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Olivella columellaris (Sowerby 1825) and O. semistriata (Gray 1839) are suspension-feeding, swash-surfing snails on tropical sandy beaches of the east Pacific. While they often are the numerically dominant macrofaunal element in their habitats, their biology is poorly understood; the two species actually have been confused in all of the few publications that address their ecology. Frequent misidentifications in publications and collections contributed also to an overestimation of the geographic overlap of the two species. To provide a sound taxonomic basis for further functional, ecological, and evolutionary investigations, we evaluated the validity of diagnostic traits in wild populations and museum collections, and defined workable identification criteria. Morphometric analysis demonstrated that shell growth is allometric in O. columellaris but isometric in O. semistriata, suggesting that the species follow distinct developmental programs. The taxonomic confusion is aggravated by the existence of populations of dwarfish O. semistriata, which originally had been described as a separate species, O. attenuata (Reeve 1851). At our Costa Rican study sites, the occurrence of such dwarfish populations correlates with low wave energies but not with predation pressure and anthropogenic disturbances, indicating significant ecological plasticity in the development of O. semistriata.

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