Item

Abstract

The shells of most anomalodesmatan bivalves are composed of an outer
aragonitic prismatic granular or columnar layer and inner nacreous layers. The family
Thraciidae is one of the very few which lack nacreous layers. In particular, the genus
Thracia is exceptional in its possession of a very distinctive, but previously unreported
microstructure, which we term ‘dendritic prisms’. Dendritic prisms,
consist of slender fibres of aragonite which radiate perpendicular to, and which stack
along, the axis of the prism. SEM and TEM study of the periostracum and its
intervening mantle, as well as of the shell, in three species of Thracia, have been used to
reconstruct the mode of shell calcification and to unravel the crystallography of the
dendritic units. The periostracum is composed of an outer dark layer and an inner
translucent layer. During the free periostracum phase the dark layer grows at the
expense of the translucent layer but at the position of the shell edge, the thickness of the
translucent layer which has not previously transformed into the dark layer mineralizes
and produces the units typical to the dendritic prismatic layer. Within each unit, the caxis
is oriented along the prismatic axis, whereas the a-axis of aragonite runs parallel to
the long axis of the fibres. The six-rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its
distinctive appearance, the dendritic prismatic layer of Thracia is homologous to the
outer granular prismatic or prismatic layer of other anomalodesmatans, while the
nacreous layer present in most anomalodesmatans has been suppressed.