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

Abstract

The shells of 2 sspp. of the mussel G. demissa, were examined with the scanning EM. In both subspecies, the shells contain an inner nacreous and middle and outer prismatic layers. The inner nacreous layer is composed of laminae of contiguous, hexagonal, aragonite crystals. The regularity of nacre crystal orientation is explained on a screw dislocation model. The middle layer is composed of aragonite prisms arranged in a flabellate pattern. The thin outer layer also contains the purple pigment found commonly in G. d. demissa but rarely in G. d. granosissima. Two subspecific differences in ultrastructure reflecting the gross morphological subspecific variation in ribbing, were evident in the middle prismatic layer. In G. d. granosissima which has narrow granulated ribs, the orientation of the aragonite prisms with respect to the nacre changes from 0.degree. to 90.degree. from 1 side to the other of each granule; G. d. demissa has no granules and the prisms are always perpendicular to the nacre. In G. d. demissa the lip of the shell is a continuous band of the prismatic layer. In G. d. granossisima the lip is primarily nacre interspersed with clusters of aragonite prisms corresponding to the underside of a granulated rib. The subspecific difference in the deposition of prismatic aragonite is genetic rather than environmental.