Physical habitat, disturbances, and the population ecology of the migratory snail Neritina virginea (Gastropoda: Neritidae) in streams of Puerto Rico

Diadromous fauna, migrating between marine and coastal streams are dominant in the neotropics, but factors controlling their populations are poorly understood. The migratory snail Neritina virginea (Gastropoda: Neritidae), abundant in estuaries and coastal rivers in the Caribbean, was studied to understand those factors. The objectives were three-fold: (1) to identify the factors controlling the distribution of diadromous fauna at different spatial scales, (2) to determine patterns and causes of massive upstream migrations and their importance for population persistence, and (3) to integrate this knowledge into conservation and management plans.; After a survey of 32 coastal streams around Puerto Rico (Greater Antilles), it was evidenced that physical variables operating at different levels of the spatial hierarchy controlled longitudinal distribution of N. virginea . These variables were river-ocean connectivity (regional scale), instream barriers and water chemistry (stream-network scale), habitat hydraulics (reach scale) and nearbed-flow roughness and depth (habitat scale). However, microhabitat scale distribution depended upon habitat and reach scale context. At habitat scale, water depth in pools, and nearbed flow roughness in riffles were the most important controls. At the reach scale, spatial heterogeneity among and within streambed patches was related to flow refugia, therefore controlling sensitivity of N. virginea density to flooding disturbances.; The role of structures (bridges) and maintenance activities (downstream channel realignment) associated to road crossings over streams was assessed. By splitting the stream channel and deflecting the flow, bridge pilings altered upstream migration routes. Migratory individuals used more frequently boulder-and-cobble riffle reaches than gravel-bed run reaches. Impact assessment of channel realignment over a 70-m reach in lower Rio Mameyes (NE Puerto Rico) showed that increase of fine sediment patches contributed to local population depletion and high mortality of juvenile individuals migrating upstream. As a consequence a population located 100 m upstream became isolated and did not recover from a major storm flood occurred after the channel realignment.; In conclusion, distribution and population size of N. virginea in coastal streams is influenced by hierarchically organized variables. By affecting variables at microhabitat to reach scales, road crossings over streams contribute to reduce longitudinal connectivity, and isolate upstream populations of N. virginea.