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

Phosphate mineralization as a skeletal material is uncommon in invertebrate animals and rare in Mollusca. Remarkably, apatite minerals were first reported more than 30 years ago in the periostracum of two species of the mytilid bivalve Lithophaga where shells are mostly constructed of calcium carbonate. This discovery extended the range of biominerals secreted by molluscs but has attracted no subsequent research. In this study we review the occurrence of phosphate mineralization in Lithophaga and putatively allied taxa. Lithophagine bivalves, particularly Lithophaga and the more diverse Leiosolenus species, are well known for their endolithic chemical dissolution of calcareous rocks and corals with calcium-binding lipoproteins secreted by mantle glands. Fluorapatite was identified by X-ray diffraction in an outer layer of the periostracum in six species of Lithophaga. Morphological study by scanning electron microscopy of four species showed the fluorapatite crystals embedded in periostracal material in a layer 10-20 mu m thick. Dilute bleach treatment revealed the crystals as densely packed euhedral prisms 250-400 nm in size. The succeeding inner layers of the periostracum were unmineralized. Observations of the developing periostracum of Lithophaga teres suggest that the initial mineralization is in the form of amorphous granules that coalesce and transform into euhedral crystals. Periostracal phosphate was not recorded in other members of the Lithophaginae - Leiosolenus, Botula or Zelithophaga species. Leiosolenus species characteristically have extraperiostracal aragonitic encrustations that can be thick and structurally complex. Published molecular phylogenies of Mytilidae bivalves show a division into two major clades with Lithophaga species in one clade and Leiosolenus species in the other, indicating that the subfamily Lithophaginae as presently understood is polyphyletic. This result implies that the two genera have independent evolutionary pathways to endolithic occupation of calcareous substrates although using similar mantle gland secretions to excavate their crypts. Because fluorapatite is considerably less soluble and harder than calcium carbonate, it is suggested that the phosphate layer of Lithophaga is a functional adaptation to protect their shells from self-dissolution from their rock-dissolving glandular secretions and may also act as defence against other shell-eroding organisms.