DYNAMICS OF UPSTREAM MIGRATION IN 2 SPECIES OF TROPICAL FRESH-WATER SNAILS

Much of the fauna of the Rio Claro on the Osa Peninsula of Costa Rica, including snails, fishes, and freshwater shrimp, migrates upstream, sometimes in large aggregations. We investigated the dynamics of the migratory behavior of the snails Neritina latissima and Cochliopina tryoniana, which both migrate upstream in huge, mixed aggregations (>500,000 individuals) during the dry season. Yet these two species have very different life histories. Neritina latissima has pelagic larvae which are swept downstream to the ocean. Metamorphosed individuals migrate upstream. Cochliopina tryoniana has direct development, with no marine stage. We analyzed the size distribution of the migrating, stationary, and drifting portions of the populations of these two species. In the drift, we collected only larval N. latissima. Analysis of the size distribution of the migrating and stationary snails suggested that the migration takes >1 yr for N. latissima and perhaps <1 yr for C. tryoniana. In both species, the migration behavior was size and age dependent. Juveniles and small adults composed the bulk of the migrating populations, whereas large adults were almost never observed to migrate. The mean size of individuals in migrating aggregations increased over time and distance upstream. Sphoeroides annulatus, a marine molluscivorous fish, was found only in the lower reaches of the stream. Where S. annulatus was common, N. latissima was less abundant and located on the underside of rocks. The distance migrated may be a balance between size-dependent energetic constraints on upstream movement and increased predation pressure downstream.