Simple SummaryChitons are a charismatic group of mollusks that live underneath rocks on shores throughout the world, where they experience variations due to environmental factors. Temperature may cause animals to grow to a larger size in colder climates, a trend originally ascribed to endothermic animals. In this work, advanced techniques were used to describe both shape and size of chitons living under different temperature regimes on Pacific shores. The results show that chitons are larger in colder zones, as occurs in endothermic vertebrates. Respiration, reproduction and/or feeding strategies may explain this trend in chitons.AbstractBergmann’s rule relates the trend of increasing body size with higher latitudes, where colder climates are found. In the Mexican Pacific, three marine ecoregions are distinguishable across a latitudinal gradient.Stenoplax limaciformisis an abundant chiton species that is distributed on rocky shores in these ecoregions. Geometric morphometric analyses were performed to describe the shape and size variation ofS. limaciformisbetween marine ecoregions that vary in sea surface temperature with latitude, thus testing Bergmann’s rule. Individuals’ body shape ranged from elongated to wide bodies. Although there was variation in chitons’ body shape and size, the was no evidence of allometry among localities. The Gulf of California is the northernmost ecoregion evaluated in this work, where larger chitons were observed and lower sea surface temperature values were registered. The results suggest thatS. limaciformisfollows a trend to Bergmann’s rule, such as endotherms. These mollusks do not need heat dissipation, but they do need to retain moisture. In addition, larger chitons were observed in zones with high primary productivity, suggesting that chitons do not delay their maturation due to food shortage.