PS 7-90 - CANCELLED - Ecological drivers of morphological variation in Galápagos land snails

Abstract: Morphological variation in land snails (e.g., shell shape and size, sculpture, aperture size, color) is frequently attributed to selection by environmental factors from a necessity to regulate body temperature and minimize water loss to avoid desiccation. While shell morphology has been prominently studied due to their longevity and presence in the fossil record, softer body parts, like the fleshy snail foot, have received less attention. In addition, the effect of microhabitat and environmental zones on snail morphology have rarely been studied simultaneously. We use Naesiotus, a land snail genus with over 60 described species found across the Galápagos Archipelago to test 1) the relationship between foot size and other morphological traits and 2) if species traits follow morphological patterns relating to their respective environmental zones and microhabitats. We sampled twenty species of Naesiotus across their range on three islands in the Galápagos Archipelago (Santa Cruz, San Cristóbal, and Isabela islands) in June and July 2021. We recorded mass, shell size, and surface foot area for every individual in the study. To assess the environmental zone, we evaluated each study site to determine if they were an arid or humid site based on available raster layers from the Charles Darwin Foundation. For microhabitat designations, terrestrial species were defined as those exclusively found on and under rocks, logs, and other debris while arboreal species were those observed climbing vegetation.

Our results demonstrate allometric relationships between foot size and other morphological traits. Snail foot size was positively correlated with body mass and size, indicating these morphological traits are likely ecologically linked. There is also evidence of morphological variation in shape and size across different environmental zones and microhabitats in Naesiotus. In general, smaller snails were found in arid zones and terrestrial habitats, compared to larger snails which were more commonly found in humid zones and arboreal habitats. Therefore, morphological variation in traits may be associated with distinct geographical trends in the environment and microhabitat. While most snail species follow general trends relating morphology to habitat sites, there are notable exceptions. Given Naesiotus is a recent adaptive radiation, some species may continue to change morphologically through time. Additional work is needed to parse apart snail habitat use and morphological constraints and selective pressures driving differences in shell shape and foot size.