Effect of culture depth on the shell thickness of the blue mussel Mytilus edulis: a practical case study comparing direct and indirect methods of measurement

Mytilus species have a fundamental role in coastal environments and constitute an important aquaculture resource. Their shell has a protective function and is affected by multiple factors, such as the size of the animal, density, environmental characteristics and presence of predators. It is crucial to accurately estimate shell thickness in order to understand the effects of the ever-changing environment on mussel physiology, morphology and ecology. This study compares a newly developed direct method for measuring shell thickness, using scanning electron microscopy (SEM), and a widely used indirect method (shell thickness index, STI), in the context of a practical case study of the blue mussel Mytilus edulis. In this study, STI did not appear as highly correlated to direct thickness measurement as in previous studies. This study also showed a weaker relationship between measured shell length and STI. In the case study, assessing the effect of culture depth on M. edulis growth and condition, STI showed a significant variation with depth, while SEM measured thickness did not. Therefore, the choice to use STI vs direct measurement of shell thickness drastically changed the results and interpretation of the case study. This work highlights the importance of a carefully designed method for measuring shell thickness and emphasizes that, before application, indirect methods should be quality controlled for the population studied. However, when high precision is needed for shell thickness measurements the application of SEM-based techniques may be necessary. This is the first study where SEM-based techniques have been used to measure shell thickness in M. edulis.