Development and Evolution of Shell Sculpture in Gastropods

• Author / Creator

  Webster, Nicole B

• The shells of molluscs are a beautiful and intriguing tool for studying both the evolution and development of novel morphologies. The mantle secretes a logarithmically spiraled shell through accretionary growth at the apertural margin, not unlike a 3D printer adding material layer by layer. Shell form has been modeled extensively, and the basic mechanics of shell secretion are understood. Shelled molluscs also have an excellent fossil record, which permits historical studies of morphological evolution. One aspect of shell growth — shell sculpture — has been sorely understudied. This thesis focuses on its evolution and development. Specifically, I examine the evolution and development of the most elaborate form of sculpture, varices — periodic axial shell thickenings, that vary from elaborate wings and spines, to subtle scars. I focus primarily on the gastropod family Muricidae, which exemplifies a diversity of shell sculpture, especially varices and the superficially similar lamellae. Prior to this work, varices lacked a comprehensive definition, which this thesis provides. I describe all 41 separate evolutionary origins of periodic varices. Overall, varices are more prevalent a) where predation pressure is stronger: in warm, shallow marine waters, b) on high‑spired shells and c) in clades with axial ribs. Many origins of varices were clumped phylogenetically, and most arose after the mid‑Mesozoic. Although half of all lineages with varices had three or fewer genera, diversification rates in the Tonnoidea correlated positively with the advent of varices. In many cases, varices are remarkably well aligned between whorls, producing a regular synchronized pattern of sculpture. A physical feedback mechanism, where previous varices guide the placement of future ones was the primary explanation. This hypothesis wasiii tested in Ceratostoma foliatum, which has three aligned alate varices per whorl. By selectively removing or artificially attaching specific varices, I showed that previous shell sculpture was neither necessary nor sufficient to trigger future varix production. Interestingly, new varices grew slightly past their normal position when the physical cue was lacking, and apertural damage caused at least a temporary disruption of synchrony where varices were grown earlier than the normal placement. Overall, some internal regulatory mechanism seems responsible for the synchronization of varices in C. foliatum, with some possible fine‑tuning by sensing previously grown physical shell cues — varices. A form of sculpture superficially similar to varices are lamellae — sharp axial, bladelike upliftings. To compare their development to the development of varices, I studied the growth and positioning of lamellae in Nucella lamellosa. In contrast to varices, lamellar growth was fast and plastic. Lamellae were produced in one to two weeks, with no evidence of a varix‑like growth hiatus. Lamellar spacing increased with increasing shell growth rate, and spacing was irregular, unlike in most varices. Just like varices, previous lamellae need to be removed to permit future shell growth. Removing lamellae experimentally had no effect on the subsequent shell growth rate or lamellar spacing. So, the process of dissolving previous shell sculpture to permit new shell growth does not appear to be rate limiting. Although the basics of shell secretion are generally understood, little is known about how the mantle changes to produce shell sculpture. Examining the mantle of Nucella ostrina, which has both spiral‑ribbed and smooth shell forms, showed a relatively straightforward process. Ribs are formed by extending the mantle region responsible for rib formation, thus producing more cells to secrete more shell, and producing a thicker rib than the adjacent inter‑rib mantle tissue. This process was examined with multiple techniques: histology,iv histochemistry, SEM, TEM, and 3D reconstruction, to understand it from multiple perspectives. The overall results of this thesis demonstrate the great potential of gastropod shell sculpture as a model to examine developmental patterns and the evolution of novel traits. It has opened a number of avenues for future work. I determined that the mechanisms involved in shell sculpture formation and control may be both more simple and more complicated than previously imagined. Shell secretion is a worthwhile model to examine accretionary growth of hard parts – a mode of skeletal growth common to several animal phyla. It is also a developmental system that differs from most other developmental systems generally studied, which will provide a new perspective on the development and evolution of morphological diversity.

• Subjects / Keywords

  • Lamellae
  • Gastropoda
  • Shells sculpture
  • Varices

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3DN40B1M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.