# Periostracum Formation in Sepia officinalis and Loligo vulgaris and Homology with Other Molluscs

## TL;DR

This study compares the formation of the periostracum in cuttlefish and squid to that in other molluscs, finding structural similarities that suggest homology.

## Contribution

The study provides the first detailed description of periostracum formation in internal shells of cuttlefish and squid.

## Key findings

- The periostracum in cuttlefish and squid forms similarly to that in bivalves and gastropods.
- Secretions from columnar and cuboidal cells create a dense and translucent layer, respectively.
- Unlike other molluscs, cuttlefish and squid lack a pellicle and specialized glandular cells in the periostracal groove.

## Abstract

In molluscs with external shells, such as gastropods and bivalves, an organic layer known as the periostracum is typically present, covering the outer surface of the mineral shell. The periostracum protects the shell against dissolution and bioerosion and, more importantly, plays a key role during the initial stages of mineralization by acting as both a template for mineral deposition and a barrier to the external environment. This study focuses on the formation of the organic layer in the internal shells of cuttlefish and squid, with the aim of comparing it with periostracum development in other molluscan groups. Using optical microscopy and transmission electron microscopy, we found that both the structure and the mode of formation of these organic layers are similar to those of the periostracum in bivalves and gastropods. Accordingly, their homology can be inferred from morphological criteria.

The periostracum is the outermost shell layer and the first produced during shell formation in molluscs. This organic layer isolates the extrapallial space from the external environment and provides a scaffold for subsequent calcification. In cephalopods with an internal shell, some organic shell structures are putatively homologous to the periostracum of other molluscan groups. However, neither their detailed structure nor their mode of formation has been described, leaving the extent of this homology unresolved. To address this issue, we investigated the morphology and formation of the organic layer of the dorsal shield and the gladius in embryos of the cuttlefish Sepia officinalis Linnaeus, 1758, and the squid Loligo vulgaris Lamarck, 1798, respectively, using light microscopy and transmission electron microscopy. In both species, the periostracum forms within a periostracal groove located along the lateral and anterior margins of the shell sac. As in other molluscs, secretions from columnar cells at the bottom of the groove produce a dense layer, while a translucent layer is subsequently added beneath it through secretions from cuboidal cells. The main difference is the absence of both a pellicle and of the specialized glandular cells that typically secrete it at the bottom of the periostracal groove.