Living dangerously: predicting survival of the limpet <i>Patella vulgata</i> based on the probability of projectile impact

Abstract The limpet Patella vulgata increases in size throughout its life, leading to a distribution of sizes for a given population and location. This paper presents a theoretical model in which the increase in shell size after maturity is modelled as a response to risks which are stochastic in nature and size-related. Among these is the risk of impact damage caused by rocks and other projectiles during storms. Data were collected to determine the size distribution and growth rates of limpet shells as well as the sizes of potential projectiles at a site on the Irish coast. Impact strength (energy to cause failure) was measured as a function of shell size. A theoretical model was developed which was able to accurately predict the size distribution (relative number of shells of a given size) for mature limpets (shell length of 25 mm and above). The model was able to demonstrate the value of continued growth: it predicted that if growth were to stop at maturity (25 mm), then the density of limpets at this site would decrease by 37%. Comparison with published data from other sites showed a consistent relationship between size distribution and the potential for storm-related impacts. Possible effects of climate change (increasing the number and energy of storms) could also be estimated. The model can potentially be expanded to cover other size-related risks (predation, food supply, etc.). This work is a contribution to our understanding of the strategies that organisms have evolved to resist the effects of mechanical forces.