Vertical Size Gradients and Migratory Patterns of Two Nerita Species in the Northern Gulf of California

This study addresses patterns of vertical zonation, shell-size variation, and migratory behavior exhibited by two herbivorous intertidal gastropods in the northern Gulf of California, Sonora, Mexico. While actively foraging, Nerita scabricosta inhabited a zone from +3.1 m to +4.6. m above mean low water, whereas Nerita funiculata ranged from below + 1.2 m to + 4.6 m. Both species significantly increased in shell size with increasing tidal height. The size of N. scabricosta increased faster with tidal height than N. funiculata. In addition, N. funiculata increased faster with tidal height when inhabiting the zone of overlap with N. scabricosta than when in the non-overlap zone. When displaced up or down a tidal gradient, both species actively migrated in the direction of the habitat from which they were taken. This finding suggests that migratory behavior is in part responsible for the maintenance of interspecific zonation patterns.