New remarkable sexually dimorphic miniature species of Hyphessobrycon (Characiformes: Characidae) from the upper Rio Tapajós basin

New small-sized species of Astyanax (Characiformes: Characidae) from the upper rio Paraguai basin, Brazil, with discussion on its generic allocation

Abstract A new species of Astyanax is described from the rio Salobra, tributary of rio Cuiabá, rio Paraguai basin. The new taxon can be distinguished from its congeners by having a well-defined dark midlateral stripe on body extending from the posterior margin of the opercle to the base of middle caudal-fin rays and a single vertical elongate humeral blotch. Although the new species is described in Astyanax, some specimens present an incomplete or a discontinuous series of perforated scales in the lateral line. Therefore, a discussion on its generic allocation is presented. Comments on different patterns of coloration among dark-striped species of Astyanax are also provided. The discovery of a new species in an underwater tourist point relatively near a large urban center underscores that even fish species daily observed by hundreds of people in limpid waters may lack a formal taxonomic identity. Such finding also highlights how the megadiverse Brazilian freshwater ichthyofauna still needs efforts and investments to identify and describe new taxa.

Osteology of Tucanoichthys tucano Géry and Römer, an enigmatic miniature fish from the Amazon basin, Brazil (Teleostei: Characiformes: Characidae)

Miniaturization, the evolution of extremely small adult body size, is a common phenomenon across the lineages of freshwater fishes, especially in the Neotropics where over 200 species are considered miniature (≤26 mm in standard length [SL]). Close to 30% of all miniature Neotropical freshwater fishes belong to the family Characidae, several of which are of uncertain phylogenetic placement within the family. We investigate the skeletal anatomy of Tucanoichthys tucano, a species of uncertain phylogenetic position from the upper Rio Negro basin, reaching a maximum known size of 16.6 mm SL. The skeleton of Tucanoichthys is characterized by the complete absence of ten skeletal elements and marked reduction in size and/or complexity of others, especially those elements associated with the cephalic latero-sensory canal system. Missing elements in the skeleton of Tucanoichthys include those that develop relatively late in the ossification sequence of the non-miniature characiform Salminus brasiliensis, suggesting that their absence in Tucanoichthys can be explained by a simple scenario of developmental truncation. A number of the reductions in the skeleton of Tucanoichthys are shared with other miniature characiforms, most notably species of Priocharax and Tyttobrycon, the latter a putative close relative of Tucanoichthys based on molecular data.

A new species of Moenkhausia (Characiformes: Characidae) from the rio Madeira basin, Brazil, with comments on the evolution and development of the trunk lateral line system in characids

Abstract A new species of Moenkhausia is described from the rio Machado drainage, Amazon basin, Brazil. It is diagnosed from congeners by its color pattern, consisting of the concentration of chromatophores on the anterior portion of body scales, the horizontally elongate blotch on caudal peduncle, a bright golden coloration of the dorsal portion of eye when alive, and a dark line crossing the eye horizontally. The new species has variable morphology regarding trunk lateral-line canals. Most fully grown individuals do not have enclosed bony tube in many lateral line scales, resembling early developmental stages of tube formation of other species. This paedomorphic condition is interpreted as a result of developmental truncation. Such evolutionary process may have been responsible for the presence of distinct levels of trunk lateral line reductions in small characids. Variation in this feature is common, even between the sides of the same individual. We reassert that the degree of trunk lateral-line tube development must be used with care in taxonomic and phylogenetic studies, because reductions in the laterosensory system may constitute parallel loss in the Characidae. We suggest the new species to be categorized Near Threatened due to the restricted geographical distribution and continuing decline in habitat quality.

A new species of Hyphessobrycon Durbin (Characiformes: Characidae) from the western Amazon basin in Colombia and Peru

Hyphessobrycon chiribiquete n. sp. is described from the Río Caquetá drainage in Colombia and the Río Ucayali drainage in Peru, western Amazon. The new species is diagnosed from its congeners by having the following combination of characters: a conspicuous narrow midlateral stripe, starting on the sides of the body behind the opercle near the lateral line; lateral stripe overlapped anteriorly with a vertically elongated humeral blotch; inner premaxillary teeth pentacuspid; margin of anal fin falcate in mature males.

On a remarkable sexual dimorphic trait in the Characiformes related to the olfactory organ and description of a new miniature species of Tyttobrycon Géry (Characiformes: Characidae)

Among the order Characiformes, secondary sexual dimorphism is commonly associated to the occurrence of bony hooks on fins, shape and length of the dorsal and anal fins, and sexual dichromatism. The analysis of a new miniature Characidae species of the genus Tyttobrycon, described herein, yielded to the discovery of a sexually dimorphic trait related to nostril aperture and number of olfactory lamellae. In this type of dimorphism, mature males present larger nostril aperture and higher number of olfactory lamella than females. A dimorphic olfactory organ is for the first time recorded and described for a member of the Characiformes. Gross morphology and development of brain and peripheral olfactory organ of Tyttobrycon sp. n. are described and compared to other species of Characidae. It is hypothesized that such dimorphic trait is related to male-male detection during cohort competition in small characids. The new species of Tyttobrycon is diagnosed from its congeners by the number of branched anal-fin rays (19-21) and the absence of a caudal-peduncle blotch. It occurs in a small tributary of Rio Madeira basin, near to the limit between Brazil and Bolivia, Acre State, Brazil.