Shell Layers and Structures in the Brackish Water Bivalve, Corbicula japonica

Abstract

Bivalve shells contain a great deal of information about the animal's growth history and local environment. Opaque and translucent layers are observed in the outer shell of Corbicula japonica. Detailed imaging with scanning electron microscopy and laser microscopy show that microstructural features are different between the two layers. The difference is mainly determined by the content of organic materials ; the translucent layer contains less organic matrix than the opaque layer. A marking experiment revealed that the translucent layers were formed in the period from early summer to winter, varying between individuals. If there is no formation of an opaque layer and shell growth rates are low in autumn, the translucent layer is accompanied by an annual ring on the shell surface. The period of formation of the layers is synchronized with the annual cycles of stable isotope values, especially with oxygen (δ18O). By combining the observation of the shell structure with stable isotope analysis, the shell of C. japonica can be used to provide information for reconstructing ecology and estuarine environments in the past.