PRELIMINARY CHARACTERIZATION OF OYSTER CHALKY DEPOSITS USING X-RAY COMPUTED TOMOGRAPHY

Both shell morphology and geochemical archives stored in bivalve shell carbonate can provide invaluable information towards studying molluscan ecology and the environment in which they precipitate. Oysters are of particular importance for studying estuarine systems because they exhibit widespread spatial range in modern systems and have a paleontological record from the Triassic to the present. Unlike most other bivalves, however, oyster shells do not form distinct growth bands and it is difficult to recognize isochronous structures within shell carbonate. The shells of many oyster species, including the commercial Eastern oyster Crassostrea virginica and Pacific oyster Crassostrea gigas, are largely composed of foliated calcite irregularly interspersed with chambers. Often these chambers are filled with so-called chalky calcite composed of a crystal structure distinct from the harder surrounding folia. Recent breakthroughs indicate that oyster folia formation is tightly regulated by a series of cellular mechanisms. However, many outstanding questions remain regarding the formation of chalky deposits, including mechanism, timing, and function of these unique shell features.

The strongest candidate for investigating chalky deposit formation in oysters is C. gigas. This Pacific species is commonly farmed on the California coast and is well known for producing chalky deposits. Specimens of C. gigas were grown from juveniles stages (~0.50 cm) to a size of ~7.0 cm in cages suspended from a dock in Bodega Harbor (Bodega Bay, California). Select individuals were collected and x-ray computed tomography (CT) was performed on a single specimen to begin characterizing the geometry, distribution, and overall morphology of chalky deposits. Preliminary results indicate that chalky deposits form complex interconnected chambers within individual valves. In addition, chalky deposits themselves have spatially variable internal structures. Results presented here provide important insights for understanding the development of chalky deposits found in the extant Pacific oyster, which has important implications for understanding the ecology, growth, and geochemical archives of fossil bivalves with chalky deposits.