Identifying environmental drivers of shell shape and size variation in a widely distributed marine bivalve along the Atlantic Patagonian coast

Since the impacts of climate change on marine benthic organisms are already detectable in many aspects, it is crucial to address the potential responses of marine organisms and their interactions with the environment. In particular, the study of phenotype traits along latitudinal gradients is a powerful tool for exploring species re-sponses in a changing environment. This work aims to: i) Quantify the variation in shell shape and size of the stripped clam Ameghinomya antiqua (King, 1832) along a 1600 km latitudinal gradient in the Atlantic Patagonian coast using geometric morphometric (GM) methods; ii) Identify environmental effects on shell shape and size change, using generalized additive mixed models GAMMs; iii) Estimate the effect size of different factors influencing shell shape and size variation. The combination of GM and GAMM allowed the quantification and identification of previously unknown relationship between morphological axes of variation and environmental variables. Results suggest that salinity is the most important factor determining shell shape, mainly affecting the outline while temperature has a substantial but weaker effect. More elongated shells and longer ligaments are observed under (less favorable) conditions of low salinity and temperature. GAMM showed that chlorophyll-a concentration, used as a proxy for food supply, has the most significant effect on shell size, and shell growth does not match the somatic soft-tissue growth. These results may help to predict the effects of climate change on the size and shape of marine benthic species and bivalves in particular. Furthermore, they provide new insights for using shell size and shape as proxies for the reconstruction of past environments and climates.