The shelled pteropods, members of the phylum Mollusca (Gastropoda), are one of the three main calcifiers in the ocean that are responsible for almost all sequestration of CaCO3 from the surface water. Their aragonite shell is composed of densely packed curved nanofibers, a unique microstructure that was not documented in any other molluscan groups.
The curved aragonite microstructure was examined by a range of techniques including SEM, TEM and X-ray diffraction analysis, but has not been unequivocally established yet (1-3) . 3D reconstructions of the shell microstructure are required in order to visualize the aragonite fibers organization through shell thickness. For this aim, we use FIB-SEM serial surface view (SSV). Using high pressure freeing (HPF), freeze substitution and embedding allows to preserve the sample as close to its native state as possible, without damaging the shell. Using this sample preparation technique and conducting FIB-SEM in the serial surface view (SSV) mode, we are able to generate a 3D reconstruction of the aragonite nanofiber organization through the shell. Based on our preliminary observation of our 3D data we hypothesize that curved aragonite fibers assemble in layers in a non-continuous manner. Further examination of the data and the shell microstructure is needed to completely solve the shell microstructure.
It was shown previously that larvae mollusk (Bivalve) use transient amorphous calcium carbonate (ACC) as a precursor for crystalline aragonite, suggesting that ACC plays an important role in mollusk shell formation. To shed light on the shell formation process in shelled pteropods, we study the mineral composition of the shell of living larvae and adult Creseis clava using micro-Raman spectroscopy. Pteropods shell grows and thickens throughout the animal’s life and our results show clearly that the aragonite shell in C. clava is formed via disordered transient nascent aragonite. This phase transforms to a more ordered phase of aragonite in older parts of the shell. The maturation process is continuous and progresses during the lifetime of the animal.
References
- Zhang T, et al. (2011) Structure and mechanical properties of a pteropod shell consisting of interlocked helical aragonite nanofibers. Angew. Chem. 123:10545-10549.
- Willinger MG, Checa AG, Bonarski JT, Faryna M, & Berent K (2016) Biogenic Crystallographically Continuous Aragonite Helices: The Microstructure of the Planktonic Gastropod Cuvierina. dv. Funct. Mater. 26:553-561.
- Li L, Weaver JC, & Ortiz C (2015) Hierarchical structural design for fracture resistance in the shell of the pteropod Clio pyramidata. Nat. Commun. 6.
