A new species of Parotocinclus is described from the upper rio Piranhas-Açu basin, northeastern Brazil. The new species differs from all its congeners, except P. bidentatus, P. muriaensis (both from rio Paraíba do Sul basin, southeastern Brazil), and P. spilurus (rio Jaguaribe basin, northeastern Brazil) by presenting the adipose fin rudimentary or absent. The new species differs from P. bidentatus, P. muriaensis, and P. spilurus mainly by presenting the abdomen region extensively naked, with few reduced rounded dermal platelets between the pectoral girdle and the anus. Parotocinclus seridoensis is probably an endemic species of the semi-arid Caatinga, region where the genus presents high species richness.Uma espécie nova de Parotocinclus é descrita da bacia do alto rio Piranhas-Açu, nordeste do Brasil. A espécie nova difere de todas as congêneres, exceto P. bidentatus, P. muriaensis (ambas da bacia do rio Paraíba do Sul, sudeste do Brasil) e P. spilurus (bacia do rio Jaguaribe, nordeste do Brasil), por apresentar nadadeira adiposa rudimentar ou ausente. A espécie nova difere de P. bidentatus, P. muriaensis e P. spilurus principalmente por apresentar no abdômen extensas áreas nuas, com poucas placas dérmicas reduzidas e arredondadas entre a cintura peitoral e o ânus. Parotocinclus seridoensis é provavelmente uma espécie endêmica da Caatinga semi-árida, região onde o gênero apresenta elevada riqueza de espécies.

Parotocinclus spilurus (Fowler) was originally described as a species of Plecostomus based on six specimens collected in the rio Salgado, Icó Municipality, Ceará State, northeastern Brazil. Because the original description of P. spilurus is brief and uninformative, a detailed redescription of this endangered species based on the type series and on freshly collected topotypes was made. Data on osteology, intraspecific variation, sexual dimorphism and color pattern in life are presented for the first time, aiming at providing the necessary characters to differentiate the species from its congeners, as well as providing data on the distribution, habitat and conservation.Parotocinclus spilurus (Fowler) foi descrita originalmente como uma espécie do gênero Plecostomus com base em seis espécimes coletados no rio Salgado, Município de Icó, Estado do Ceará. Devido à descrição original de P. spilurus ser breve e pouco informativa, uma redescrição detalhada desta espécie ameaçada com base na série tipo e em topótipos recentemente coletados foi feita. Dados de osteologia, variação ontogenética, dimorfismo sexual e padrão de coloração em vida são apresentados pela primeira vez, com o objetivo de fornecer caracteres necessários para distinguir a espécie das suas demais congêneres, assim como fornecer dados de distribuição, habitat e conservação.

Parotocinclus haroldoi is separated from most of its congeners by having an abdomen covered by relatively large plates (vs. naked abdomen or with small plates). This species occurs in Parnaíba River basin with two other congeners, P. cabessadecuia and P. cearensis. Parotocinclus haroldoi is distinguished from P. cabessadecuia for having developed adipose fin (vs. rudimentary or absent adipose fin) and P. cearensis, for having abdomen covered by entirely plates (vs. almost naked abdomen with small sparse plates). Parotocinclus haroldoi is known from the Parnaíba river basin in the state of Piauí, Maranhão-Piauí.

Poecilia vivipara, a small euryhaline guppy is reported at the Maceió River micro-basin in the Fernando de Noronha oceanic archipelago, northeast Brazil. However, the origin (human-mediated or natural dispersal) of this insular population is still controversial. The present study investigates how this population is phylogenetically related to the surrounding continental populations using the cytochrome oxidase I mitochondrial gene from eleven river basins in South America. Our phylogenetic reconstruction showed a clear geographical distribution arrangement of P. vivipara lineages. The Fernando de Noronha haplotype fell within the 'north' clade, closely related to a shared haplotype between the Paraíba do Norte and Potengi basins; the geographically closest continental drainages. Our phylogenetic reconstruction also showed highly divergent lineages, suggesting that P. vivipara may represent a species complex along its wide distribution. Regarding to the insular population, P. vivipara may have been intentionally introduced to the archipelago for the purpose of mosquito larvae control during the occupation of a U.S. military base following World War II. However, given the euryhaline capacity of P. vivipara, a potential scenario of natural (passive or active) dispersal cannot be ruled out.

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