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

Abstract

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.