SHELL ULTRASTRUCTURE IN TWO SUBSPECIES OF THE RIBBED MUSSEL,GEUKENSIA DEMISSA(DILLWYN, 1817)

Abstract

1. The shells of two subspecies of the mussel, Geukensia demissa, were examined with the scanning electron microscope. In both subspecies, the shells contain an inner nacreous and middle and outer prismatic layers.2. The inner nacreous layer is composed of laminae of contiguous, hexagonal, aragonite crystals.3. The middle layer is composed of aragonite prisms arranged in a flabellate pattern.4. Two subspecific differences in ultrastructure, reflecting the gross morphological subspecific variation in ribbing, were evident in the middle prismatic layer. First, in G. d. granosissima, which has narrow granulated ribs, the orientation of the aragonite prisms with respect to the nacre changes from 0° to 90° from one side to the other of each granule; G. d. demissa has no granules and the prisms are always perpendicular to the nacre.Secondly, in G. d. demissa, the lip of the shell is a continuous band of the prismatic layer. In G. d. granossisima, however, the lip is primarily nacre interspersed with clusters of aragonite prisms corresponding to the underside of a granulated rib.5. The subspecific difference in the deposition of prismatic aragonite is genetic rather than environmental.6. The thin outer layer could only be observed after specific staining for calcite. This layer also contains the purple pigment found commonly in G. d. demissa but rarely in G. d. granosissima.7. A model involving screw dislocation is presented to explain the regularity of nacre crystal orientation.