Osteology of Priocharax and remarkable developmental truncation in a miniature Amazonian fish (Teleostei: Characiformes: Characidae)

Two new species of miniature tetras of the fish genus Priocharax from the Rio Juruá drainage, Acre, Brazil (Teleostei: Characiformes: Characidae)

Two new miniature tetra species of the Neotropical characid genus Priocharax Weitzman and Vari, 1987 are described, raising the known species diversity to seven. Both species occur in the Rio Juruá system, Cruzeiro do Sul municipality, Acre State, Brazil. Priocharax toledopizae sp. nov. occurs in streams flowing to the lower Rio Moa, a tributary of Rio Juruá, and is distinguished from congeners by a combination of presence of claustrum and infraorbitals 1 and 2, absence of infraorbital 3, and presence of five branched pelvic-fin rays. Priocharax marupiara sp. nov. is known from Igarapé Canela Fina, tributary of Rio Juruá, and is diagnosed by a combination of fewer maxillary teeth (21–27 vs. 27–58 in remaining species), fewer branched anal-fin rays (18–23 vs. 22–27 in two species) and colour pattern. Both species differ from each other in the general body shape: Priocharax toledopizae is more robust with deep body and Priocharax marupiara more elongate. DNA barcode data support the specific distinctness of the two new species and that of the other five species in the genus. We describe a remarkable sexual dimorphism of the pelvic girdle of Priocharax toledopizae in which the pelvic musculature is enlarged forming a pedicel for the fin in mature males. Most localities where these species were found suffer from significant degradation mainly due to litter accumulation and suppression of the riparian forest, raising concerns about their conservation status.

Estimated richness and environmental correlates of miniature fish assemblages in the rio Jacundá, Brazil

Abstract South America is home to more miniature fishes (<26 mm in standard length) than any other continent. Despite this diversity, the ecology of miniature fishes is poorly studied. To promote the study of miniature fish ecology, we investigated patterns in total richness, assemblage structure and environmental correlates for miniature fishes in the rio Jacundá drainage of the Lower Amazon River basin, Pará State. Based on multi-pass dip-netting of leaf litter at 20 locations distributed across two sites, we collected miniature species and used rarefaction to estimate 9 to 14 species might be present. The miniature fish assemblage at the upstream site was a nested subset of the downstream site, and water pH and canopy cover, two features known to be altered by deforestation, correlated most strongly with assemblage variation. Our work represents one of the first quantitative assessments of environmental correlates with miniature fish assemblages and highlights research topics that should be investigated further to promote conservation and preservation of the overlooked and understudied Amazonian diminutive freshwater fish fauna.

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 miniature Pristella (Actinopterygii: Characiformes: Characidae) with reversed sexual dimorphism from the rio Tocantins and rio São Francisco basins, Brazil

A new species of the genus Pristella Eigenmann, 1908 (Pristella crinogi sp. nov.) is described from the middle rio Tocantins and middle rio São Francisco basins, Brazil. The new species can be diagnosed from its two congeners, Pristella ariporo Conde-Saldaña, Albornoz-Garzón, García-Melo, Villa-Navarro, Mirande, and Lima, 2019 and Pristella maxillaris (Ulrey, 1894), by a combination of color pattern and teeth morphology characters. A phylogenetic analysis of the genus recovered P. crinogi as the sister taxa of P. ariporo. Pristella crinogi, along with P. ariporo, are the first characiform fishes, and one of the first bony fishes, to be reported as presenting a reversed sexual dimorphism, with females presenting a more developed color pattern than males. Comments on the miniaturization of the species, as well as remarks on the biogeography of the genus Pristella, are presented.

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 miniature species of Priocharax (Characiformes: Characidae) from the upper rio Ipixuna, Purus drainage, Brazil

Abstract A new species of miniature fish of the characid genus Priocharax is described from a small lake near the rio Ipixuna, rio Purus drainage, Amazonas State, Brazil. It is distinguished from all congeners except P. pygmaeus by the lower number of teeth on the maxilla and dentary. It differs from P. pygmaeus by the presence of two postcleithra and 22–27 branched anal-fin rays (vs absence and 19–22). The new species is further distinguished from other species of Priocharax by a combination of characters involving the number of pelvic-fin rays and branched anal-fin rays, the number of postcleithra, the shape of postcleithrum 3, and the absence of the claustrum. Molecular evidence based on COI sequences of all valid species of Priocharax also corroborates the validity of this new species.

A new miniature species of Odontocharacidium (Characiformes: Crenuchidae) from the Río Orinoco basin, Venezuela

ABSTRACT A new species of Odontocharacidium is described from the upper Río Orinoco basin, in Venezuela. The new species is distinguished from its only congener, Odontocharacidium aphanes, by the presence of: the antorbital, the parietal branch of the supraorbital laterosensory canal, the postcleithrum 1, the conspicuous bars extending ventrally below the middle portion of the body posteriorly, and two dark round blotches at the tip of the caudal peduncle. With the recognition of an additional species of Odontocharacidium the diagnostic characters of the genus and the variability in the number of maxillary teeth in specimens are discussed.

The cephalic lateral-line system of Characiformes (Teleostei: Ostariophysi): anatomy and phylogenetic implications

The lateral-line system has been traditionally recognized as an important source of phylogenetic information for different groups of fishes. Although extensively studied in Siluriformes and Cypriniformes, the lateral-line system of Characiformes remained underexplored. In the present study, the anatomy of the cephalic lateral-line canals of characiforms is described in detail and a unifying terminology that considers the ontogeny and homologies of the components of this system is offered. Aspects of the arrangement of lateral-line canals, as well as the number, location and size of canal tubules and pores, resulted in the identification of novel putative synapomorphies for Characiformes and several of its subgroups. The study also revised synapomorphies previously proposed for different characiform families and provided comments on their observed distribution across the order based on extensive taxon sampling. Information from the ontogenetic studies of the cephalic lateral-line canal system and a proposal for the proper use of these data to detect truncations in the development of the lateral-line canals across the order is also offered.

A new sexually dichromatic miniature Hyphessobrycon (Teleostei: Characiformes: Characidae) from the Rio Formiga, upper Rio Juruena basin, Mato Grosso, Brazil, with a review of sexual dichromatism in Characiformes

Hyphessobrycon myrmex sp. nov., is described from the Rio Formiga, upper Rio Juruena, upper Rio Tapajós basin, Mato Grosso, Brazil. The new species can be distinguished from congeners by having the lower half of the body deeply pigmented with dark chromatophores, chromatophores concentrated above the anal fin and forming a broad, diffuse, dark midlateral stripe and by having a dense concentration of dark chromatophores along unbranched dorsal-fin rays and distal portions of the two or three subsequent branched rays. In life, H. myrmex exhibits a conspicuous sexual dichromatism, with adult males red to orange and females and immatures pale yellow. A list containing 108 sexually dichromatic taxa in six families of Characiformes is provided and the distribution of this poorly known type of dimorphism across the Characiformes is discussed.

Comparative development in Moenkhausia pittieri and Paracheirodon innesi (Ostariophysi: Characiformes) with comments on heterochrony and miniaturization in the Characidae

Developmental heterochronies between the small-sized Moenkhausia pittieri and the miniature Paracheirodon innesi were determined by describing and comparing external morphological development in both species. Despite being classified as miniature, P. innesi was larger and more developed in the flexion and post-flexion larval stages than M. pittieri. Broader patterns within the Characidae suggest that body size reduction in P. innesi is tied to accelerated development and higher growth rate along with early maturation and precocious cessation of growth, which are characteristic of miniature species. Comments on adipose-fin loss in Characidae as a consequence of developmental truncation are included.

New species of the miniature genus Gelanoglanis (Siluriformes: Auchenipteridae) from the Tocantins River basin (Brazil) and osteological description of G. nanonocticolus

A new species of Gelanoglanis is described based on specimens collected in the Tocantins River, a tributary of the Amazon Basin. The new species is distinguished from congeners by unusual features to the genus such as the pelvic fin falling short of the urogenital opening by a distance larger than the anal-fin length, the first unbranched dorsal and pectoral-fin rays not ossified as spines, the dorsal-fin spinelet absent and the adipose fin in adults preceded by a long fleshy ridge. The new species is further distinguished from congeners by its soft, sinuous snout tip and morphometric proportions of the body and head. The new species shares most putative synapomorphies of the genus, as well as the miniaturized condition evidenced by a diminutive body size and reduced ossifications and lateral sensory system. An elongate dorsal process of the cleithrum is posited as an additional synapomorphy for the genus. An osteological description of Gelanoglanis nanonocticolus based on high-resolution X-ray computed tomography is also provided. http://zoobank.org/urn:lsid:zoobank.org:pub:BA3FE72D-FEF84-7A7-B751-2A15EA52C203.

Morphological novelty and modest developmental truncation in Barboides, Africa's smallest vertebrates (Teleostei: Cyprinidae)

Miniaturization, the evolution of extremely small adult body size, is widespread amongst animals and commonly associated with novel ecological, physiological, and morphological attributes. The phenotypes of miniaturized taxa are noteworthy because they combine reductions and structural simplifications with novel traits not developed in their larger relatives. Previous research on miniature cyprinid fishes (focused predominantly on South and South East Asian taxa of a single subfamily) has identified two distinct classes of miniature taxa: proportioned dwarves and developmentally truncated miniatures. Miniaturization has also occurred independently in the subfamily Cyprininae, particularly in African lineages. We investigate the skeletal anatomy of Barboides, a genus of miniature African cyprinids that includes Africa's smallest known species of vertebrates, to assess whether miniaturization has resulted in similar organismal outcomes in different lineages of the Cyprinidae. The skeleton of Barboides is characterized by the complete absence of a number of dermal and endochondral ossifications, and marked reduction in size and/or complexity of other skeletal elements, particularly those of the dermatocranium. Absent skeletal elements in Barboides include those which develop relatively late in the ossification sequence of the non-miniature African relative 'Barbus' holotaenia suggesting that their absence in Barboides can be explained by a simple scenario of developmental truncation. In contrast to this theme of loss and reduction, the os suspensorium of Barboides is enlarged and the outer arm distally trifid and associated with a novel bulbous muscle in males. An evaluation of the skeleton of Barboides provides further evidence for a link between developmental truncation and evolutionary morphological novelty in Cyprinidae. In the spectrum of miniature cyprinids ranging from proportioned dwarves with few bones missing to highly progenetic taxa with dozens of missing bones, the two species of Barboides range roughly in the middle showing that the extremes are connected by intermediate levels of truncatedness.

A new miniature of Xenurobryconini (Characiformes: Characidae) from the rio Tapajós basin, Brazil

Abstract A new species of Xenurobrycon isdescribed from tributaries of the rio Tapajós, Pará, Brazil. It is diagnosed from all congeners by the dark color of the posterior half of both caudal-fin lobes, the presence of infraorbitals one, two, and three, the presence of a set of lamellar processes on eighth principal caudal-fin ray of mature males, the presence of bony hooks similar in size or decreasing posteriorly on last unbranched and first 9-12 branched anal-fin rays of mature males, the presence of only conical teeth on both jaws, the lack of the adipose fin and the presence of 15-18 predorsal scales. The description of the new species increases the number of species in Xenurobrycon to six. An updated key to the species of the genus is provided.