Studies on Shell Formation

Abstract

The submicroscopic structure of the growing surface of the shell of the oyster, Crassostrea ~rgin~a, was studied by means of shadowed replicas. The outer edge of the prismatic region consists of a fine grained matrix enclosing crystals, the surfaces of which show a finely pebbled structure. Crystal size varies continously from 0.01/~ to 8/~. The matrix surface shows no evidence of fibrous structure. The outer portions of the prismatic region exhibit a tile-like arrangement of large crystals separated by granular matrix 0.02 to 0.08/~ in thickness. The exposed crystal surfaces have indentations of varying form which appear as roughly parallel grooves spaced at intervals of approximately 0.3/~. The final form of this region is believed to result from the random distribution of crystal seeds, which grow without orientation and through coalescence and growth come into contact, producing polygonal areas. The crystal arrangement of the nacreous region is one of overlapping rows of crystals in sideto side contact, and with one end of each crystal free, permitting continued increase in length. Crystal angles and plane indices are presented.The structure of mollusk shell has been comprehensively studied with respect to its microscopic appearance (3, 2, 4, 16, 17, 22, 23), crystallography (3, 21, 24-26), and chemical composition (2, 5); and experimental studies, old and recent, have dealt with growth and repair (1,2,13,15,18,27, 28,32,33). However, the mechanisms of normal shell growth still remain to be clarified. The information that electron microscopy is providing on the structure of shell (8,9, 11,31) and pearl (29, 30) strongly indicates that our understanding of growth mechanisms will be advanced by the further application of this technique to shell studies. The

In situ distribution and characterization of the organic content of the oyster shell Crassostrea gigas (Mollusca, Bivalvia)

“…One of the first microstructural studies of the shell was performed using TEM sections of the right valve of Crassostrea virginica (Tsujii et al, 1958). Only two layers were described in this study: the outer prismatic and the inner one, called "nacre" (although both are calcitic).…”

Formation of the prismatic layer in the freshwater bivalveHyriopsis cumingii: the feedback of crystal growth on organic matrix

“…This phenomenon is explained by the squeezing process first hypothesized by Tsuji et al . () and later developed by Carriker et al . ().…”

“…Tsuji et al . () and Carriker et al . () hypothesized that prism growth induces squeezing of organic matter and that the interprismatic membranes are the result of this squeezing.…”

Biology of the Integument

“…The prisms are either calcitic or aragonitic and are encased in an organic sheath, and typically show a honeycomb pattern on their horizontal surface. 1) are about 90-100 ~m in diameter (Tsujii et al 1958, Kobayashi 1971 and the interprismatic matrix wall is about 0.5 to 8 11m thick. The prisms are calcite in many gastropods and marine pelecypods, or aragonite as in the Unionacea and Trigonacea (Gregoire 1972), and Pandoracea, Pholadomyacea and Poromyacea (Kennedy et al 1969).…”

“…The foliated layer is built oflaminae consisting of parallel and elongate blades dipping uniformly over large portions of the depositional surface (Carter 1980). The blades are joined adjacently and the surface resembles a shingled roof (Tsujii et al 1958, Watabe et al 1958). …”