Trans-zonal movements in winkles, Littorina littorea (L.): Reasons and consequences

Two years of monthly samples of L. littorea at three levels on an estuarine shore at Southampton, U.K. have provided evidence of long-term movements up and down the shore. Increased population densities at the middle level of winkles in their second year of growth indicate movements of young winkles toward the center of their zonation range. Increases in population densities of older winkles at the higher and lower levels indicate later dispersion away from the center of the zonation range. Evidence of movements between the middle level and the lower level is provided by the presence or absence in the shell of bore-holes made by the polychaete Polydora ciliata (Johnston) which recruits mainly on the lower shore. At the upper level, fluctuating population densities and observations of feeding fronts in the spring indicate seasonal migrations, down in winter and up in spring. Advantages and disadvantages of living at upper or lower levels are discussed and related to differences in growth rates and predation risk.