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

We demonstrate that nature has produced a close-packed helical twisted filamentous material in the biomineralization of the mollusk. In liquid crystals, twist–bend nematics have been predicted and observed. We present and analyze evidence that the helical biomineral microstructure of mollusk shells may be formed from such a liquid-crystal precursor.

Figure 1

Scanning electron micrographs of the aspect of the helical microstructure as seen in fracture surfaces in four species of cavolinioidean mollusks: (a) Diacria trispinosa, (b) Clio sp., (c) Cavolinia inflexa, and (d) Cuvierina columnella.

Figure 2

Three-dimensional (3D) reconstructions and anaglyph images from scanning electron micrography of three cavolinioidean mollusk species: (a) Diacria trispinosa, (b) Creseis acicula, and (c) Cavolinia inflexa. Further examples together with videos are provided in the Supplemental Material [66].

Figure 3

Scanning electron micrographs of the aspect of the helical microstructure as seen in growth surfaces in three species of cavolinioidean mollusks: (a) and (b) Clio sp., (c) Cavolinia longirostris, and (d) Cuvierina columnella.

Figure 4

Packing twisted filaments. Helicoidal pasta: fusilli lunghi (left) in the uncooked, unpacked state and (right) cooked to become flexible and stretchable so that it may be packed in a space-filling array. The radius of the container is 4.5 cm and the wavelength of the pasta 7.5 mm.

Figure 5

Isometric (constant spacing) packing. (a) Model of a helix. [(b)–(d)] Packing of helical fibers cut along the axis of the helix along two orthogonal planes. All fibers are in full contact with each other, demonstrating the deformation produced in an isometric packing. The colors of the different fibers are to facilitate the identification of the individual fibers.

Figure 6

Isomorphic (constant shape) packing. (a) Model of a helix. [(b)–(d)] Packing of helical fibers cut along the axis of the helix along two orthogonal planes. The helices are slightly in contact with each other, spaced such that identical filaments do not overlap in the bundle interior in an isomorphic packing. The colors of the different fibers are to facilitate the identification of the individual fibers.

Figure 7

Twist–bend nematic. Schematic representation of the twist–bend nematic liquid crystal structure.

Figure 8

Box plot of the period of the structure measured in five different species: Cavolinia inflexa, Clio sp., Creseis acicula, Cuvierina columnella, and Diacria trispinosa.

Figure 9

Scanning electron micrographs of the aspect of the helical microstructure as seen in fracture surfaces in Creseis acicula: (a) from a juvenile where the pattern seems to be purely helical, (b) and (c) showing the intermediate S-shaped stage unique to this species, and (d) an example of tendril perversion in Passiflora caerulea.