Population differences in behaviour and morphology in the snail Littorina saxatilis: phenotypic plasticity or genetic differentiation?

Abstract

The ovoviviparous Littorina saxatilis (Olivi) can be found in many intertidal habitats. Winkles from different habitats have different morphologies and behaviours, which are usually argued to reflect adaptations. Whether the different forms are caused by genetic differentiation or due to phenotypic plasticity is, however, less discussed. Our aim was to document morphological and behavioural differences among one exposed rock poulation, one sheltered boulder population, and one mud flat population, and we reared offspring in a common laboratory environment to see if the differences persisted.We used principal component analysis (PCA), based on 19 measurements, to compare size and shape differences. We also used PCA and linear regression to study allometry and compare growth trajectories. Behavioural differences were studied in three laboratory trials. Littorina saxatillis from the rock were relatively small, had small aperture lips, thin shells, blunt spires, and wide columellae. They preferred to stay in cracks and to be above the water surface, and they quickly emerged out of their shells after disturbance. L. saxatillis from the boulder shore were relatively large, had large lips, thick shells, pointed spires, and narrow columellae. They preferred open surfaces, submergence, and after disturbance they slowly emerged out of the shell. L. Saxatilis from the mud flat were relatively small, had small lips, think shells, pointed spires, and narrow columellae and most behaved similarly to the boulder shore snails. Although less pronounced, most of these morphological and behavioural differences persisted in the snails that we reared in the common environment. This indicates some genetic differentiation. All groups grew allometrically, and most groups differed in growth trajectories, Despite similar ages, the reared females were generally larger than the reared males, indicating higher growth rate in females. Sexual maturation seemed to be reached at a certain size rather than at a certain age. Theories predict genetic diferentiation in intertidal snail species with low dispersal, but some investigations show results that contradict the theories, while others agree with them. Our study confirms the prediction that L. saxatilis should be genetically differentiated.