Body Size Variation In Polyplacophoran Molluscs: Geographical Clines And Community Structure Along The South-Eastern Pacific

Aim To evaluate the latitudinal pattern of body size within and among chiton species employing phylogenetically structured analyses and examine the role ofgeographical variation in temperature, productivity and oxygen availability as potential environmental drivers. Location Coastal habitats of the south-eastern Pacific along a latitudinal range of nearly 6,000 km, from the equator to Patagonia (c. 2 degrees to 56 degrees S). Time period Present (2011-2017). Major taxa studied Thirty-one species of polyplacophoran molluscs. Methods We measured the body length of 6,162 individuals collected in 62 sites, and reconstructed the phylogeny of this group based on two mitochondrial and one nuclear gene regions. We combined this information with data of sea surface temperature, chlorophyll-a concentration-as a proxy of primary productivity-and dissolved oxygen, and assessed which variables best explain the variation in size both within and among species employing phylogenetic generalized least squares (PGLS) and a model comparison approach. Main conclusions Our analyses show that body size increases consistently with latitude, both within and among species, following Bergmann's rule. Variation in sea surface temperature along the latitudinal gradient provided a substantially better fit than chlorophyll-a and dissolved oxygen. Our results support the temperature-size rule for this lineage and suggest that similar processes could underlie the emergence of intra- and interspecific gradients in body size of polyplacophorans. At the community level, chiton species richness was higher at intermediate latitudes and positively correlated with body size variation, suggesting that heterogeneity in size may reduce interspecific competition and contribute to species coexistence in this group. Overall, our study demonstrates that historical events, macroecological adaptive trends and local processes at the community level contribute to the distribution and size variation of polyplacophoran species throughout the south-eastern Pacific.