Amazonian ecology: tributaries enhance the diversity of electric fishes.

Neotropical rivers support a diverse array of endemic taxa, including electric fishes of the order Gymnotiformes. A comprehensive survey of the main channels of the Amazon River and its major tributaries (>2000-kilometer transect) yielded 43 electric fish species. Biogeographical analyses suggest that local mainstem electric fish diversity is enhanced by tributaries. Mainstem species richness tends to increase downstream of tributary confluences, and species composition is most similar between tributaries and adjacent downstream mainstem locations. These findings support a "nodal" or heterogeneous model of riverine community organization across a particularly extensive and diverse geographical region.