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* Correspondence: hresmaeili22@gmail.com Sexual dimorphism is a topic of lasting interest to biologists (e.g., Darwin, 1871; Lewin 1988), is common in nature, and has the potential to increase intraspecific variation in performance and patterns of resource use (McGee and Wainwright, 2013). Many vertebrate taxa exhibit sexual dimorphism, and explanations for its cause abound and are sometimes contradictory: e.g., “biological complexity is such that one can find evidence and devise arguments for proposing that large size leads to decreased survivorship ... or to increased survivorship...’ (Murray 1984). Sexual dimorphism, or phenotypic divergence between the sexes, is a common and often substantial form of intraspecific phenotypic variation (McGee and Wainwright, 2013). Typically, in any fish, sex can be determined by visual inspection of the gonads (primary sexual characteristics), which normally requires dissection, and in mature fishes, these characteristics are quite evident. The evolution of secondary sexual characteristics is usually the result of a disparity in the parental investment of males and females (Trivers, 1972; Andersson, 1994). Sexual dimorphism is a phenotypic differentiation between males and females of the same species (Dimijian, 2005). This differentiation happens in organisms who reproduce through sexual reproduction, with the prototypical example being for differences in characteristics of reproductive organs. Other possible examples are for secondary sex characteristics, body size, physical strength and morphology, ornamentation, behavior, and other bodily traits (Dimijian, 2005). Traits such as ornamentation and breeding behavior found in only one sex imply that sexual selection over an extended period of time leads to sexual dimorphism (Dimijian, 2005). Many teleost fishes do not exhibit any sexual dimorphism, even during the spawning season, and do not show sexual characteristics or permanent ornaments. On the other hand, some fishes show permanently dimorphic traits that are not necessarily associated with internal fertilization (Rapp Py-Daniel and Fernandez, 2005). When present, sexual characteristics can be easily recognized in some species, e.g., Aphanius (Esmaeili et al., 2014; Teimori et al., 2014), whereas, in some others (e.g., some catfishes), a detailed examination is required for identification of these characteristics (Godinho, 2007). The catfishes (order Siluriformes) comprise 39 families and well over 3709 species found worldwide in fresh waters Abstract: 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.

Sexual dimorphism in two catfish species, Mystus pelusius (Solander, 1794) and Glyptothorax silviae Coad, 1981 (Teleostei: Siluriformes)