Abstract
Abstract. The rapidly intensifying process of ocean acidification (OA) due to anthropogenic CO2 is not only depleting carbonate ions necessary for calcification but also causing acidosis and disrupting internal pH homeostasis in several marine organisms. These negative consequences of OA on marine calcifiers, i.e. oyster species, have been very well documented in recent studies; however, the consequences of reduced or impaired calcification on the end-product, shells or skeletons, still remain one of the major research gaps. Shells produced by marine organisms under OA are expected to show signs of dissolution, disorganized microstructure and reduced mechanical properties. To bridge this knowledge gap and to test the above hypothesis, we investigated the effect of OA on juvenile shells of the commercially important oyster species, Magallana angulata, at ecologically and climatically relevant OA levels (using pH 8.1, 7.8, 7.5, 7.2). In lower pH conditions, a drop of shell hardness and stiffness was revealed by nanoindentation tests, while an evident porous internal microstructure was detected by scanning electron microscopy. Crystallographic orientation, on the other hand, showed no significant difference with decreasing pH using electron back-scattered diffraction (EBSD). These results indicate the porous internal microstructure may be the cause of the reduction in shell hardness and stiffness. The overall decrease of shell density observed from micro-computed tomography analysis indicates the porous internal microstructure may run through the shell, thus inevitably limiting the effectiveness of the shell's defensive function. This study shows the potential deterioration of oyster shells induced by OA, especially in their early life stage. This knowledge is critical to estimate the survival and production of edible oysters in the future ocean.
Highlights
Edible oysters belonging to the genus Magallana have a complex life cycle, in which the free-swimming larvae attach onto a suitable hard substrate and metamorphose into sessile juveniles within a few hours
As the calcitic foliated layer is the major shell structure for mechanical support in oysters (Lee et al, 2008), we examined its structural and mechanical properties by using a variety of characterization and imaging techniques such as scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and nanoindentation tests
Previous studies showed that early larval life stages of several edible oyster species were relatively physiologically tolerant of near-future ocean acidification (OA) conditions (Dineshram et al, 2013; Ko et al, 2013, 2014; Thiyagarajan and Ko, 2012), this study shows that they are still vulnerable due to the effects decreased pH has on shell characteristics, like porosity, hardness and stiffness
Summary
Edible oysters belonging to the genus Magallana have a complex life cycle, in which the free-swimming larvae attach onto a suitable hard substrate and metamorphose into sessile juveniles within a few hours The fraction of calcite rapidly increases and becomes the main component in the juvenile and adult oyster shell (Medakovicet al., 1997; Weiner and Addadi, 1997). Life stages of marine invertebrates, oysters included, are highly vulnerable to predators (Newell et al, 2007) and environmental stressors (Thomsen et al, 2015) when compared to the adult stages. Production of mechanically strong shells during larval and juvenile stages is essential to the post-larval phase because shell integrity and strength act as a protective barrier against shell-breaking and drilling predators
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