Cetopsis arcana Vari, Ferraris & de Pinna, 2005

Identification: Cetopsis arcana can be distinguished from all of its congeners by the combination of the presence of an eye, the conical teeth on the vomer and dentary, the rounded posterior nares which is distinctly separated from the contralateral nares by a distance greater than the width of posterior nares, the presence of a dark humeral spot except in some individuals which lack a pattern consisting of dark pigmentation on the pectoral and pelvic fin other than for a thin, clear margin, the presence of a posteriorly-rounded, variably-developed, bilobed patch of dark pigmentation at the base of the caudal fin, the presence of a spot of dark pigmentation on the base of the dorsal fin with prominent dark pigmentation along the membrane behind the first ray of the dorsal fin, the possession of typically 42 or 43 total vertebrae (41 vertebrae in only one individual), the presence of a distinct demarcation between the dark pigmentation on the dorsal and dorsolateral surfaces of the body versus more ventral regions, and the limitation of eye-size or larger dark spots on the lateral surface of the body to the region dorsal of the horizontal extending through the ventral margin of the caudal peduncle. Maximum size: 90 mm SL.

Range: Cetopsis arcana is only known from middle and upper portions of the Rio Tocantins basin.

Information from Vari, R. P., C. J. Ferraris Jr. & M. C. C. de Pinna. 2005. The Neotropical whale catfishes (Siluriformes: Cetopsidae: Cetopsinae), a revisionary study. Neotropical Ichthyology 3:127-238.