Stability of parallel evolution in shell traits between the littoral and sublittoral ecotypes of the Antarctic limpet Nacella concinna along a 17-year time series

Ecotypes constitute valuable markers for understanding the phenotypic responses influenced by evolutionary forces and environmental variations. Assessing the role of these processes in establishing and maintaining ecotype divergence is essential for anticipating evolutionary responses to future climate shifts. We studied the evolutionary dynamics of phenotypic population structure between Nacella concinna ecotypes through a morphometric and temporal analysis (2004–2021) in Antarctica. This approach enables the assessment of contemporary evolutionary dynamics in the presence of biotic and abiotic factors, facilitating the inference of interactions between the acting forces. Using generalized Procrustes analysis, ecotypes were successfully discriminated by their lateral shell traits consistently persisting over the analysed period. Multivariate linear and univariate mixed models revealed a significant temporal effect on shell phenotypic variation, surpassing the effect of inter-ecotype differences. However, according to phenotypic trajectory analysis, this temporal effect did not alter the direction of the phenotypic trajectory of ecotype shell shape, which evolved in parallel during the time series. Despite the environmental variation, the evidence of parallel evolution might indicate the incidence of natural selection in ecotype divergence rather than relying solely on phenotypic plasticity. This work enhances our understanding of ecotype temporal changes and microevolution, underscoring the importance of long-term phenotypic monitoring.