Impact resistance of limpet shells: a study of local adaptations

Abstract

Limpets are molluscs which have a conical shell that is well adapted to resist fracture by impact from projectiles such as rocks during storms. We hypothesised that the impact strength of the shell varies depending where the animal is located, reflecting the relative risk of high-energy impact. We quantified shell impact strength for the species Patella vulgata using a normalised energy. Limpets located in exposed places on open rock surfaces were found to be more than twice as strong as those living constantly underwater (7.34 MJ/m4.6 v 3.48 MJ/m4.6). This difference was discussed using a theoretical model based on the physics of projectiles moving through fluids. Limpets located in rocky crevices had an intermediate impact strength (5.43 MJ/m4.6), attributed to the reduced probability of impact in these locations. Differences in impact strength were found to be linked to two geometric parameters: apex thickness and the ratio of apex height to rim diameter. Combining the present results with data from previous work, we developed a theoretical model which was able to predict impact strength accurately as a function of rim diameter, apex height and apex thickness. These results demonstrate the considerable plasticity of form, which this species is capable of, helping to explain why it is so abundant. The findings may be valuable in the biomimetic development of lightweight impact resistant structures.