Niche divergence along a fine‐scale ecological gradient in sympatric color morphs of a coral reef fish

Color polymorphisms offer unique opportunities for testing the role of ecological adaptation and natural selection in the origin of species. However, the ecological conditions that facilitate the coexistence and speciation of color morphs in nature remain poorly understood. In this study, we investigate the ecological mechanisms maintaining the color polymorphism in the arc‐eye hawkfish ( Paracirrhites arcatus ), which consists of two common morphs (pink white‐striped [PWS] and melanistic [MEL]) and less common intermediates that all coexist on shallow coral reefs throughout the Indo‐Pacific. Our approach was to test whether ecologically based disruptive selection on color could explain the maintenance of and possible incipient divergence between sympatric color morphs. We compared patterns of spatial distribution, abundance, habitat use and preference, and niche overlap among color morphs at nine sites along the leeward coast of Hawaii Island. We demonstrate that despite co‐occurring at all sites, sympatric color morphs show significant niche divergence along a steep ecological gradient. Morph frequencies correlate strongly with a multivariate gradient in coral community and reef topography, which explained 81% of the variability in relative abundance of the two main color morphs. Melanistic morphs were more frequent in shallow, steep surge zones dominated by Pocillopora corals. In contrast, PWS morphs were more frequent in deeper, sub‐surge zones with higher coral cover dominated by Porites corals. Niche overlap analyses highlight how the two main morphs are partitioned on opposite ends of this continuous ecological gradient, while phenotypic intermediates are mostly restricted to intermediate habitats. The strong correlation between phenotype and environment suggests that morphs have fitness advantages in their respective habitats. We speculate that disruptive natural selection on color pattern to increase crypsis in heterogeneous visual environments has led to divergence in habitat preferences. The resulting ecological isolation provides a plausible mechanism for the persistence of multiple coexisting color phenotypes in P. arcatus . If divergence in color also affects mate choice, the evolution of reproductive isolation is likely to be promoted.