Effect of increased pCO₂ level on early shell development in great scallop (Pecten maximus Lamarck) larvae

Abstract. As a result of high anthropogenic CO₂ emissions, the concentration of CO₂ in the oceans has increased, causing a decrease in pH, known as ocean acidification (OA). Numerous studies have shown negative effects on marine invertebrates, and also that the early life stages are the most sensitive to OA. We studied the effects of OA on embryos and unfed larvae of the great scallop (Pecten maximus Lamarck), at pCO₂ levels of 469 (ambient), 807, 1164, and 1599 μatm until seven days after fertilization. To our knowledge, this is the first study on OA effects on larvae of this species. A drop in pCO₂ level the first 12 h was observed in the elevated pCO₂ groups due to a discontinuation in water flow to avoid escape of embryos. When the flow was restarted, pCO₂ level stabilized and was significantly different between all groups. OA affected both survival and shell growth negatively after seven days. Survival was reduced from 45% in the ambient group to 12% in the highest pCO₂ group. Shell length and height were reduced by 8 and 15%, respectively, when pCO₂ increased from ambient to 1599 μatm. Development of normal hinges was negatively affected by elevated pCO₂ levels in both trochophore larvae after two days and veliger larvae after seven days. After seven days, deformities in the shell hinge were more connected to elevated pCO₂ levels than deformities in the shell edge. Embryos stained with calcein showed fluorescence in the newly formed shell area, indicating calcification of the shell at the early trochophore stage between one and two days after fertilization. Our results show that P. maximus embryos and early larvae may be negatively affected by elevated pCO₂ levels within the range of what is projected towards year 2250, although the initial drop in pCO₂ level may have overestimated the effect of the highest pCO₂ levels. Future work should focus on long-term effects on this species from hatching, throughout the larval stages, and further into the juvenile and adult stages.