Evolution of adaptation through allometric shifts in a marine snail

Variation in ontogenetic development among individuals may be a major contributor to morphological variation within species. Evolution of different growth trajectories might, for example, evolve as a response to varying ecological contexts of individuals living in different environments, or by life-stage or gender differences. The intertidal periwinkle Littorina saxatilis is strongly polymorphic in shell shape. We compared ontogenetic trajectories between life stages, local populations, and sexes to understand how different morphological end points are reached during ontogeny and what might cause these differences. Applying landmark-based geometric morphometrics, we captured shell shape variation for four Swedish populations of this species. We also derived a method to visualize ontogenetic trajectories described by the relationship of size to the multivariate shape space. We found that growth trajectories differed between individuals living in different habitats, as well as between sexes and maturity stages. Males living on rocky cliffs grew isometrically throughout life, whereas females from the same habitat switched from isometric growth as juveniles to allometric growth as adults. In contrast, males and females living on boulders grew allometrically as juveniles but changed to isometric growth at sexual maturity. Thus, in this species, ontogenetic growth seems influenced by habitat-associated selection as well as by gender and age-specific selection. These differing selection regimes result in ontogenetic shifts in allometry in three of the four groups examined.