Oyster larval biomineralisation – insights from electron backscatter diffraction

Abstract

The initiation of biomineralisation is crucial to the ecology of shelled organisms, for instance providing protection during early life stages when animals are particularly vulnerable. In oysters, the processes involved in early shell deposition remain debated—whether amorphous calcium carbonate (ACC) is deposited initially and transforms into aragonite, or aragonite is directly deposited—largely due to challenges examining the youngest age classes and the limited diversity of model species. Early larval shell deposition has primarily been studied in pearl oysters (Pinctada spp.) due to commercial interests in pearl formation. Edible oyster biomineralisation, however, remains relatively unexplored, despite the commercial importance of post-settlement survival. In this study, we provide a comparative analysis of shell crystallography of a relatively unexamined, ecologically and commercially important edible species, the Hong Kong oyster (Magallana hongkongensis). We focus on three important life stages—D-larvae (3 days post fertilisation), pediveliger (14 days post fertilisation) and spat (three months post settlement)—over which the shell increases drastically in thickness and alters its microstructure. Employing Scanning Electron Microscopy-based Electron BackScatter Diffraction (SEM-EBSD), we show: (1) larval shells are made entirely of aragonite crystals with no traces of ACC detected, whereas spat exhibit calcitic shells; (2) relative to spats, larval shells show a stronger alignment of their crystal c-axes perpendicular to the shell surface, suggesting perhaps a tighter control of mineralisation processes in early life stages; (3) shell grain area increases as the oyster matures, likely linked to the aragonite-to-calcite shift, but also maturation of the larval shell. These quantitative data on ultra- and microstructural changes in oyster shell architecture advance our understanding of early biomineralisation in edible oysters; by elucidating the mechanisms of crystal deposition and organization, we provide a foundation for designing novel materials inspired by natural biomineralisation processes.