Optimizing Computed Tomography (CT) for Structurally Complex Organisms: A Case Study of Gastropod Shell Features

The internal volume of gastropod shells is a measured metric in studies of hermit crab ecology, as it determines shell adequacy and subjective value. In this study, we developed and tested an optimized computed tomography (OCT) protocol to estimate shell volume as an alternative to traditional methods, using water or sand. Gastropod species with different shell architectures and sizes were analyzed, and OCT estimates were compared with traditional methods and a previously established nonoptimized CT protocol (NOCT). We also compared the performance of the standard proprietary software (CT Viewer), which operates only on the CT machine computer, with another proprietary program (OsiriX MD), a commercial software. Results showed that OCT produced similar volume estimates to NOCT for high-spired shells and shells with small elevations on their surface (hips). For low-spired and middle-spired shells with less ornamentation, OCT provided higher values than NOCT, and the results were similar to the sand-filling method. Across species, shell size influenced all methods, with larger high-spired shells exhibiting greater variability. In contrast, low-spired shells showed lower variability across all size classes. OsiriX MD yielded estimates equivalent to OCT for the majority of the species and high accuracy and reproducibility. Overall, OCT represents a reliable and nondestructive approach for shell volume estimation, supporting its use in ecological studies. Nevertheless, the NOCT protocol may be more advantageous for species with ornamentation, prickles, or high-spired shells, as it requires less processing time and does not demand fine adjustments according to species or shell size.