Albinism is a genetic condition caused by an autosomal recessive gene that affects normal pigmentation in animals (^{Oliveira & Foresti, 1996}). The total albinism can occur in all vertebrates groups and it is characterized by whitish body and the presence of red eye (^{Sazima & Pombal Jr., 1986}), whereas partial albinism, also known as leucism, is characterized by presence of pigmentation in only some body parts (^{Lutz, 2001}).

In Neotropical freshwater fish fauna, albinism cases are extremely rare, with only 14 species recorded (^{Nobile, Freitas-Souza, Lima, Acosta, & Silva, 2016}) among more than 6000 described (^{Reis, Kullander, & Ferraris, 2003}). Of these species, 11 belong to the order Siluriformes, with representatives of Callichthyidae, Doradidae, Loriicaridae and Pimelodidae families (^{Nobile et al., 2016}).

Albinism in fish can be related to 3 different situations: heredity (^{Ueda, Ishinabe, & Jeon, 2007}), contamination by heavy metals (^{Oliveira & Foresti, 1996}) and artificial selection by aquarists (^{Sazima & Pombal Jr., 1986}). Apart from the pigmentation differences, albino fish also show behavioral differences as lower aggressiveness and reduced shoaling behavior preference (^{Slavík, Horký, & Wackermannová, 2016}).

Imparfinis mirini is a South American catfish that belongs to Heptapteridae family, which can reach up to 8.5 cm of SL and is widely distributed in the Upper Araguaia and Paraná River basins (^{Bockmann & Guazzelli, 2003}). It inhabits streams with substrates composed mostly of boulders, gravel or sand, which speculates using their barbells as sensory organ to feed on juvenile aquatic insects, especially Diptera, Trichoptera and Ephemeroptera (^{Casatti, Langeani, & Castro, 2001}).

In July 2016, we collected a totally albino specimen of I. mirini (SL 37 mm) with a seine net in a second order stream (23°2’18” S, 48°39’36” W) which belongs to the Rio Pardo basin, Itatinga, São Paulo (Fig. 1). In the same site, we collected more than 50 conspecifics animals with normal pigmentation. The environmental characteristics of the site are in Table 1. The albino specimen was deposited in the “Coleção do Laboratório de Biologia e Genética de Peixes”, UNESP, Botucatu (Voucher: LBP-22256).

Figure 1. Individuals of Imparfinis mirini with normal pigmentation (left) and total albinism (right) collected in Rio Pardo Basin, Itatinga, São Paulo, Brazil. 

Table 1 Values of stream’s environmental variables sampled in the local where the albino specimen of Imparfinis mirini was collected. 

The Heptapteridae family are mostly cryptic and nocturnal fish (^{Reis et al., 2003}), which allows the species without pigmentation be less susceptible to predation by visual oriented predators and recognition by their preys (^{Sazima & Pombal Jr., 1986}). Furthermore, in headwater streams there are lower amounts of piscivorous fish and birds (^{Uieda & Motta, 2007}), which may favor the growth of the albino fish.

In vertebrates, albinism is caused by an autosomal recessive gene (^{Smallcombe, 1949}), therefore, a possible explanation for the occurrence of total albinism in this individual could be the presence of deleterious genes caused by the high endogamy rate in small isolated populations (^{Peltonen, Palotie, & Lange, 2000}), such as in fish found in headwaters (^{Clarke, MacNally, Bond, & Lake, 2008}).

Another possible explanation may be related to a genetic disorder caused by heavy metal contamination, as shown in streams located in eucalyptus monocultures suffer contamination of their substrate by Cadmium and Chromium (^{Baggio & Horn, 2009}).