Abstract
Coloration in vertebrates is determined by the presence or absence of pigments, whose alteration may result in atypical conditions such as albinism, dilution, schizochromatism, and leucism. These anomalies usually originate from genetic mutations, influenced by environmental factors, exposure to pollutants, or artificial selection. Leucism is expressed in partially or completely white individuals, but with normally pigmented eyes and claws, which distinguishes it from albinism. In teleost fishes, these alterations have been documented in multiple marine and freshwater species, with behavioral and ecological implications. In particular, some Siluriformes exhibit gregarious advantages, such as efficient resource use and protection against predators. Genidens barbus, an anadromous and migratory marine catfish distributed from Brazil to Argentine and Chilean Patagonia, represents a vulnerable resource due to its complex reproductive cycle. In Argentina, two stocks are recognized: northern and Patagonian. For the former, a previous record of albinism has been reported. This study reports the first case of leucism in the Patagonian stock of G. barbus, corresponding to a juvenile measuring 305 mm in total length, captured in November 2022 at El Cóndor, Río Negro. The specimen exhibited a complete lack of body pigmentation, but normal eyes. Among 372 individuals sampled since 2017, this represents the only case recorded in Patagonia and the second in Argentina, confirming the rarity of this condition.
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