The shell matrix and microstructure of the Ram's Horn squid: Molecular and structural characterization

Area of Science:

• * Marine Biology
• * Biomineralization
• * Cephalopod Research

Background:

• * Molluscs exhibit remarkable diversity, with most species forming external calcified shells via mantle epithelium secretion.
• * Shell biomineralization is orchestrated by extracellular macromolecules known as the organic matrix.
• * Organic matrix composition is well-studied in bivalves and gastropods, but less so in cephalopods.

Purpose of the Study:

• * To investigate the physical and biochemical properties of the internal shell of the Ram's Horn squid (Spirula spirula).
• * To characterize the organic matrix components involved in the biomineralization of this unique internal shell.
• * To compare the shell matrix proteins of S. spirula with those of other molluscs and cephalopods.

Main Methods:

• * Scanning Electron Microscopy (SEM) for microstructural analysis.
• * Fourier-transform infrared and solid-state nuclear magnetic resonance (NMR) spectroscopies for biochemical characterization.
• * Proteomics and in silico searches against metazoan models and a S. spirula transcriptome.

Main Results:

• * SEM revealed a complex microstructural organization of the S. spirula shell.
• * Spectroscopies indicated a high saccharide content (80% of insoluble fraction), including β-chitin and other polysaccharides.
• * Proteomic analysis identified unique peptides, with limited similarity to known molluscan shell proteins and no clear phylogenetic signal within cephalopods.

Conclusions:

• * Spirula spirula utilizes a distinct set of shell matrix proteins for its internal shell formation.
• * The identified matrix components, particularly polysaccharides, highlight unique biomineralization strategies in this species.
• * The findings challenge assumptions about conserved phylogenetic signals in cephalopod skeletal matrices.