The Marsh’s Membrane: A Key-Role for a Forgotten Structure

Abstract

Recent imaging methods applied to the growing edge of the Pinctada margaritifera shell allow for a better appreciation of ancient structural data. Growth of the Pinctada shell (both lateral extension and thickness increase) is a coordinated mechanism involving a series of clearly identified steps in contrast to the prevailing concept of a direct "self-assembly" process.

Prism substructures in the shell of Pinna nobilis (Linnaeus, 1758), Mollusca – Evidence for a three-dimensional pulsed-growth model

“…With respect to size, shape and position, these circular units are similar to the structures previously described on the periostracum of the Pinctada margaritifera shell, the Polynesian pearl oyster ( Cuif and Dauphin, 2018 ). This species also produces an outer shell layer built by calcitic prisms although, compared with the Pinna shells, the respective importance of the outer and inner shell layers are in reverse proportions.…”

“…It is also worth to note that the mineralizing surface of the prisms is covered by the Marsh's membrane. This continuous translucent organic film is so closely attached to the shell surface that the ridges and grooves remain visible, although attenuated ( Figure 7 h) ( Cuif and Dauphin, 2018 ). Existence of corresponding substructures in this membrane is emphasized by epipolarization ( Figure 7 i).…”

Synchrotron-Based HR-Fluorescence and Mineralogical Mapping of the Initial Growth Stages of Polynesian Cultivated Pearls Disprove the ‘Reversed Shell’ Concept

“…This point is important because, after being incorporated into the shell growing edge, these calcite units are acting as substrates for crystallization of the prisms. The amorphous calcium carbonate simultaneously secreted by the mantle cells will crystallize by following the crystalline orientation of the underlying disks [18]. Note must be made that from this point, growth of the prisms is simultaneous [16,19], whereas periostracal disks individually grow.…”

“…In the development of the shell growing edge, it has been shown that periostracum is acting only as a conveyor belt moving the calcite disks from the deeper part of the outer mantle grove up to the shell border [16]. Crystalline orientation of the calcite disks is given by the initial organic centers deposited onto the internal surface of the periostracum [18]. In contrast, the organic layer secreted by the pearl sac and progressively covering the nucleus (Figure 7A-C) has a leading role regarding crystalline orientation of the tiny mineral particles it is bearing.…”

Non‐spherical pearl layers in the Polynesian ‘black‐lipped’ Pinctada margaritifera : The non‐nacreous deposits compared to microstructure of the shell growing edge

“…From a biomineralization viewpoint, these calcareous discs are remarkable because in spite of freely crystallizing (no contact between them) and a roundshaped morphology they exhibit a single-crystal behaviour (Figure S1F). With respect to their future function as mineralizing substrate for growth of the calcite prisms, this crystallographic property is an essential pattern (Cuif & Dauphin, 2018c).…”