Calcitic Nacreous Ultrastructures in Bryozoans: Implications for Comparative Biomineralization of Lophophorates and Molluscs.

The occurrence of nacreous-type fabrics in bryozoans has broad implications for our understanding of similarities between lophophorate and molluscan biomineralization. Calcitic semi-nacreous ultrastructures in the skeletons of several species of cyclostome bryozoans belonging to the suborders Tubuliporina and Articulata are described. In these taxa semi-nacre is the dominant ultrastructure of the walls, and often succeeds a precursory fabric of tiny wedge-shaped crystallites. The semi-nacre comprises laminar sheets of irregularly stacked, typically six-sided tablets with frequent screw dislocations. Tablets usually have inclined edges and an upper surface with a central depression. The surfaces of etched six-sided tablets have six subtriangular sectors: three soluble sectors alternating with three less-soluble sectors. The less-soluble sectors have outward-sloping edges and sometimes have slightly longer sides than the more soluble sectors, which have inward-sloping edges. Newly seeded crystallites comprise only the three less-soluble sectors separated by a `trilete' suture. They are not nucleated on a particular sector type. Similar fabrics have previously been recorded in the cyclostome suborder Cancellata. The substructure of the calcitic semi-nacreous tablets of cyclostomes resembles molluscan aragonitic nacre, particularly that of bivalves, but differs in the precise arrangement of the sectors. Morphologically, the bryozoan semi-nacre is at least as similar to bivalve nacre as bivalve nacre is to gastropod and cephalopod nacre. The existence of a distinct substructure in the component tablets of molluscan nacre has been used as evidence that a greater evolutionary potential characterizes molluscan rather than lophophorate (bryozoans and brachiopods) biomineralization. This claim is contradicted by the recognition of substructured semi-nacre in cyclostome bryozoans.