This study demonstrated that the increased partial pressure of CO 2 (pCO 2 ) in seawater and the attendant acidification that are projected to occur by the year 2300 will severely impact the early development of the oyster Crassostrea gigas. Eggs of the oyster were artificially fertilized and incubated for 48 h in seawater acidified to pH 7.4 by equilibrating it with CO 2 -enriched air (CO 2 group), and the larval morphology and degree of shell mineralization were compared with the control treatment (air-equilibrated seawater). Only 5% of the CO 2 group developed into normal 'Dshaped' veliger larvae as compared with 68% in the control group, although no difference was observed between the groups up to the trochophore stage. Thus, during embryogenesis, the calcification process appears to be particularly affected by low pH and/or the low CaCO 3 saturation state of high-CO 2 seawater. Veliger larvae with fully mineralized shells accounted for 30% of the CO 2 -group larvae, compared with 72% in the control (p < 0.005). Shell mineralization was completely inhibited in 45% of the CO 2 -group larvae, but only in 16% of the control (p < 0.05). Normal D-shaped veligers of the control group exhibited increased shell length and height between 24 and 48 h after fertilization, while the few D-shaped veligers of the CO 2 group showed no shell growth during the same period. Our results suggest that future ocean acidification will have deleterious impacts on the early development of marine benthic calcifying organisms.

We investigated the effects of seawater equilibrated with CO 2 -enriched air (2000 ppm, pH 7.4) on the early development of the mussel Mytilus galloprovincialis. Mussel embryos were incubated for 144 h (6 d) in control and high-CO 2 seawater to compare embryogenesis, larval growth and morphology with ordinary light, polarized light, and scanning electron microscopy. Embryogenesis was unaffected by exposure to high-CO 2 seawater up to the trochophore stage, but development at the trochophore stage was delayed when the shell began to form. All veliger larvae of the high-CO 2 group showed morphological abnormalities such as convex hinge, protrusion of the mantle and malformation of shells. Larval height and length were 26 ± 1.9% and 20 ± 1.1% smaller, respectively, in the high-CO 2 group than in the control at 144 h. These results are consistent with our previous findings of CO 2 effects on early development of the oyster Crassostrea gigas, although the severity of CO 2 damage appears to be less in M. galloprovincialis, possibly due to differing spawning seasons (oyster: summer; mussel: winter). Results from this and the previous study indicate that high CO 2 (2000 ppm) interferes with early development, particularly with larval shell synthesis, of bivalves; however, vulnerability to high CO 2 differs between species. Taken together with recent studies demonstrating negative impacts of high CO 2 on adult mussels and oysters, results imply a future decrease of bivalve populations in the oceans, unless acclimation to the predicted environmental alteration occurs.

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