Tube ultrastructure of Pomatoceras americanus (Polychaeta, Serpulidae): implications for the tube formation of serpulids

The inner tube layer of Pomatoceros americanus has a complex oriented ultrastructure, which cannot be explained by the standard granular secretion model, predicting a largely unoriented structure of the tube. In the lamello-fibrillar structure of the inner tube layer, the crystallization axis of crystals has a uniform orientation, which is not continuous through successive growth increments. The complex biomineral structures of P. americanus suggest a matrix-controlled crystallization model rather than solidification of a slurry with calcareous granules deposited on the tube aperture.