Male-specific asymmetric curvature of anal fin in a viviparous teleost, Xenotoca eiseni.

Xenotoca eiseni is a viviparous teleost from the family Goodeidae. Internal fertilization occurs in this species; therefore, the male fish must transfer their sperm to the females. To this end, the males use their anal fins as external genitalia during mating. However, in goodeid species, there are a few reports of anal fin modifications functioning as genitalia. In the present study, I describe a male-specific lateral curvature with fin robe thickening on the two most posterior rays of the anal fin of X. eiseni. Morphological and behavioral analyses suggested that the lateral curvature provides a directional preference for coupling of male X. eiseni. The modification appears to be a male secondary sex characteristic. Other goodeid species Xenotoca melanosoma and Chapalichthys pardalis also possess the lateral curvature, but without robe thickening. Furthermore, Zoogoneticus quitzeoensis exhibited only curvature structure either smaller than those of other species or absent. Therefore, the anal fin curvature may not necessarily be used for internal fertilization in all species investigated in this study. However, it is still possible that it provides novel advantages during mating in certain goodeid species.

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