Ancistrus is the most speciose genus of the tribe Ancistrini, with 58 valid species and many yet to be described. Cytogenetic studies were conducted on five apparently undescribed species from the Amazon basin, which showed different diploid numbers: Ancistrus sp. Purus (2n = 34); Ancistrus sp. Macoari (2n = 46); Ancistrus sp. Dimona (2n = 52); Ancistrus sp. Vermelho (2n = 42) and Ancistrus sp. Trombetas (2n = 38). All species possessed only one pair of NOR-carrying chromosomes, but with extensive variation in both the location on the chromosome as well as in the position of the ribosomal sites on the karyotype. The karyotypic evolution of Ancistrus species seems to be based on chromosomal rearrangements, with a tendency to a reduction of the diploid number. Two new instances of XX/XY sex chromosomes for Ancistrus species, based on the heteromorphism in the male karyotype, were also recorded. The large karyotypic diversity among Ancistrus species may be related to biological and behavioural characteristics of these fish that include microhabitat preferences, territoriality and specialized reproductive tactics. These characteristics may lead to a fast rate of fixation of chromosomal mutations and eventually speciation across the basin.

Loricariid catfishes show a predominance of homomorphism in sex chromosomes, but cases of simple and multiple systems were also found. Here we describe two cases of multiple sex chromosome systems in loricariids from Brazilian Amazonia. Males of Ancistrus sp.1 "Balbina" have a modal number of 2n = 39 chromosomes, fundamental number (FN) of 78, and karyotypic formula of 27 m + 10 sm + 2 st; females have 2n = 38 chromosomes, FN = 76, and 26 m + 10 sm + 2 st. Ancistrus sp.2 "Barcelos" has 2n = 52 chromosomes for both sexes, FN = 80 for males and FN = 79 for females. Karyotypic formula is 12 m + 12 sm + 4 st + 24a for males and 11 m + 12 sm + 4st + 25a for females. The two species show different arrangements of constitutive heterochromatin blocks, which are coincident with NORs and absent in sex chromosomes. We suggest a XX/XY(1)Y(2) mechanism for Ancistrus sp.1 "Balbina", and a Z(1)Z(1)Z(2)Z(2)/Z(1)Z(2)W(1)W(2) mechanism for Ancistrus sp.2 "Barcelos". The XX/XY(1)Y(2) mechanism here reported is the second known occurrence of this type of multiple sex chromosomes for Loricariidae and the third for Neotropical fishes; the mechanism Z(1)Z(1)Z(2)Z(2)/Z(1)Z(2)W(1)W(2) represents the first record among fishes. The presence of different sex chromosome systems in Ancistrus indicates a probable independent origin and suggests that the differentiation of sex chromosomes is evolutionarily recent among species in this genus.

We present karyotypic characteristics and report on the occurrence of ZZ/ZW sex chromosomes in Ancistrus ranunculus (rio Xingu) and Ancistrus sp. "Piagaçu" (rio Purus), of the Brazilian Amazon. Ancistrus ranunculus has a modal number of 2n=48 chromosomes, a fundamental number (FN) of 82 for both sexes, and the karyotypic formula was 20m+8sm+6st+14a for males and 19m+9sm+6st+14a for females. Ancistrus sp. "Piagaçu" presented 2n=52 chromosomes, FN= 78 for males and FN= 79 for females. The karyotypic formula was 16m+8sm+2st+26a for males and 16m+9sm+2st+25a for females. The high number of acrocentric chromosomes in karyotype of Ancistrus sp. "Piagaçu" differs from the majority of Ancistrini genera studied so far, and may have resulted from pericentric inversions and translocations. The lower number of chromosomes in A. ranunculus indicates that centric fusions also occurred in the evolution of Ancistrus karyotypes. We conclude that karyotypic characteristics and the presence of sex chromosomes can constitute important cytotaxonomic markers to identify cryptic species of Ancistrus. However, sex chromosomes apparently arose independently within the genus and thus do not constitute a reliable character to analyze phylogenetic relations among Ancistrus species.Nós apresentamos características cariotípicas e registramos a ocorrência de cromossomos sexuais ZZ/ZW em Ancistrus ranunculus (rio Xingu) e Ancistrus sp. "Piagaçu" (rio Purus), da Amazônia Brasileira. Ancistrus ranunculus teve um número modal de 2n=48 cromossomos, um número fundamental (NF) de 82 para ambos os sexos, e a fórmula cariotípica 20m+8sm+6st+14a para machos e 19m+9sm+6st+14a para fêmeas. Ancistrus sp. "Piagaçu" apresentou 2n=52 cromossomos, NF=78 para machos e NF= 79 para fêmeas. A fórmula cariotípica foi de 16m+8sm+2st+26a para machos e 16m+9sm+2st+25a para fêmeas. O alto número de cromossomos acrocêntricos no cariótipo de Ancistrus sp. "Piagaçu" difere da maioria dos gêneros de Ancistrini estudada até o momento, e pode ter resultado de inversões pericêntricas e translocações. O número mais baixo de cromossomos em A. ranunculus indica que fusões cêntricas também ocorreram na evolução dos cariótipos de Ancistrus. Nós concluímos que as características cariotípicas e a presença de cromossomos sexuais podem constituir marcadores citotaxonômicos importantes para identificar espécies crípticas de Ancistrus. Entretanto, cromossomos sexuais aparentemente surgiram de forma independente dentro do gênero e, deste modo, não constituem um caráter confiável para ser usado em análises de relações filogenéticas entre espécies de Ancistrus.

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.

The neotropical freshwater systems have a high number of catfish species (Siluriformes), and many of those are denominated "cascudos" in Brazil. Cytogenetic data about three "cascudos" species fished in the rio Araguaia are described in the present study. The Pterygoplichthys joselimaianus showed 2n=52, with 28 metacentrics (M) chromosomes, 16 submetacentrics (SM) and 8 subtelocentrics/acrocentrics (ST/A) in both sexes. Hemiancistrus spinosissimus showed 2n=52, with karyotype formulae 26M+22SM+4ST, in both sexes. Hemiancistrus spilomma also showed 2n=52, but in this species a ZZ/ZW sex chromosome system (25M+21SM+6ST in females and 24M+22SM+6ST in males) was observed. The cells from H. spinosissimus and P. joselimaianus showed one chromosome pair bearing Ag-NORs, while in the H. spilomma three chromosome pairs bearing AgNORs were detected. The data showed in this work reveal particular chromosomal characteristics, important for a good recognition of both Hemincistrus species, and also show the importance of the insertion of cytogenetic data on taxonomic phylogenetic studies.Os sistemas de água doce neotropicais possuem um alto número de espécies de peixes Siluriformes, muitas das quais popularmente conhecidas como cascudos no Brasil. Dados citogenéticos sobre três espécies de cascudos, capturadas na bacia do rio Araguaia são descritos no presente trabalho. Pterygoplichthys joselimaianus possui 2n=52, com 28 cromossomos metacêntricos (M), 16 submetacêntricos (SM) e 8 subtelecêntricos/acrocêntricos (ST/A) em ambos os sexos. Hemiancistrus spinosissimus revelou 2n=52, com fórmula cariotípica 26M+22SM+4ST, nos dois sexos. H. spilomma também possui 2n=52, porém, nesta espécie foi observado um sistema de cromossomos sexuais do tipo ZZ/ZW (25M+21SM+6ST para fêmeas e 24M+22SM+6ST para machos). As células de H. spinosissimus e P. joselimaianus mostraram um par de cromossomos portadores de Ag-RONs. Em H. spilomma, três pares de cromossomos portadores de Ag-RONs foram observados. Os dados apresentados neste trabalho revelam características cromossômicas particulares, importantes para um bom reconhecimento de ambas as espécies de Hemiancistrus e mostram a importância da inserção dos dados citogenéticos nos estudos taxonômicos e filogenéticos.