Originally described from the upper Rio das Velhas, a tributary of the Rio São Francisco, state of Minas Gerais, Brazil, Hysteronotus megalostomus was recently collected in many tributaries of the Rio São Francisco north of the type locality. The specimens of the population samples collected outside the type locality share the morphological features present in the type material except for the presence of an adipose fin found only in two specimens within the more recently collected material. Presence or absence of the adipose fin has been extensively used by fish taxonomists to characterize different species and even genera, but in H. megalostomus the character is not consistent, indicating its use alone is not diagnostic. The species is redescribed and its phylogenetic relationships and conservation status are briefly discussed.
Key words: conservation; fresh water; neotropical; sexual dimorphism; systematics.
INTRODUCTION
Hysteronotus megalostomus Eigenmann 1911 was known until recently only from its type locality in the Rio das Velhas, a tributary of the middle Rio São Francisco basin, in the Brazilian state of Minas Gerais (Eigenmann, 1908, 1911). During expeditions carried out between 1987 and 1994, specimens of a probable congeneric new species (Weitzman &Menezes, 1998) were found in tributaries near the type locality along the upper and middle courses of the Rio São Francisco, in the State of Minas Gerais and
Bahia. The supposed new species would be characterized mainly by the absence of an adipose fin present in H. megalostomus. Extensive analysis of all available specimens from all localities, however, revealed that presence or absence of the adipose fin is rather an intraspecific variation since no other morphological differences were found. The purpose of this contribution is to redescribe H. megalostomus based on all available specimens, report its extended geographic range, and to comment on its relationships and conservation status.
MATERIALS AND METHODS
All the procedures about taking measurements and counts, presentingmeristic andmorphometric data and identification of mature specimens to sex are those described by Fink &Weitzman (1974) and Menezes & Weitzman (2009) except for vertebral and supraneural counts that were taken from clear and stained specimens (C&S). Description is based on all population samples combined since no significant morphological differences were found among the material collected from different localities.
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