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Genital morphology and allometry differ by species and sex in Malawi cichlid fishes

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The African cichlid fishes show great diversity in mating displays and reproductive strategies, yet species differences in genital morphology have been little studied. Observational notes have described broad sex differences in external genital shape between males and females, but these differences have not been quantified. We examined three aspects of genital morphology (relative anogenital distance, relative vent length, and relative external genital area) in two riverine and eleven Lake Malawi African cichlid species from eight genera. We find the most sexually distinct morphology in the Lake Malawi rock cichlids and the least sexual dimorphism in the riverine outgroup; additionally, diversity in metrics within genus indicates that these traits are recently evolving. Sexual dimorphism in morphology is present in most species, and, in the Lake Malawi species, multivariate discriminant analysis allows for accurate assignment of gonadal sex based on genital morphology and body size. This will serve as a useful method for sexing fish in a nonlethal fashion and provides a starting point for further examination of the evolution of genital morphology in this diverse group of fishes.

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The authors would like to thank the North Carolina State University and the WM Keck Center for Behavioral Biology for funding; David M. Reif for statistical consultation and comments; and M. Kaitlyn Stanley and Natalie B. Roberts for assistance with photography.

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Correspondence to Emily C. Moore.

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Guest editors: S. Koblmüller, R. C. Albertson, M. J. Genner, K. M. Sefc & T. Takahashi / Advances in Cichlid Research II: Behavior, Ecology and Evolutionary Biology

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Online Resource. 1 Raw measurements (in pixels) of anus area and external genital area by standard length (mm) for four Lake Malawi species. A linear relationship is present between standard length and raw genital area (b) for both sexes, but not for standard length and raw anus area (a) (EPS 1088 kb)

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Moore, E.C., Roberts, R.B. Genital morphology and allometry differ by species and sex in Malawi cichlid fishes. Hydrobiologia 791, 127–143 (2017).

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