Revisiting Amazonian water types: experimental evidence highlights the importance of forest stream hydrochemistry in shaping adaptation in a fish species

Abstract

Following Alfred Russel Wallace, Amazonian freshwaters are classified into three types: black, white, and clear. Such waters have been demonstrated to affect adaptation and gene flow of the aquatic fauna. However, this classification focuses on large rivers and fails to acknowledge the importance of small forest streams that flow through upland terra firme forests, known as igarapes. Igarapes are surrounded by a distinct floristic composition when compared to the floodplains and contribute with a much greater water load to the Amazon basin than the Amazon River itself. We assessed the importance of blackwater, whitewater, and igarape water on spawning and habitat choice in Crenuchus spilurus, a small fish composed of distinct genetic lineages that occur in igarapes flowing to blackwater and whitewater systems. Lineages from igarapes connected to the Rio Negro (blackwater) and Amazon River (whitewater) basins have greater spawning success in igarape water than in the water from their surrounding floodplain. Habitat choice trials showed active selection of igarape water by both lineages. Our results indicate that the hydrochemical condition of igarapes shapes adaptation in the physiology and behavior of C. spilurus. We suggest expanding upon the current classification to include igarapes as a distinct water type and environment.