Why are salmonids pink?

Pigmented flesh, a trait uniquely present in four of the 10 genera of salmonid fishes, is caused by the ubiquitous capacity of these genera for accumulating dietary carotenoids in muscle tissue. Here we combine life history reasoning with current knowledge of carotenoid metabolism to suggest a possible evolutionary scenario accounting for the emergence and maintenance of this phenotype. We claim that a basal carotenoid metabolism and a hormonally controlled targeted degradation of muscle tissue during maturation were established before the anadromous life style and that these proximal features were concomitantly exploited by natural selection in an ancestral anadromous and nest-making salmonid. Three different selection regimes are suggested to account for available empirical data concerning emergence, as well as maintenance, of the flesh pigmentation trait: a primary one driven by the need for enhanced somatic maintenance accompanying nest making under starving conditions, a secondary one driven by sexual selection, and a tertiary one driven by exposure to a meager carotenoid diet in nonanadromous freshwater populations. Finally, an experimental research program thought to be instrumental for fully resolving this evolutionary puzzle is suggested.