Description: Ocean acidification (OA)—a process describing the ocean's increase in dissolved carbon dioxide ( pCO2) and a reduction in pH and aragonite saturation state (Ωar) due to higher concentrations of atmospheric CO2—is considered a threat to bivalve mollusks and other marine calcifiers. While many studies have focused on the effects of OA on shell formation and growth, we present findings on the separate effects of pCO2, Ωar, and pH on larval feeding physiology (initiation of feeding, gut fullness, and ingestion rates) of the California mussel Mytilus californianus. We found that elevated pCO2 delays initiation of feeding, while gut fullness and ingestion rates were best predicted by Ωar; however, pH was not found to have a significant effect on these feeding processes under the range of OA conditions tested. We also modeled how OA impacts on initial shell development and how feeding physiology might subsequently affect larval energy budget components (e.g. scope for growth) and developmental rate to 260 µm shell length, a size at which larvae typically become pediveligers. Our model predicted that Ωar impacts on larval shell size and ingestion rates over the initial 48 h period of development would result in a developmental delay to the pediveliger stage of 〉4 d, compared with larvae initially developing in supersaturated conditions (Ωar 〉 1). Collectively, these results suggest that predicted increases in pCO2 and reduced Ωar values may negatively impact feeding activity and energy balances of bivalve larvae, reducing their overall fitness and recruitment success.

Description: Anthropogenic modification watersheds and climate change have altered export from fluvial systems causing changes to the carbonate chemistry of river-influenced near shore environments. To determine the possible effects of riverine discharges on the mussel Perumytilus purpuratus, we performed in situ transplant experiments between river-influenced and open coastal habitats with contrasting seawater carbonate chemistries (i.e., pCO2, pH, Omega ar) across four regions covering a wide latitudinal range (32°55'S-40°10'S). The river-influenced habitats selected for transplant experiments were different than open coastal habitats; with higher pCO2 (354-1313 µatm), lower pH (7.6?7.9) and Omega ar values (0.4?1.4) than in open coastal area. Growth, calcification, metabolism were measured in a reciprocal transplant experiment to determine physiological responses associated with river-influenced sites and non-influenced control sites. Growth and calcification rates were higher in river-influenced habitats; however the organisms in this area also had lower metabolic rates, possibly due to enhanced food supply from river systems. Further analysis of carbon isotopic composition (delta 13C) indicated that the relative contribution of seawater dissolved inorganic carbon (DIC) to the carbonate shells of P. purpuratus was much higher than respiratory carbon. Nevertheless, P. purpuratus incorporated between 7% and 26% of metabolic carbon in the shell depending on season. There was a strong, significant relationship between delta 13C POC and delta 13C Tissue, which likely influenced the isotopic composition of the shell carbon.

Description: Anthropogenic carbon dioxide (CO2) emissions reduce pH of marine waters due to the absorption of atmospheric CO2 and formation of carbonic acid. Estuarine waters are more susceptible to acidification because they are subject to multiple acid sources and are less buffered than marine waters. Consequently, estuarine shell forming species may experience acidification sooner than marine species although the tolerance of estuarine calcifiers to pH changes is poorly understood. We analyzed 23 years of Chesapeake Bay water quality monitoring data and found that daytime average pH significantly decreased across polyhaline waters although pH has not significantly changed across mesohaline waters. In some tributaries that once supported large oyster populations, pH is increasing. Current average conditions within some tributaries however correspond to values that we found in laboratory studies to reduce oyster biocalcification rates or resulted in net shell dissolution. Calcification rates of juvenile eastern oysters, Crassostrea virginica, were measured in laboratory studies in a three-way factorial design with 3 pH levels, two salinities, and two temperatures. Biocalcification declined significantly with a reduction of ~0.5 pH units and higher temperature and salinity mitigated the decrease in biocalcification.

Description: Increasing anthropogenic carbon dioxide is altering marine carbonate chemistry through a process called ocean acidification. Many calcium carbonate forming organisms are sensitive to changes in marine carbonate chemistry, especially mollusk bivalve larvae at the initial shell building stage. Rapid calcification, limited energy reserves, and more exposed calcification surfaces, are traits at this stage that increase vulnerability to ocean acidification through our previously argued kinetic-energetic hypothesis. These developmental traits are common to broadcast spawning bivalve species that are the focus of most ocean acidification studies to date. Some oyster species brood their young, which results in slower development of the embryos through the initial shell formation stage. We examined the responses of the brooding Olympia oyster, Ostrea lurida, during their initial shell building stage. We extracted fertilized eggs from, O. lurida, prior to shell development, then exposed developing embryos to a wide range of marine carbonate chemistry conditions. Surprisingly, O. lurida showed no acute negative response to any ocean acidification treatments. Compared to the broadcast spawning Pacific oyster, Crassostrea gigas, calcification rate and standardized endogenous energy lipid consumption rate were nearly 10 and 50 times slower, respectively. Our results suggest that slow shell building may lessen the energetic burden of acidification at this stage and provides additional support for our kinetic-energetic hypothesis. Furthermore, these results may represent an example of exaptation; fitness conveyed by a coopted trait that evolved for another purpose, a concept largely lacking in the current perspective of adaptation and global climate change.

Description: Ocean acidification (OA) has had significant negative effects on oyster populations on the west coast of North America over the past decade. Many studies have focused on the physiological challenges experienced by young oyster larvae in high pCO2/low pH seawater with reduced aragonite saturation state (Omega arag), which is characteristic of OA. Relatively few, by contrast, have evaluated these impacts upon fitness traits across multiple larval stages and between discrete oyster populations. In this study, we conducted 2 replicated experiments, in 2015 and 2016, using larvae from naturalized 'wild' and selectively bred stocks of the Pacific oyster Crassostrea gigas from the US Pacific Northwest and reared them in ambient (~400 µatm) or high (1600 µatm) pCO2 seawater from fertilization through final metamorphosis to juvenile 'spat.' In each year, high pCO2 seawater inhibited early larval development and affected the timing, but not the magnitude, of mortality during this stage. The effects of acidified seawater on metamorphosis of pediveligers to spat were variable between years, with no effect of seawater pCO2 in the first experiment but a 42% reduction in spat in the second. Despite this variability, larvae from selectively bred oysters produced, on average, more (+ 55 and 37%) and larger (+ 5 and 23%) spat in ambient and high pCO2 seawater, respectively. These findings highlight the variable and stage-specific sensitivity of larval oysters to acidified seawater and the influence that genetic factors have in determining the larval performance of C. gigas exposed to high pCO2 seawater.

Description: Increasing atmospheric carbon dioxide threatens to decrease pH in the world's oceans. Coastal and estuarine calcifying organisms of significant ecological and economical importance are at risk; however, several biogeochemical processes drive pH in these habitats. In particular, coastal and estuarine sediments are frequently undersaturated with respect to calcium carbonate due to high rates of organic matter remineralization, even when overlying waters are saturated. As a result, the post-larval stages of infaunal marine bivalves must be able to deposit new shell material in conditions that are corrosive to shell. We measured calcification rates on the hard clam, Mercenaria spp.,in 5 post-larval size classes (0.39, 0.56, 0.78, 0.98, and 2.90 mm shell height) using the alkalinity anomaly method. Acidity of experimental water was controlled by bubbling with air-CO2 blends to obtain pH values of 8.02, 7.64, and 7.41, corresponding to pCO2 values of 424, 1120, and 1950 µatm. These pH values are typical of those found in many near-shore terrigenous marine sediments. Our results show that calcification rate decreased with lower pH in all 5 size classes measured. We also found a significant effect of size on calcification rate, with the smaller post-larval sizes unable to overcome dissolution pressure. Increased calcification rate with size allowed the larger sizes to overcome dissolution pressure and deposit new shell material under corrosive conditions. Size dependency of pH effects on calcification is likely due to organogenesis and developmental shifts in shell mineralogy occurring through the post-larval stage. Furthermore, we found significantly different calcification rates between the 2 sources of hard clams we used for these experiments, most likely due to genotypic differences. Our findings confirm the susceptibility of the early life stages of this important bivalve to decreasing pH and reveal mechanisms behind the increased mortality in post-larval juvenile hard clams related to dissolution pressure, that has been found in previous studies.

Description: Three cohorts of Pacific oyster (Crassostrea gigas) larvae at Whiskey Creek Shellfish Hatchery (WCH) in Netarts Bay, Oregon, were monitored for stable isotope incorporation and biochemical composition: one in May 2011 and two in August 2011. Along with measures of growth and calcification, we present measurements of stable isotopes of carbon in water, algal food, and the shell and tissue, and nitrogen in food and tissue across larval development and growth. These relatively unique measures through larval ontogeny allow us to document isotopic shifts associated with initiation and rate of feeding, and the catabolism of C-rich (lipid) and N-rich (protein) pools. Similar ontological patterns in growth and bulk composition among the cohorts reinforce prior results, suggesting that the creation of the initial shell is energetically expensive, that the major carbon source is ambient dissolved inorganic carbon, and that the major energetic source during this period is maternally derived egg lipids. The May cohort did not isotopically reflect its food source as rapidly as the August cohorts, indicating slower feeding and/or higher catabolism versus anabolism. Our measurements also document differences in bulk turnover of organic carbon and nitrogen pools within the larvae, showing far greater conservation of nitrogen than carbon. These stable isotope and bulk biochemical measurements appear to be more sensitive indicators of sub-lethal environmental stress than the commonly used metrics of development and growth.

Description: Ocean acidification results in co-varying inorganic carbon system variables. Of these, an explicit focus on pH and organismal acid–base regulation has failed to distinguish the mechanism of failure in highly sensitive bivalve larvae. With unique chemical manipulations of seawater we show definitively that larval shell development and growth are dependent on seawater saturation state, and not on carbon dioxide partial pressure or pH. Although other physiological processes are affected by pH, mineral saturation state thresholds will be crossed decades to centuries ahead of pH thresholds owing to nonlinear changes in the carbonate system variables as carbon dioxide is added. Our findings were repeatable for two species of bivalve larvae could resolve discrepancies in experimental results, are consistent with a previous model of ocean acidification impacts due to rapid calcification in bivalve larvae, and suggest a fundamental ocean acidification bottleneck at early life-history for some marine keystone species.

Description: The pteropod Limacina helicina frequently experiences seasonal exposure to corrosive conditions (Omega arag  〈 1) along the US West Coast and is recognized as one of the species most susceptible to ocean acidification (OA). Yet, little is known about their capacity to acclimatize to such conditions. We collected pteropods in the California Current Ecosystem (CCE) that differed in the severity of exposure to Omega arag conditions in the natural environment. Combining field observations, high-CO2 perturbation experiment results, and retrospective ocean transport simulations, we investigated biological responses based on histories of magnitude and duration of exposure to Omega arag  〈 1. Our results suggest that both exposure magnitude and duration affect pteropod responses in the natural environment. However, observed declines in calcification performance and survival probability under high CO2 experimental conditions do not show acclimatization capacity or physiological tolerance related to history of exposure to corrosive conditions. Pteropods from the coastal CCE appear to be at or near the limit of their physiological capacity, and consequently, are already at extinction risk under projected acceleration of OA over the next 30 years. Our results demonstrate that Omega arag  exposure history largely determines pteropod response to experimental conditions and is essential to the interpretation of biological observations and experimental results.