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Octocorals sclerites are ideal structures to study the emergence of complex shapes

from particular arrangements of crystallites in biominerals. Sinularia polydactyla

sclerites have been studied by polarizing microscopy, scanning electron microscopy,

and electron backscattered diffraction. Small sclerites (<100 μm) are simple mesocrystalline

structures, with similarly oriented sub-micrometer crystallites arranged in elongated

fibers, with only a low degree of ordered misorientations between them. Large sclerites

(∼2 mm) are composite structures: (1) at their center, small proto-sclerites (≤20 μm)

act as nuclei around which growth proceeds. (2) The large sclerite axial frame is made

of crystallites with c axes arranged in three directions, at an inclination of about 22°

from the long sclerite axis. The axial frame displays a center of symmetry, and crystallites

are arranged in an opposite trihedral arrangement with respect to this center. (3) Inside

the large sclerite, deeply rooted tubercles develop with crystallite c axes close to perpendicular

to the long sclerite axis. The tubercles grow by a branching process and display crystallite

misorientations ordered around the three a axes of the hexagonal unit cell of calcite.

Crystallites in both the frame and the tubercles form trigonal inverse pyramids resulting

from sudden or progressive changes of crystallite orientations. A crystallographic model

emphasizes the importance of the trigonal inverse pyramid as a structural pattern.

Concerning the sclerite morphology, small crystallite sizes, ordered misorientations

and mesotwinning are important features to achieve concave shapes. The sclerite surface

morphology also results from the regulation of mesocrystalline growth and patterning

by cells or vacuoles. In this respect, the sclerite morphology is the product of internal

and external forces, among which crystallographic order and molding play important roles.
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