Electron microscopy of molluscan crossed-lamellar microstructure

Abstract

The morphology of the molluscan shell microstructure cross-lamellar structure (XLM) was studied by a scanning electron microscopy (SEM) examination of a variety of species ( Patella vulgata, Littorina littorea, L. nigrolineata, L. obtusata, L. saxatilis, Oliva sayana, Conus litteratus, C. magus, Pomatias elegans, Acanthopleura brevispinosa , and Arca tetragona ). Two of the gastropod species ( O. sayana and L. littorea ) were also examined by transmission electron microscopy (TEM) to determine the crystallographic relationships between the microstructural units. SEM studies reveal that although the basic structure of XLM is broadly similar in all species, there is quite a degree of variation present in size of unit and regularity in form, even within a single specimen. Transitions from other aragonitic microstructures to XLM are gradational in nature, whereas the boundary between XLM and a calcitic microstructure is always sharp. The basic morphological units in Oliva sayana are third order lamellae that are bounded by (001) and (110) faces. They are profusely twinned on (110) and have very minor (110) twinning. The crystallographic relationship between first order lamellae in O. sayana maintains an approximate parallelism between the (110) twinning planes in adjacent crossed bands, and the third order lamellae in the bands are rotated by approximately 45° with respect to each other about the [001] direction. The basic morphological units in Littorina littorea are third order lamellae with (001) and (100) faces, again with ubiquitous (110) twinning. The crystallographic relationship between first order lamellae is an approximately 50° rotation about [100], making the [001] direction in the (100) face on a third order lamella of one band almost parallel to the [011] direction in the (100) face of a lamella of an adjacent band. The organic interlamellar sheets are thought to control the inexact epitaxial relationships observed in both species, and our results indicate the likelihood of a unique matrix composition for each.