Nanocrystalline fluorapatite mineralization in the calciphile rock-boring bivalve Lithophaga: functional and phylogenetic significance.

This article focuses on the occurrence, structure, and evolutionary significance of phosphate mineralization in the periostracum (outer shell layer) of Lithophaga species, a genus of mytilid bivalves that bore into calcareous substrates. Using X-ray diffraction and electron microscopy, fluorapatite—a calcium phosphate mineral—was identified as a dense, crystalline layer embedded within the periostracum of six Lithophaga species, contrasting with related genera such as Leiosolenus that lack this phosphate layer but possess extraperiostracal aragonitic encrustations. Molecular phylogenetic analyses reveal that Lithophaga and Leiosolenus belong to separate clades within the family Mytilidae, indicating independent evolutionary pathways to their endolithic lifestyles. The fluorapatite layer in Lithophaga is proposed to function as a protective adaptation against shell dissolution caused by their own rock-boring secretions and possibly as a defense against shell-eroding organisms.