Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XIX, No.3


Mineralization and Architecture of the Larval Shell of Haliotis discus hannai Ino, (Archaeogastropoda)

Iwata, Keiji

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Mineralization process of the larval shell was studied in the recent archaeogastropod, Haliotis discus hannai Ino, with electron microscope in order to investigate the mechanism of biomineralization during early ontogeny. Prior to shell mineralization, invagination of ectoderm occurs and a thin periostracum is formed. The periostracum does not mineralize itself. Small organic spherules are secreted on the periostracum and crystal nuclei appear heterogeneously within the organic spherules. Crystal nuclei grow into flaky and granular crystallites which aggregate to form spherulites and fill up the organic spherules. These aggregations of crystallites are joined with one another and form a thin mineralized layer. Morphology of larval shell is almost completed throughout the planktonic veliger life. The protoconch consists of minute granular architecture, in which crystallites are arranged without particular orientation. After the larvae sink to bottom, inner layer of protoconch and peristomal shell are accreted. The inner layer of the protoconch consists of intersecting blocks which are composed of acicular crystals. The architecture of the inner layer is similar to the crossed-blocky structure in the early stage of pleurotomariids. The nacreous layer is absent in the protoconch.