Two new miniature translucent catfish species of the rare genus Tridens (Siluriformes: Trichomycteridae) from the Madeira River basin, northern Brazil.

Two new miniature species of the trichomycterid genus Tridens are described from the Madeira River drainage, Acre and Rondônia States, Brazil. Until this work, Tridens was a monotypic genus composed solely of Tridens melanops, from the Putumayo/Içá River drainage, upper Amazonas River basin. Tridens vitreus sp.n. is known from upper and middle Madeira River drainage and differs from all other congeners by the lack of pelvic fins and girdle and by vertebra and dorsal-fin ray counts. Tridens chicomendesi sp.n. is known from Abunã River, middle Madeira River drainage and is distinguished from all other congeners by the number of vertebrae, dorsal-fin ray count and anal-fin base colouration pattern. Tr. chicomendesi sp.n. is further distinguished from T. vitreus by a combination of character states regarding the position of urogenital opening, dorsal-fin position, anal-fin position, maxillary barbel length, number of premaxillary teeth, number of dorsal-fin rays, number of anal-fin rays, number of lateral-line system pores, frontal bone anatomy, degree of ossification of maxilla, anatomy of quadrate-hyomandibular joint, size of posterodorsal process of hyomandibula, length of opercular patch of odontodes, number of interopercular odontodes, proportion of upper hypural plate cartilage relative to its area ; by the absence of a proximal, distal and ventral cartilages on ventral hypohyal; by the absence of a lateral process on basibranchial 4; by the presence of a cartilage block on the lateral process of autopalatine, the presence of a well-developed ossification on proximal margin of ventral hypohyal, the presence of hypobranchial foramen; and by the presence of an anterior cartilaginous joint between quadrate and base of posterodorsal process of hyomandibula. This work represents the first species description for the subfamily Tridentinae in more than 30 years and for the genus Tridens since its original description in 1889.

A new species of Characidium (Characiformes: Crenuchidae) from the rio Madeira basin, Brazil.

This study describes Characidium nambiquara, a new species from the upper rio Guaporé, rio Madeira basin, Brazil. The new species differs from most congeners by the presence of isthmus and area between the contralateral pectoral-fin bases completely naked. From congeners with some degree of scaleless ventral surface of the body C. nambiquara differs by having 10 circumpeduncular scales. The new species is also distinguished from congeners by the spotted colour pattern on body of the large-sized specimens and by having black dashes on all fins and conspicuous midlateral longitudinal dark stripe or conspicuous vertical bars absent. Characidium nambiquara further differs from most congeners by the presence of 34-36 pored scales on the lateral line, 3 horizontal scale rows above the lateral line and 3 horizontal scale rows from the lateral line to the midventral scale series. Remarks on intraspecific colour variation within the genus, not related to sexual dimorphism, are also provided.

A new species of Odontostilbe cope (Characiformes: Cheirodontinae) from rio Madeira basin diagnosed based on morphological and molecular data.

A new species of Odontostilbe is described from the rio Jaciparaná, rio Madeira basin, Rondônia, Brazil. Odontostilbe pacaasnovos differs from all its congeners, except O. pequira, by the colour pattern. Additionally, it differs from its congeners by the terminal mouth, number of cusps in the teeth of the premaxilla (5-7), number of branched rays in the anal fin (19-22), by the shape of dentary teeth (5-7 cusps with central cusp larger and longer than laterals cusps) and by the number of lamellae of the olfactory rosette (17-18 in male and 14 in female). Morphological and molecular comparisons corroborate the distinctiveness between O. pacaasnovos and its congeners, justifying its recognition as a new species.

A new rheophilic South American darter (Crenuchidae: Characidium) from the rio Juruena basin, Brazil, with comments on morphological adaptations to life in fast-flowing waters.

Characidium iaquira, a new species from the upper rio Juruena, rio Tapajós basin, Brazil, is described. The new species can be promptly distinguished from all congeners by having a unique v-shaped dark mark lying along the caudal-fin extension, in medium- and large-sized specimens, and a remarkable iridescent green colouration in life. Characidium iaquira is closely related to Characidium crandellii and Characidium declivirostre by sharing unambiguous synapomorphies such as branchiostegal membranes united to each other across the isthmus, a scaleless area extending from the isthmus to the pectoral girdle, and dermal flaps surrounding anterior and posterior naris independent, but touching each other distally. Morphological specializations of the paired fins in the three riffle-dwellers species are discussed, including the wing-like shape, robustness, and inclination of the pectoral fin.

Hyphessobrycon lucenorum (Characiformes: Characidae), a new species from the rio Madeira basin, Rondônia State, Brazil.

A new species of characid is described from the upper rio Machado, a tributary of the rio Madeira basin, Rondônia, Brazil. Hyphessobrycon lucenorum can be distinguished from all congeners by the unique combination of the presence of a conspicuous rounded humeral blotch and a broad and diffuse longitudinal stripe. The new species is included within the Hyphessobrycon agulha group. The high number of endemic species of fishes occurring in the headwater tributaries of three river systems (rio Madeira, rio Tapajós, and rio Paraguai) draining the watershed defined by the Chapada dos Parecis is discussed.

Redescription of Astyanax guaporensis Eigenmann, 1911 (Characiformes: Characidae), a small characid from the rio Madeira basin.

During recent expeditions to several localities in the rio Madeira basin, a poorly known species of Characidae described more than a century ago, Astyanax guaporensis Eigenmann, was collected. The species is herein redescribed based on the type material and many recently collected specimens. The species seems to occur only in the rio Madeira basin, rio Amazonas drainage. The species is mainly recognized among its congeners by having five to nine maxillary teeth, a series of V-shaped marks on the midlateral line of body, and the absence of a black mark on the caudal peduncle.

A new rheophilic species of Knodus Eigenmann (Characiformes: Characidae: Stevardiinae) from the upper rio Juruena, rio Tapajós basin, Chapada dos Parecis, Mato Grosso, Brazil.

Knodus ytuanama, new species, is described from the upper rio Juruena, rio Tapajós drainage, Amazon basin, Mato Grosso, Brazil. The new species differs from its congeners by presenting the interradial membranes of the caudal fin thickened, forming folds, and also differs from most congeners by the presence of a dark, wide midlateral stripe extending from the posterior margin of opercle to the middle caudal-fin rays, the absence of a humeral blotch in adults, and by having four rows of scales between the lateral line and the pelvic-fin origin, among another features. We also provide a discussion on the presence of membranous flaps on the fins as an adaptation for living in fast-water environments in Knodus ytuanama n. sp. as well as in a congener, K. tiquiensis.

Geophagus pyrineusi: a new species from the rio Teles Pires, rio Tapajs basin, Brazil (Cichliformes: Cichlidae: Geophagini).

Geophagus pyrineusi, new species, is described from the rio Teles Pires drainage, rio Tapajs basin, Brazil. The new species can be diagnosed from the majority of the congeners by presenting a complete infraorbital bar. Additionally, it differs from all other congeners by several coloration-related character states: flank bars 5 and 67 as dark as the infraorbital bar, and almost as dark as the midlateral spot; retention of dorsal melanophoric patch 6 as a distinct mark (not connected to any lateral melanophoric patch), and almost as dark as the midlateral spot in adults; retention of lateral melanophoric patch 1p and almost complete absence of dorsal- and caudal-fin color patterns in adults, among other characters.

A database of freshwater fish species of the Amazon Basin.

The Amazon Basin is an unquestionable biodiversity hotspot, containing the highest freshwater biodiversity on earth and facing off a recent increase in anthropogenic threats. The current knowledge on the spatial distribution of the freshwater fish species is greatly deficient in this basin, preventing a comprehensive understanding of this hyper-diverse ecosystem as a whole. Filling this gap was the priority of a transnational collaborative project, i.e. the AmazonFish project - https://www.amazon-fish.com/. Relying on the outputs of this project, we provide the most complete fish species distribution records covering the whole Amazon drainage. The database, including 2,406 validated freshwater native fish species, 232,936 georeferenced records, results from an extensive survey of species distribution including 590 different sources (e.g. published articles, grey literature, online biodiversity databases and scientific collections from museums and universities worldwide) and field expeditions conducted during the project. This database, delivered at both georeferenced localities (21,500 localities) and sub-drainages grains (144 units), represents a highly valuable source of information for further studies on freshwater fish biodiversity, biogeography and conservation.

A new species of Eigenmannia Jordan & Evermann (Gymnotiformes: Sternopygidae) from rio Tapajós, Brazil, with discussion on its species group and the myology within Eigenmanniinae.

A new species of Eigenmannia is described from the rio Mutum, tributary of upper rio Juruena, rio Tapajós basin, Comodoro, Mato Grosso, Brazil. The new species is distinguished from all congeners by coloration pattern, position of the mouth, number of scales rows above lateral line, number of premaxillary and dentary teeth, number of precaudal vertebrae, orbital diameter, mouth width, relative depth of posterodorsal expansion on infraorbitals 1+2 and relative size of coronomeckelian bone. Comments on potentially useful characters in phylogenetic studies derived from musculature, discussion on Eigenmannia species-group and the first dichotomous key for Eigenmannia are provided.

A new species of Microschemobrycon (Characiformes: Characidae) from Rio Xingu basin, Brazil.

A new species of Microschemobrycon with a restricted distribution was recently discovered in the Rio Curuá, Rio Xingu basin, Pará, Brazil. Microschemobrycon cryptogrammus new species can be promptly distinguished from all congeners by the presence of a rounded, dark, conspicuous caudal-peduncle blotch and by a subjacent dark midlateral stripe visible in life. Additionally, the new species can be distinguished from all its congeners, except M. elongatus, by the presence of a longitudinal dark stripe along the lower jaw. The new species can be distinguished from M. elongatus by presenting lateral line with 36-38 pored scales, anterior and posterior nostrils coalescend, dark chromatophores concentrated along the ventral margin of the caudal peduncle and by the presence in life of a dark spot at the base of the dorsal-fin origin.

Hyphessobrycon rheophilus, a new species from rapids of the Amazon and Orinoco river basins (Characiformes: Characidae: Stethaprioninae).

A new species from rapids of Rio Aripuanã, Rio Madeira basin, in Brazil, and from the same type of habitat in the upper Rio Negro and upper Rio Orinoco basins in Brazil, Colombia, and Venezuela is described and assigned to the genus Hyphessobrycon. The new species presents an interrupted lateral line plus a single perforated scale on caudal peduncle and a small dark blotch on dorsal procurrent caudal-fin rays, features not found in the other species of Hyphessobrycon. Comments on the phylogenetic position of the new species, its rheophilic habits, and the biogeographic implications of its distribution are presented.

Unexpected species diversity in electric eels with a description of the strongest living bioelectricity generator.

Is there only one electric eel species? For two and a half centuries since its description by Linnaeus, Electrophorus electricus has captivated humankind by its capacity to generate strong electric discharges. Despite the importance of Electrophorus in multiple fields of science, the possibility of additional species-level diversity in the genus, which could also reveal a hidden variety of substances and bioelectrogenic functions, has hitherto not been explored. Here, based on overwhelming patterns of genetic, morphological, and ecological data, we reject the hypothesis of a single species broadly distributed throughout Greater Amazonia. Our analyses readily identify three major lineages that diverged during the Miocene and Pliocene-two of which warrant recognition as new species. For one of the new species, we recorded a discharge of 860 V, well above 650 V previously cited for Electrophorus, making it the strongest living bioelectricity generator.

Phycocharax rasbora, a new genus and species of Brazilian tetra (Characiformes: Characidae) from Serra do Cachimbo, rio Tapajós basin.

A new genus and species of characid fish is described from rio Braço Norte, a tributary of rio Teles Pires, Tapajós basin, Mato Groso, Brazil. The new taxa can be diagnosed from the remaining characids by a unique combination of characters that includes the presence of a single row of relatively compressed premaxillary teeth, large teeth with four to nine cusps on premaxillary and dentary, absence of pseudotympanum, incomplete lateral line with 7-13 pored scales, sexually-dimorphic males with distal margin of anal fin approximately straight, and presence of a nearly triangular and horizontally elongated blotch from the posterior half of the body to caudal peduncle. The most parsimonious phylogenetic hypothesis, using morphological data, recovered the new genus and species in a clade including Paracheirodon axelrodi and Hyphessobrycon elachys.

Hyphessobrycon petricolus, a new species of tetra (Characiformes: Characidae) from the rio Madeira basin, Mato Grosso, Brazil.

A new species of Hyphessobrycon is described from the rio Roosevelt, rio Madeira basin, Mato Grosso State, Brazil. Hyphessobrycon petricolus sp. n. can be distinguished from its congeners by the unique combination of the following features: a well-defined, relatively narrow dark midlateral stripe on body extending from immediately behind posterior margin of opercle to the middle caudal-fin rays, relatively conspicuous humeral blotch, and 16-20 branched anal-fin rays. Comments on the remaining Hyphessobrycon species presenting a conspicuous dark midlateral stripe are presented.

A new species of Brachychalcinus (Characiformes: Characidae) from the rio Xingu basin, Serra do Cachimbo, Brazil.

Brachychalcinus reisi, a new species of characid fish, is described from the rio Curuá, tributary of rio Iriri, rio Xingu basin, Serra do Cachimbo, Pará State, Brazil. The new species can be distinguished from its congeners by the presence of a series of longitudinal black wavy stripes on the entire body and by a lower number of longitudinal scale rows between dorsal-fin origin and lateral line (7-8 vs. 8-12). Additionally, the new species differs from B. copei, B. parnaibae, and B. retrospina by the lower number of branched dorsal-fin rays (9 vs.10). This is the first description of a new species of the subfamily since the revisionary study of Stethaprioninae, published almost 30 years ago.

On the type locality of Sorubim trigonocephalus Miranda-Ribeiro, 1920 (Siluriformes: Pimelodidae).

Sorubim trigonocephalus was described in 1920 by Alípio de Miranda Ribeiro, based on a single specimen collected in a locality identified as "Porto Velho", during the "Comissão das Linhas Telegráficas Estratégicas de Mato Grosso ao Amazonas" (more commonly known as Rondon Commission). Given that the type locality is Porto Velho, the species has been referred to the Madeira River basin (Lundberg & Littmann, 2003; Littmann, 2007; Eschmeyer et al., 2016). Nevertheless, after its description, no additional specimens were collected in the Madeira basin despite several ichthyological expeditions undertaken to the area (Santos, 1996; Camargo & Giarrizzo, 2007; Rapp Py-Daniel et al., 2007; Perin et al., 2007; Pedroza et al., 2012; Casatti et al., 2013; Queiroz et al., 2013a), some of them including region of Porto Velho (Fowler, 1913; Araújo et al., 2009; Torrente-Vilara et al., 2011; Queiroz et al., 2013b).

A new long-snouted species of Corydoras Lacépède, 1803 (Siluriformes: Callichthyidae) from the rio Madeira basin.

A new species of Corydoras is described from the rio Aripuanã, rio Madeira basin, Mato Grosso State, Brazil. The new species can be distinguished from its congeners by the presence of the following features: long mesethmoid, with anterior tip well developed, conspicuously larger than 50% of bone length; posterior margin of dorsal-fin spine with serrations directed towards tip of spine; one to two longitudinal black stripes on flanks; absence of a black blotch across the eyes (mask); absence of a large black blotch on dorsal fin; and transversal black bars on caudal fin. Comments on endemism in interfluvial region between Madeira and Tapajós rivers are briefly discussed.

A new species of Hyphessobrycon Durbin (Characiformes: Characidae) from rio Aripuanã, rio Madeira basin, Brazil.

A new species of Hyphessobrycon is described from the rio Aripuanã, rio Madeira basin, Amazonas State, Brazil. Hyphessobrycon procyon can be distinguished from its congeners by the unique combination of the following features: elongation of the dorsal fin in mature males, absence of dark blotches on the dorsal fin, and presence of two humeral blotches. The possibility of a relationship of the new species within H. elachys, H. heliacus, H. loweae, H. moniliger, and H. peugeoti is discussed based on the sharing of characteristics related to the sexual dimorphism and tooth morphology.

Wrapped in flames: Corydoras hephaestus, a new remarkably colored species from the Rio Madeira basin (Teleostei: Callichthyidae).

A new species of Corydoras is described from the upper Rio Machado, Rio Madeira basin, Rondônia State, Brazil. The new species can be distinguished from its congeners by the absence of contact between the posterior process of the parieto-supraoccipital and the nuchal plate; a ventral laminar expansion of the infraorbital 1 conspicuously developed; and the posterior margin of the pectoral-fin spine with serrations along almost of its entire length, only lacking in the distal portion. Additionally, Corydoras hephaestus possesses dorsal and pectoral spines short, and exhibits an unusual color pattern in life. The conservation status of the new species and other endemic species are briefly commented.