Sensory drive in colourful waters: Morphological variation suggests combined natural and sexual selection in an Amazonian fish

Natural selection often shapes visual perception with respect to the lighting environment. For organisms that rely on visual communication for mating, environmental backscatter and bias in wavelength transmission affect the exchange of visual signals, which can mediate major changes in sexually selected traits. Based on the lighting environment, Amazon forest streams (igarapés) can be categorized into two major water types: clearwater and blackwater. The lighting environment is mostly transparent in clearwater, whereas retention of high amounts of dissolved organic carbon biases light towards the red and creates strong backscatter in blackwater igarapés. We investigated morphological differences among populations representative of lineages within the sailfin tetra Crenuchus spilurus, a sexually dichromatic Amazonian fish. We show that despite the broad geographical range of the nominal species, populations are similar for most measured morphological traits. However, eye diameter and characteristics of the fin ornaments revealed two distinct groups, corresponding to lineages from blackwater and clearwater igarapés. Our results lend support to the sensory drive hypothesis by suggesting that as a consequence of animals inhabiting different lighting conditions, natural selection affected visual perception and thus resulted in differences in sexual ornaments. © 2019 The Linnean Society of London, Biological Journal of the Linnean Society

Testing Wallace's intuition: water type, reproductive isolation and divergence in an Amazonian fish

Alfred Russel Wallace proposed classifying Amazon rivers based on their colour and clarity: white, black and clear water. Wallace also proposed that black waters could mediate diversification and yield distinct fish species. Here, we bring evidence of speciation mediated by water type in the sailfin tetra (Crenuchus spilurus), a fish whose range encompasses rivers of very distinct hydrochemical conditions. Distribution of the two main lineages concords with Wallace's water types: one restricted to the acidic and nutrient-poor waters of the Negro River (herein Rio Negro lineage) and a second widespread throughout the remaining of the species’ distribution (herein Amazonas lineage). These lineages occur over a very broad geographical range, suggesting that despite occurring in regions separated by thousands of kilometres, individuals of the distinct lineages fail to occupy each other's habitats, hundreds of metres apart and not separated by physical barrier. Reproductive isolation was assessed in isolated pairs exposed to black-water conditions. All pairs with at least one individual of the lineage not native to black waters showed significantly lower spawning success, suggesting that the water type affected the fitness and contributed to reproductive isolation. Our results endorse Wallace's intuition and highlight the importance of ecological factors in shaping diversity of the Amazon fish fauna. © 2018 European Society For Evolutionary Biology. Journal of Evolutionary Biology © 2018 European Society For Evolutionary Biolog