Resilient mantle, vulnerable shell: Ocean acidification's impact on juvenile pen shell Atrina maura

Ocean acidification results from the accumulation of CO₂ in seawater. Predictions indicate a pH decrease of 0.3–0.5 units by the year 2100 and 0.7 units by the year 2300. The effects of OA vary among species, exposure time, physiological limits, and short-term adaptability. This phenomenon can be exacerbated in many coastal marine habitats due to nutrient inputs, upwelling events, and terrestrial sources of acidity. This study analyzed the effects of pH reduction on the shell and mantle tissue structure of the juvenile pen shell (Atrina maura) living in coastal bay where the natural pH is 7.8. These organisms were exposed to a reduction of pH (7.5) for 40 days. Scanning electron microscopy (SEM) analysis of the shells revealed signs of malformation and dissolution of the calcite prismatic layer and the nacre layer at acidic pH (7.5 ± 0.2) compared to controls (7.8 ± 0.1) throughout the experiment. However, shell growth increased 13.95 % after CO₂ injection after 40 days of exposure, while the shell index and the mantle tissue remained unaffected any of the times evaluated. The results suggest that A. maura, a species naturally exposed to acidic environments, may experience effects from pH reductions primarily in their shell microstructure, while mantle morphology remains unaffected. The results obtained here prove that resilience of marine species, such as A. maura, can be compromised by ocean acidification (OA), highlighting the importance of continued exploration in this field to better understand the broader implications of OA on marine ecosystems.