at three levels of seawater pH, including a control (8.0) and two additional levels (7.9 and 7.8). Oysters were cultured through their planktonic larval period to metamorphosis and into early juvenile life. Larvae reared under pH 7.8 exhibited a 15% decrease in larval shell growth rate, and a 7% decrease in shell area at settlement, compared to larvae reared under control conditions. Impacts were even more pronounced a week after settlement, with juveniles that had been reared as larvae under reduced pH exhibiting a 41% decrease in shell growth rate. Importantly, the latter effect arose regardless of the pH level the oysters experienced as juveniles, indicating a strong carry-over effect from the larval phase. Adverse impacts of early exposure to low pH persisted for at least 1.5 months after juveniles were transferred to a common environment. Overall, our results suggest that a stringent focus on a single phase of the life cycle (e.g., one perceived as the "weakest link") may neglect critical impacts that can be transferred across life stages in taxa with complex life histories.

There is growing likelihood of minerals mining in the deep sea. (64)  Assessing the significance of resulting environmental impacts takes on urgency. (79)  The ISA is developing regulations for seabed mining which must prevent serious harm. Defining "serious harm" is critical to effective regulation of mining activities. (82)  Deep faunal vulnerabilities derive from low growth rates, species longevity and rarity. Connectivity, resilience, and cumulative impacts are key to significance assessment.