Evidence for helical microstructure of aragonite fibers in pteropod shells

Abstract

Planktonic gastropods of the suborder Euthecosomata (order Pteropoda) secrete a unique microstructure made of tightly interlocked, space-filling helically coiled aragonite fibers. This material has a high degree of ordering because all fibers are in phase at a given growth plane. The recognition of the helical nature from mere scanning electron micrographs is somewhat uncertain. In addition, three-dimensional (3D) reconstructions of individual helices are hindered by the small dimensions of fibers, and the virtual absence of intervening organic interphases. Here, we (1) reexamine the morphological characteristics of the fibers in fracture and find them compatible with a helical model, (2) provide direct evidence of the helical mode of coiling in the form of anaglyphs, and (3) develop a 3D model, which is fully consistent with the patterns observed in fractures and polished sections. Proposals based on motifs different from helices are not congruent with the observations. Graphical abstract Impact statement Some planktonic gastropods of the order Pteropoda secrete a unique microstructure that was originally described as made of tightly interlocked helically coiled continuous aragonite fibers. Despite this material having been described 50 years ago, there is still debate as to the helical design of such fibers. We provide additional unambiguous evidence that fibers describe true helical trajectories during growth. Establishing their true nature is essential in order to know the spectrum of calcified biomaterials produced by molluscs. This is also a prerequisite for future biomimetic studies aiming at producing bioinspired highly functional materials.

Helical microstructures in molluscan biomineralization are a biological example of close packed helices that may form from a colloidal liquid crystal precursor in a twist–bend nematic phase

“…Its structure was first described in detail by Bé et al in 1972 [7]. Recently, we looked again at the microstructure [8]. It consists of very thin aragonite fibers which coil helically, in all cases dextrally when viewed from the shell outer surface, from less than one turn to between three and four turns [9][10][11].…”

Biomineral mesostructure

“…Unexpected and exciting new structures, properties, and formation mechanisms are frequently discovered in natural biominerals, such as the recent dual scale, single-crystalline calcite in a sea star revealed by Li et al 20 and the latest installments of the ongoing debate on the fiendishly complicated structure of pteropod shells. [21][22][23][24][25][26][27][28] Biominerals also provide extremely useful ideas for bioinspired synthetic materials. [29][30][31][32][33][34][35][36][37][38][39][40][41] Aragonite and calcite (both polymorphs of CaCO 3 ) are the final, stable crystalline phases in many marine biominerals.…”