Structure and distribution of chalky deposits in the Pacific oyster using x-ray computed tomography (CT)

Abstract

Oysters are unusual among bivalves in that they possess chambers, often filled with soft, chalky calcite, that are irregularly scattered throughout the shell. Because the function of these so-called chalky deposits is still unclear, evaluating the growth and distribution of chalk is important for elucidating the ecological function of this unique shell trait. Specimens of the Pacific oyster Magallana gigas, an oyster well known for chalk expression, were grown in Bodega Harbor, Bodega Bay, CA. At the end of an 11 month growing period, specimens were culled and selected animals were submitted for x-ray computed-tomography imaging. Three-dimensional reconstructions of oyster shells were used to assess the overall distribution of chalk, and also to better understand the relationship between chalk and other structures within the shell. Results indicate that chalky deposits underly sculptural features on the shell exterior, such as external ridges and changes in growth direction, and also that there is a relationship between chalk formation and oyster processes of cementation. Overall, chalk is useful for a cementing lifestyle because it enables morphological plasticity needed to conform to irregular substrates, but also acts as a cheap building material to facilitate rapid growth. Many researchers have used preserved remains of shells (molluscan or otherwise) to improve scientific understanding of paleoecosystems and organismal interactions in the fossil record 1-3. Oysters (Bivalvia: Ostreidae) have utility in this setting because they are well preserved in the fossil record, and are widely geographically distributed in paleo and modern ecosystems 4. Moreover, stable isotope and elemental records derived from bivalve shell carbonate reflects the environmental conditions in which the shell grew 5-9. These geochemical datasets have been used to develop proxies, reconstruct paleoclimate, and characterize the environment in which fossil oysters lived (e.g. estuarine versus fully marine) 10-13. Stable isotopes have also been used to assess growth rate, season of death, and the occurrence of phytoplankton blooms in both modern and fossil Ostreids 14-16. Oyster shells are composed primarily of foliated calcite interspersed with lens shaped chambers 17,18. These chambers are irregularly interspersed throughout the shell and are often filled with porous calcite, also referred to as chalk or chalk deposits. It is important to note that chalk is distinct from the vesicular calcite formed by some members of Gryphaeidae (Ostreoidea), which consists of hollow columnar spaces composed of granular calcite, as opposed to the interconnected calcite laths that are characteristic of chalk 19,20. Chambers and chalky deposits are found in both extant and fossil oysters, though the function of these features, particularly chalk and how it forms, has been the subject of much scientific inquiry (e.g. 21-23). Ontogenetically, the formation of chalk commences after the transition to the dissoconch stage when the oyster settles as spat, and co...

Directional fabrication and dissolution of larval and juvenile oyster shells under ocean acidification

“…Cross section of the juvenile shell shows two prominent micro-structures—the prismatic layer just below the periostracum, and folia layer which forms the major fraction of the juvenile shell (figure 2 f ). The chalky layer could not be observed in these early juvenile shells [51].…”

Secretory and transcriptomic responses of mantle cells to low pH in the Pacific oyster (Crassostrea gigas)

“…In contrast to most bivalves, C. gigas shell is made up of a shell-like foliae layer and lenses of chalk (Marie et al, 2011;Checa et al, 2018), a highly porous and poorly organized microstructure that is only found in Ostreidae (Mouchi et al, 2016;Checa et al, 2018). Recent structural and crystallographic observations have depicted that growth lines are continuous between the foliae and chalk, suggesting both layers are made simultaneously by different zones of the mantle tissue, where the foliated layer is made by the mantle edge and the chalky layer by the mantle pallial (Banker and Sumner, 2020). In oysters kept in low pH conditions, the outer epithelium of the mantle edge shows fewer AB-PAS positive cells compared with the mantle pallial zone, suggesting a decrease in the secretory activity in mantle cells underlying the foliated layer that possibly affects its formation.…”

Tracing timing of growth in cultured molluscs using strontium spiking

“…This suggests that the success of Sr labelling does not depend only on shell mineralogy, but that the mode of shell growth and the location of measurement plays an important role. In contrast to cockles and mussels, oysters show highly irregular growth patterns with frequent changes in growth rate and direction (Carriker et al, 1980;Banker and Sumner, 2020). Furthermore, oysters may resorb parts of their shell to maintain pH within the extrapallial fluid during anaerobic respiration, which may affect the timeline recorded in their outer shell layers (Kent, 1992).…”