Differences in spatial structures between juveniles and adults of the gastropod Hydrobia ulvae on an intertidal mudflat (Marennes–Oléron Bay, France) potentially affect estimates of local demographic processes

Spatial structures of juvenile and adult cohorts of the gastropod Hydrobia ulvae were investigated over a surface area of several km 2 in an intertidal mudflat at the end of the recruitment period. Three cohorts (average shell lengths of 3.9±0.89 mm, 1.8±0.43 mm and 1.1±0.18 mm for cohorts I, II and III, respectively) were identified by a statistical analysis and their spatial distributions were characterised. Cohorts of juveniles (cohorts II and III) showed a patchy distribution pattern, whereas the cohort of adults (cohort I) was homogeneously distributed over the study area. These marked differences in spatial distribution of cohorts are likely to have a strong effect on estimates of local demographic processes, and hence of population dynamics.