STRUCTURE AND DISTRIBUTION OF CHALKY DEPOSITS IN OYSTER SHELLS USING X-RAY COMPUTED TOMOGRAPHY (CT)

The morphology and chemistry of molluscan shells are largely the result of selective factors that act on organisms over evolutionary timescales, as well as influences from the particular setting in which an organism lived. Oysters (Bivalvia: Ostreidae) are useful windows into the past because of their stratigraphic range from the Triassic to the present, excellent preservation in the fossil record, and widespread distribution in ecosystems. Oyster shells are composed primarily of foliated calcite, though the shells of certain species are interspersed with lens shaped chambers that are often filled with porous calcite, also referred to as chalk or chalky deposits. Characterizing the growth and structure of chalky deposits in modern oysters is an important step towards understanding the ecological and evolutionary significance of this shell feature. To characterize the growth and structure of chalky deposits in Crassostrea gigas, the Pacific oyster, specimens were grown from juvenile stages (~0.50 cm) to a size of ~7.0 cm in cages that were suspended from a dock in Bodega Harbor (Bodega Bay, California). Shells of select oysters were imaged using a micro X-Ray Computed Tomography (CT) scanner, which provided high-resolution 3D visualization of shells, chambers, and chalk. Resulting reconstructions, which yielded 46 x 46 x 46 µm voxels, allowed for straightforward differentiation between foliated calcite and the relatively less dense chalky deposits. Analyses show that chalk preferentially underlies ridges expressed on the shell exterior, suggesting chalk enables the formation of these sculptural features. Chalky deposits are abundant in the umbo and appear irregularly on the interior surface as well as near the lip of the shell. In addition, some chambers are connected to one another and to the shell exterior, indicating continued interaction with the external environment. Last, chalk growth is extensive where oysters recemented onto a hard substrate, which implies that the controls on chalk formation are similar to those that govern cementation. Overall, chalk appears to be an important tool used by C. gigas to change growth direction and shell shape, perhaps as an adaptation to a sessile lifestyle.