Evidence of countergradient variation in the growth of an intertidal snail in response to water velocity

Growth rates in rocky intertidal snails can vary considerably across wave exposure gradients, and have both plastic and genetic bases. However, little is known regarding whether genetic and environmental influences on variation in growth act in the same (cogradient) or in an opposing (countergradient) direction. Determining how genetic and environmental influences on growth covary with one another may improve our understanding of how habitat-specific variation in growth emerges. This study utilized laboratory flumes to examine the effects of high and low water velocities on the growth of intertidal snails Littorina obtusata from a wave-exposed and a sheltered shore. Both flow velocity and source population significantly influenced all measures of growth (shell length, shell thickness, shell mass and tissue mass). Snails from both populations exhibited greater growth in low versus high flow velocity. In addition, snails from the wave-exposed population grew more than snails from the sheltered population regardless of flow treatment. This result yielded a pattern of countergradient variation in growth and suggests that genetic differentiation between the 2 populations was responsible for the more rapid growth of wave-exposed snails. This greater growth potential of wave-exposed snails was particularly evident when they were raised in an environment conducive to rapid growth (i.e. low flow velocity). Most examples of countergradient variation in the growth of intertidal gastropods have involved temperature effects on latitudinally separated populations. This study provides evidence that countergradient variation in growth can occur on localized spatial scales in response to environmental cues other than temperature. On rocky intertidal shores, countergradient variation in growth may reflect selection for fast-growing genotypes to offset limitations on foraging time imposed by increased hydrodynamic stress on wave-exposed shores.