Geology constrains biomineralization expression and functional trait distribution in the Mountainsnails (Oreohelix)

Aim: Geographic variation in metabolic resources necessary for functional trait expression can set limits on species distributions. For species that need to produce and maintain biomineralized traits for survival, spatial variation in mineral macronutrients may constrain species' distributions by limiting the expression of biomineralized traits. Here, we examine whether threatened, heavily biomineralized Oreohelix land snails are restricted to CaCO₃ rock regions, if they incorporate greater amounts of CaCO₃ rock carbon in their shell than less biomineralized smooth forms, and if ornamentation increases shell strength.

Location: Western United States

Methods: We used random forest (RF) classification models at multiple spatial resolutions to evaluate the contribution of topographic, vegetation, climate, and geologic variables in predicting the presence of heavily biomineralized shell ornaments. We then measured and compared shell biometric variables, ¹⁴C/¹²C ratios, and peak force for fracture for ornamented and smooth forms from calcareous and non-calcareous substrates.

Results: Distance to CaCO₃ rock was the most important variable in all trait distribution models and was highly associated with local ornamentation classification and forecasted distribution. Pairwise comparisons of ¹⁴C/¹²C ratios in closely occurring ornamented vs. smooth population pairs revealed ornamented forms incorporate greater CaCO₃ rock carbon than smooth forms. Ornamented types measured in this study were generally heavier and required greater peak force for fracture than smooth snails, except when compared to smooth forms sampled from CaCO₃ rock.

Main conclusions: Biomineralization expression, species distribution, and trait function appear to be constrained by mineral supply in a highly diverse group of land snails. This trait-environment relationship suggests similar CaCO₃ macronutrient constraints may modulate biomineralization expression and restrict species distribution in other terrestrial molluscs and has a direct impact on the management of Oreohelix species.