The Pteropod Creseis acicula Forms Its Shell through a Disordered Nascent Aragonite Phase

Shelled pteropods are holoplanktonic mollusks that build lightweight shells containing aragonite crystals. The complex shell architecture is composed of well-aligned, curved aragonitic fibers. Each curved fiber is continuously crystalline. We used in vivo micro-Raman spectroscopy to study the mineral composition of shells of living Creseis acicula pteropods at the larval (veliger) and adult stages. The spectra obtained from the growing edge have weak and broad peaks indicative of a highly disordered nascent aragonite phase. The disordered precursor phase is detected in the newly formed regions both at the shell edge, and during thickening in the internal part of the shell. As the shell grows and thickens throughout the life of the animal, the mineral matures from a disordered transient precursor phase to crystalline aragonite. We conclude that the shell of C. acicula is formed via a disordered nascent form of aragonite, which, being isotropic, facilitates the formation of the convoluted morphology in the continuously crystalline fibers of aragonite.