Massive upstream migrations by a tropical freshwater neritid snail

While upstream movements of benthic invertebrates have been reported, gregarious upstream migrations of freshwater snails have never been described. We report a possible adaptation to downstream drift by a population of the Neritid (Archeogastropoda) snail, Neritina (Clypeolum) latissima (Broderip, 1833), in the Rio Claro, a small coastal river in Corcovado National Park, Costa Rica. We observed large numbers of these snails in gregarious upstream migration. Snails migrated in narrow lines up to 32 meters long, containing thousands of snails. The migration covered over 1 km of the stream. Larval and juvenile snails are more susceptible to washout by current, and differential mortality of these age classes in the estuarine and ocean environment may provide the selective pressure for the evolution of this behavior.