Aim: To evaluate the latitudinal pattern of body size within and among chiton species employing phylogenetically structured analyses, and to examine the role of geographic variation in temperature, productivity and oxygen availability as potential environmental drivers.

Location: Coastal habitats of the Southeastern Pacific along a latitudinal range of nearly 6,000 km, from the Equator to Patagonia (~ 2º to 56º S).

Time Period: Present (2011 – 2017).

Major taxa: 31 species of polyplacophoran mollusks.

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 generalised 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 along the Southeastern Pacific.

Chitons sampling

Field samplings were conducted in 62 localities ranging from the equator (~ 2.3 °S) to the Magellan strait (~ 56 °S), along the Southeastern Pacific (SEP) (Figure 1A). Sampling was conducted mostly in the spring-summer period between July 2011 and January 2017. We collected a total of 6,162 individuals belonging to 31 chiton species (Table S1). Chitons were collected by hand during low tide at intertidal habitats, and in shallow subtidal habitats by snorkeling and/or SCUBA (2 – 10 m). All surveys were performed during at least two-hour intervals at each location in order to homogenize sampling efforts. Specimens were identified to the species-level following different taxonomic guides (e.g. Kaas & Van Belle, 1987; Schwabe et al., 2006; Kaas, Van Belle, & Stack, 2006; Schwabe, 2009). Body length (BL, mm) –including the girdle– was measured for each specimen using a digital caliper (to the nearest 0.1 mm).