Breaking down shell strength: inferences from experimental compression and future directions enabled by 3D printing

Abstract

Mollusc and brachiopod shells have served as biological armour for hundreds of millions of years. Studying shell strength in compression experiments can provide insights into macroevolution, predator–prey dynamics, and anthropogenic impacts on aquatic ecosystems. These studies have been conducted across fields including palaeontology, ecology, conservation biology and engineering using a range of techniques for a variety of purposes. Using this approach, studies have demonstrated that predators can cause changes in prey shell morphology in the laboratory over both short timescales and over longer evolutionary timescales. Similarly, environmental factors such as nutrient concentration and ocean acidification have been shown to influence shell strength. Experimental compression tests have been used to study the functional morphology of shell‐crushing predators and to test how the taphonomic state of shells (e.g. presence of drill holes, degree of shell degradation) may influence their likelihood of being preserved in the fossil record. This review covers the basic principles and experimental design of compression tests used to infer shell strength. Although many investigations have used this methodology, few provide a detailed explanation of how meaningfully to interpret data generated using compression experiments for those unfamiliar with this method. Furthermore, this review provides a compilation of the findings of studies that have employed these experimental methods to address specific themes: taphonomy, morphology, predation, environmental variables, and climate change. Many authors have used experimental compression tests, however, disparities among methodologies (e.g. in experimental design, taxa, specimen preservation, etc.) limit the applicability of findings from taxon‐specific studies to broader eco‐evolutionary questions. The review highlights confounding factors, such as shell thickness, size, damage, microstructure, and taphonomic state, and address how they can be mitigated using three‐dimensional (3D)‐printed model shells. 3D prints have been demonstrated as valuable proxies for understanding aspects of shell morphology that cannot otherwise be experimentally isolated. Using 3D printed models allows simplification of complex biological systems for idealized experimental studies. Such studies can isolate specific aspects of shell morphology to establish fundamental relationships between form and function. Establishing standardized methods of testing shell strength in this way will not only permit comparison across studies but also will enable investigators systematically to add complexity to their models.

Geology constrains biomineralization expression and functional trait distribution in the Mountainsnails

“…Mass standardized shell strength was greater for all ornamented types than smooth forms except when comparing smooth forms that were calcareous rock residents to commarginal ornaments (Figure 5, Supplementary Material Appendix II, Table S6). These results are consistent with previous studies documenting increased shell strength along the axis of ornamentation expression (Johnson, 2021;Posilović & Bajraktarević, 2010) as shell strength during lateral crushing was higher for keeled and antimarginal shell forms which have greater shell thickness along the axis of crushing compared to smooth or commarginal ornaments.…”

Did shell-crushing predators drive the evolution of ammonoid septal shape?

“…4B). This observation verified that the models’ peak loads were appropriate proxies for the relative strengths of different shell morphologies (Johnson 2020, 2021). Additionally, we used the shapes of the load-displacement curves to assess any differences in the mechanical behavior of various septal shapes upon loading.…”

“…Using flat plates is the most general way to test prey strength in bulk compression without introducing numerous assumptions about the placement of point loads from specific predator dentitions. Shell strength varies based upon loading location (Johnson 2021). As the current literature lacks physical testing that isolates septal morphologies and relative strength, the goal of this study was to develop a first-order approximation for these relationships in bulk compression.…”

Morphometric study of Lola Rochia nilotica (Linnaeus 1767) shells from natural harvest found in Indonesian

“…Shell size is also influenced by food availability and food quality, also the presence of natural predators can cause changes in shell morphology (Capinpin Jr. 2018;Johnson 2021). From this statement, it can be said that the availability of feed affects the weight of snails that live in that habitat.…”