Phosphatic Shell Formation in Atremate Brachiopods

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Abstract
The atremate brachiopods are unique in that they possess shells of calcium phosphate. In Lingula adamsi and Gloltidia pyramidata, the shell mineral is (CO3 + F)-containing apatite and is crystallo-chemically similar but not identical to the mineral francolite. The shell of Glottidia consists of a thin periostracum, a mineralized thick primary layer, and alternating mineralized layers and less mineralized chitin layers. The basic unit of the crystals is the spherulite. Proteinaceous and glycosaminoglycan (GAG) matrices are present in the primary and mineralized layers. The GAGS in the chitin layer are morphologically different from those of the other layers. The GAGS are intimately associated with the apatite crystals. Shell formation appears to be mediated by three different types of cells in the outer epithelium. The cells primarily involved in the mineral formation are characterized by many vacuoles with electron-dense granular inclusions containing Ca, P, and S. The connective tissue at the anterior edge of the mantle also contains fine granules with Ca, P, and S. Those granules are considered to be a mineral reserve for shell formation. Some problems of the mechanisms of shell formation are discussed.

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