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

Abstract

Shell Layers and Structures in the Brackish Water Bivalve, CorbiculajaponicaKeiko Yamaguchi1), Koji Seto2), Katsumi Takayasu3) and Morihiro Aizaki1)1) Faculty of Life and Environmental Science, Shimane University2) Research Center for Coastal Lagoon Environments, Shimane University3) Shimane University(Received April 7, 2006)(Accepted June 19, 2006)Bivalve shells contain a great deal of information about the animal'sgrowth history and local environment. Opaque and translucent layersare observed in the outer shell of Corbicula japonica. Detailed imagingwith scanning electron microscopy and laser microscopy show thatmicrostructural features are different between the two layers. Thedifference is mainly determined by the content of organic materials; the translucent layer contains less organic matrix than the opaquelayer. A marking experiment revealed that the translucent layerswere formed in the period from early summer to winter, varying betweenindividuals. If there is no formation of an opaque layer and shellgrowth rates are low in autumn, the translucent layer is accompaniedby an annual ring on the shell surface. The period of formation ofthe layers is synchronized with the annual cycles of stable isotopevalues, especially with oxygen (?18O). By combining the observationof the shell structure with stable isotope analysis, the shell ofC. japonica can be used to provide information for reconstructingecology and estuarine environments in the past.Keywords:brackish water, bivalve, Corbicula japonica, microstructure,shell growth, translucent layer, stable isotopes