A case of complete albinism in the catfish Cambeva guareiensis (Siluriformes: Trichomycteridae)

Abstract: Albinism has been recorded in Neotropical freshwater fishes, mostly for nocturnal or cryptobiotic species. We report herein a case of albinism in the catfish Cambeva guareiensis (Trichomycteridae) from the Guareí River basin, Upper Paraná River basin, southeastern Brazil. The albino fish was caught with seven individuals with typical color pattern of the species. The features of the albino fish in life and shortly after preservation are described and illustrated.

During a field work (Azevedo-Santos et al. 2020) at the Guareí River basin, an important tributary of the Upper Paranapanema River, Brazil, a complete albino specimen of the catfish Cambeva guareiensis Katz & Costa 2020 (Trichomycteridae) was caught together with normal individuals. Herein we describe and illustrate the case.

Material and Methods
Cambeva guareiensis individuals were collected on 20 September 2017 in the Corrente stream (-23.434932°-48.388694°), in the Guareí River basin, Upper Paranapanema River, São Paulo State, southeastern Brazil (Azevedo-Santos et al. 2020). The Corrente stream presents clear waters along its course. In the sampled site the substrate is composed mostly of rocks; the riparian vegetation is relatively well preserved, especially if compared to other streams in the Guareí River basin. Specimens of this trichomycterid catfish were collected in a stretch above a waterfall, with rapid waters and rocky substrate. Collection methods (hand net), euthanasia, fixation and preservation of the specimens are fully described in Azevedo-Santos et al. (2020).
The eight specimens caught in the same stretch had their standard length (SL, 0.1 mm) measured under stereomicroscope and were deposited in the collection of the Departamento de Zoologia e Botânica, UNESP, São José do Rio Preto, SP, Brazil (DZSJRP 23090). Names for the varied forms of albinism followed mostly Henle et al. (2017) for amphibians, as we found no comparable definitions and descriptions for fishes.

Results
The complete albino individual of Cambeva guareiensis has 28.3 mm SL and the normally pigmented individuals measure between 30.1 and 56.0 mm SL. The body proportions of the albino are similar to those of the normal specimens. In addition, meristics are within the range expected for normal specimens. In life, the albino had pink eyes, a yellowish color dorsally and pink laterally, and the opercle and the region immediately after pectoral fins were red (Figure 1a-b). Due to its small size and the photographic perspective, the eyes are best viewed on the right side of the albino in dorsal view (Figure 1b). Shortly after fixed in formalin, the specimen showed a yellowish color dorsally and laterally. The ventral region was slightly yellow to white. The eyes and the operculum were whitish, the rays of all fins were light yellow (Figure 2a). Pigmented individuals had brown spots dorsally and laterally, with a midline composed of larger spots. The ventral region was yellowish to white ( Figure 2b).
The yellowish tinge and red eyes are typical traits of complete albinos (Sazima & Pombal-Jr. 1986;Oliveira & Foresti 1996;Silva et al. 2013). Therefore, the color of our specimen should not be confused with xanthism or flavism (sensu Henle et al. 2017), since xanthic individuals have pigmented eyes.
Cambeva guareiensis, a recently described species (Katz & Costa 2020), seems to be the second complete albino trichomycterid recorded to date. In a population of the cave-dwelling Trichomycterus itacarambiensis studied by Trajano (1997), there are depigmented individuals with red eyes (Carvalho & de Pinna 1986), which qualify them as complete or true albinos. However, it is difficult to qualify complete albinism in species that lack eyes, a typical trait in troglobiont organisms. This is the case of eyeless fish dwellling in caves and other types of hypogeal waters (Shibatta et al. 2007;Felice et al. 2008).

Addendum
We refrained here from labelling our report on Cambeva guareiensis as a first case of albinism within the genus, even if it seems to be the only record of an albino of this species, and the second case of complete albinism within Trichomycteridae (Carvalho & de Pinna 1986;Trajano 1997). To illustrate our point, an albino described by Sazima & Pombal Jr (1986)

Discussion
The lack of pigmentation and pink eyes qualify the atypical Cambeva guareiensis individual as a complete albino (sensu Henle et al. 2017).  We take the opportunity to clear the facts about I. mirini albinos (Sazima & Pombal Jr. 1986;Manoel et al. 2017), and recommend that the word "first" should be used with care whatever the case or occurrence reported.