Sexual dimorphism of two catfish, Mystus pelusius (Solander, 1794) and Glyptothorax silviae Coad, 1981, is presented based on the examination of external morphology of the fish and morphology of the gonads. We observed sexual dimorphism for M. pelusius in the shape and position of the genital papilla. In the male, the genital papilla was a small, fleshy, pointed and broad-based structure. The pelvic fin was short and did not reach back to the genital papilla. In females of M. pelusius the opening of the genital structure was without any protrusion or with a very small, dot-like structure in a few specimens, which the pelvic fin reached and overlapped. In females of G. silviae, the base of the round genital structure was wider and had a small round protrusion. The color of the female was also darker in comparison with the male. In males the base of the round genital structure was narrower and had a longer pointed protrusion. The male body color was lighter than that of females.

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Among vertebrates, hybridization is common phenomena in cyprinid fishes particularly among closely related species/genera which share common habit and habitat (niche) requirements and desires. In this paper we report presence of two hybrids of Luciobarbus barbulus x Luciobarbus kersin and Luciobarbus barbulus x Luciobarbus xanthopterus from the Tigris River tributaries (the Persian Gulf basin), southwestern Iran. The hybrid specimens were identified on the basis of their intermediate morphological characteristics with the parent species and the position of them in the clade and close genetic distance with their maternal sequences. Our studies showed intermediate characteristics between putative hybrids and their parents. In the putative hybrid of L. barbulus and L. kersin, the general characteristics and morphometric measurements are similar to one parent especially paternal characteristics (L. kersin), although most of the meristic characteristics are similar to maternal characteristics (L. barbulus). The other putative hybrid between L. barbulus and L. xanthopterus show the general appearance for L. barbulus (the putative father) and some meristic characteristics were similar to L. xanthopterus (the putative mother). Low abundance of the hybrids shows that the hybridization might be a random event.

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