Phenotypic plasticity at fine-grained spatial scales: the scorched mussel Perumytilus purpuratus growing on Patagonian rocky salt-marshes

Understanding phenotypic plasticity of species at different spatial scales is vital in the current context of an increasing pace of environmental changes. Through this knowledge, it is possible to predict their potential to adapt and/or evolve in face of new environmental conditions such as climate change, and/or to understand their ecological range expansion. In Patagonian rocky salt-marshes, one of the most abundant invertebrate species is the scorched mussel Perumytilus purpuratus. In this system, this mussel can be found inhabiting both vegetated and non-vegetated patches, which differ in critical environmental conditions. We performed a field study evaluating whether mussels growing in vegetated patches differ in shell shape from those growing in adjacent non-vegetated patches. We sampled individuals from both patch types and assessed their shell shape and size using geometric morphometrics. The results showed that mussels from vegetated patches had shells that were more dorsoventrally expanded, anterodorsally restricted and globose in shape than those from non-vegetated patches, which showed the opposite traits resulting in a more elongated shell. The differences found could be driven by the different conditions of temperature, desiccation rate, wave action and population density to which mussels are exposed in each patch type. These results revealed the striking phenotypic plasticity of shell form of this native species at a fine-grained scale, which could be one of the explanations for its success in its ecological range expansion.