COMPARATIVE ANALYSIS OF THE MICROSTRUCTURE OF SPIRAL SHELL RIBS IN TWO BIVALVE AND THREE GASTROPOD SPECIES

Spiral ribs are among the most common morphological features in mollusk shells and previous studies have shown them to have functional significance with expected evolutionary consequences. Many previous studies, however, have treated these features as potentially analogous across taxa, without examining whether they may have important constructional dissimilarities. Mollusk shells are made of multiple layers of calcite or aragonite which may exhibit different microstructure or microstructure orientations which may in turn impact their mechanical properties. In this study, five specimens of marine mollusks with spiral ribs, including three turritellid gastropods and two bivalves, were examined under SEM to examine microstructure of ribbed region in comparison of non-ribbed region. SEM imaging revealed differences in the number and thickness of distinct microstructural layers of each shell and allowed comparisons to be made between the ribbed and non-ribbed region of each specimen, providing a greater understanding of how these ribs were constructed during shell deposition.

Ribs in all specimens are formed through the thickening of single or multiple crossed-lamellar layers, but differences in rib ultrastructures were found among species and different ribs of same species, showing great diversity and complexity of constructional mechanisms. This diversity in rib construction might indicate heterology in the development of shell sculpture, especially mechanisms for differences in concurrently deposited rib strength. This is especially notable for turritellids where the pattern of onset of spiral ornamentation is phylogenetically informative, suggesting homology of rib identity. Further study will be conducted on turritellid gastropods in different lineages to explore the taxonomic meaning of different rib constructional mechanisms.