A new species of rheophilic armored catfish of Rineloricaria (Siluriformes: Loricariidae) from the Vaupés River, Amazonas basin, Colombia.

A new rheophilic species of the genus Rineloricaria is described for the Amazon basin in Colombia. Rineloricaria cachivera n. sp. differs from its congeners by having anterior to the first predorsal plate, an inconspicuous saddle-like mark; the presence of dark, diffuse blotches, present as unified dark colouration along most of the dorsal portion of the head, without bands or spots on the head; a long snout that occupies more than half the head length (HL), between 58.0% and 66.3% HL; a naked portion on the cleithral area from the border of lower lip reaching the origin of pectoral fin; and by having five series of lateral plates in longitudinal rows below the dorsal fin. The new species is morphologically similar to Rineloricaria daraha; however, it can be distinguished by the presence of six branched pectoral fin rays (vs. seven) and the lower lip surface with short thick papillae (vs. long finger papillae). An identification key to the Rineloricaria species of the Amazon River basin in Colombia is provided. The new species is herein categorized as Least Concern, following the IUCN criteria.

Phylogenetic relationships and description of two new species of Diapoma (Characidae: Stevardiinae) from the La Plata River basin

Herein we describe two new species of Diapoma, one from the Negro River, a tributary of the Uruguay River in Brazil and Uruguay, and one from the Iguaçu River, in Brazil and Argentina. The new species from the Negro River basin is distinguished from its congeners by the following combination of characters: a black narrow and conspicuous line restricted to the body horizontal septum, incomplete lateral line, tricuspid teeth in the inner series of the premaxilla, and a lower body depth at vertical through the dorsal-fin origin (29.3-32.8% SL in males and 27.7-33.3% SL in females). The new species from the Iguaçu River basin is distinguished from its congeners by the following combination of characters: a discontinuous lateral line, adipose fin hyaline, longer anal-fin base (26.5-32.4% SL), and a longitudinal black stripe along the median region of caudal-fin rays. Additionally, we updated the molecular phylogeny of the genus, including new sequences from these two new species and Diapoma thauma. An identification key for species of Diapoma is presented, modified from previous study.(AU)

Neste trabalho descrevemos duas espécies novas de Diapoma, uma do rio Negro, afluente do rio Uruguai, Brasil e Uruguai, e a segunda do rio Iguaçu, Brasil e Argentina. A espécie nova do rio Negro é diagnosticada de suas congêneres pela combinação das seguintes características: uma linha preta, estreita e conspícua restrita ao septo horizontal do corpo, linha lateral incompleta, dentes da série interna da pré-maxila tricuspidados, e baixa altura do corpo na vertical que passa pela origem da nadadeira dorsal (29,3-32,8% SL em machos e 27,7-33,3% SL em fêmeas). A espécie nova do rio Iguaçu é diagnosticada de suas congêneres pela combinação das seguintes características: linha lateral descontínua, nadadeira adiposa não pigmentada de preto, base da nadadeira anal longa (26,5-32,4% SL) e raios médios da nadadeira caudal com uma linha preta longitudinal. Adicionalmente, atualizamos a filogenia molecular do gênero, incluindo novas sequências destas duas espécies novas e de Diapoma thauma. Uma chave de identificação para as espécies de Diapoma é apresentada, modificada de estudo anterior.(AU)

New Ecuadorian records of the eyeless banjo catfish Micromyzon akamai (Siluriformes: Aspredinidae) expand the species range and reveal intraspecific morphological variation.

Two specimens of Micromyzon akamai, an eyeless and miniaturized species previously known only from the deep channels of the eastern Amazon basin in Brazil, are reported from the Curaray River, a tributary of the Napo River in Ecuador. The new specimens are the first records of Micromyzon in the headwaters of the Amazon River and the first records of M. akamai outside Brazil. External morphological characters and a phylogenetic analysis of cytochrome c oxidase I (coI) gene support the identification of the new specimens as M. akamai. Nevertheless, the new specimens also indicate that some features previously hypothesized to be apomorphic for M. akamai are intraspecifically variable.

Evidence on the paleodrainage connectivity during Pleistocene: Phylogeography of a hypoptopomatine endemic to southeastern Brazilian coastal drainages

The coastal basins of southeastern Brazil are influenced by climatic changes that caused sea-level oscillations during the Pleistocene. These marine transgressions and regressions can generate isolation and connection among coastal rivers. In this region, freshwater fishes are excellent models for phylogeographic studies because their distributions may have been affected by geographical and ecological changes resulting from these processes. Therefore, the main objective of this study was to evaluate the effects of Pleistocene sea-level changes on the genetic structure of the loricariid Hisonotus leucofrenatus throughout its area of occurrence. Two genes were sequenced: Cytochrome Oxidase subunit 1 (mitochondrial gene) and rpS7 ribosomal protein gene intron 1 (nuclear gene) from specimens representing 14 river drainages. The genetic data corroborate a divide for freshwater fish by the Serra do Tabuleiro mountain in Santa Catarina State. This divide determines two main genetic groups in H. leucofrenatus: one group to the south and one to the north of this mountain range. The genetic structure observed coincide with the limits of estimated paleodrainage systems for the region, supporting that marine transgressions and regressions during the Pleistocene influenced the biogeographical history of H. leucofrenatus.(AU)

As bacias costeiras do sul do Brasil são influenciadas pelas mudanças climáticas que causaram oscilações no nível do mar durante o Pleistoceno. Essas transgressões e regressões marinhas geraram isolamento e conexão entre os rios. Nessa região, as espécies de peixe são excelentes modelos para estudos filogeográficos, pois suas distribuições podem ter sido afetadas por mudanças históricas e ecológicas decorrentes desses processos. Portanto, o objetivo principal deste estudo foi testar os efeitos das alterações do nível do mar durante o Pleistoceno na estrutura genética das populações do loricarídeo Hisonotus leucofrenatus ao longo de sua área de ocorrência. Dois genes foram sequenciados: Citocromo Oxidase subunidade 1 (gene mitocondrial) e o intron 1 da proteína ribossomal rpS7 (gene nuclear) de espécimes representando 14 bacias de drenagens. A estrutura genética observada corrobora uma divisão para peixes de água doce separada pela Serra do Tabuleiro, em Santa Catarina. Essa divisória determina dois grupos principais genéticos em H. leucofrenatus: um grupo ao sul e outro ao norte desse divisor. A estrutura genética também coincide com os limites dos sistemas de paleodrenagens estimados para a região, sustentando que as transgressões e regressões marinhas durante o Pleistoceno influenciaram a história biogeográfica de H. leucofrenatus.(AU)

Species delimitation in a range-restricted group of cascudinhos (Loricariidae: Epactionotus) supports morphological and genetic differentiation across coastal rivers of southern Brazil.

Epactionotus species are known for inhabiting the rocky-bottom stretches of fast-flowing rivers in a limited geographic area along the Atlantic coast of southern Brazil. These species are endemic to single coastal river drainages (two neighbouring drainages for Epactionotus bilineatus) isolated from each other by the coastal lacustrine environments or the Atlantic Ocean. E. bilineatus is from the Maquiné and Três Forquilhas River basins, both tributaries of the Tramandaí River system, whereas E. itaimbezinho is endemic to the Mampituba River drainage and Epactionotus gracilis to the Araranguá River drainage. Recent fieldwork in the Atlantic coastal drainages of southern Brazil revealed new populations in the Urussanga, Tubarão, d'Una and Biguaçu River drainages. Iterative species delimitation using molecular data (cytochrome c oxidase subunit I) and morphology (morphometrics and meristics) was applied to evaluate species recognition of isolated populations. With regard to new data, the genus was re-diagnosed, the status of Epactionotus species/populations was re-evaluated, formerly described species were supported and population structure was recognized. As for the newly discovered populations, both morphological and molecular data strongly support the population from the Biguaçu River drainage, in Santa Catarina State, as a new species. Molecular data revealed strong per-basin population structure, which may be related to species habitat specificity and low or no dispersal among drainages.

Taxonomical study of Trichomycterus (Siluriformes: Trichomycteridae) from the Ribeira de Iguape River basin reveals a new species recorded in the early 20th century.

A new species of Trichomycterus endemic to the Ribeira de Iguape River basin, southeastern Brazil, was studied based on morphological and molecular evidence. This species had an outer layer of coloration composed of scattered, round, black or dark-brown spots smaller or equivalent in size to the circumference of the eye; eight pectoral-fin rays; 28-29 opercular odontodes; 54-56 interopercular odontodes; and supraorbital line of the laterosensory system not interrupted, with pores s2 absent. Two other species of Trichomycterus from the Ribeira de Iguape River basin are recorded, and their taxonomic status is discussed: Trichomycterus alternatus and Trichomycterus jacupiranga were not differentiated using molecular analysis but may be consistently distinguished based on morphology. The phylogenetic relationships of the co-occurring species, T. alternatus and Cambeva zonata, were inferred using mitochondrial data, reinforcing the taxonomic status of these recently revised species that have a complex taxonomy. In addition, a new combination for Trichomycterus taroba with its inclusion in the genus Cambeva is recommended.

Using community phylogenetics to assess phylogenetic structure in the Fitzcarrald region of Western Amazonia

Here we explore the use of community phylogenetics as a tool to document patterns of biodiversity in the Fitzcarrald region, a remote area in Southwestern Amazonia. For these analyses, we subdivide the region into basin-wide assemblages encompassing the headwaters of four Amazonian tributaries (Urubamba, Yuruá, Purús and Las Piedras basins), and habitat types: river channels, terra firme (non-floodplain) streams, and floodplain lakes. We present a robust, well-documented collection of fishes from the region including 272 species collected from 132 field sites over 63 field days and four years, comprising the most extensive collection of fishes from this region to date. We conduct a preliminary community phylogenetic analysis based on this collection and recover results largely statistically indistinguishable from the random expectation, with only a few instances of phylogenetic structure. Based on these results, and of those published in other recent biogeographic studies, we conclude that the Fitzcarrald fish species pool accumulated over a period of several million years, plausibly as a result of dispersal from the larger species pool of Greater Amazonia.(AU)

Aquí exploramos el uso de la filogenética de comunidades como herramienta para documentar patrones de biodiversidad en la región de Fitzcarrald, un área remota en el suroeste de la Amazonía. Para estos análisis subdividimos la región en grupos de toda la cuenca que abarcan las cabeceras de cuatro tributarios del Amazonas (cuencas Urubamba, Yuruá, Purús y Las Piedras) y en los tipos de hábitat: canales fluviales, arroyos de tierra firme (sin planicie aluvial) y lagos de planicie aluvial. Presentamos una colección de peces robusta y bien documentada que incluye 272 especies, colectadas a lo largo de cuatro años y 63 días de campo, en 132 puntos de monitoreo. Convirtiéndose en la colección más extensa de peces de esta región hasta la fecha. Realizamos un análisis filogenético preliminar de la comunidad basado en esta recopilación y recuperamos resultados en gran medida estadísticamente indistinguibles de la expectativa aleatoria, con sólo unos pocos casos de estructura filogenética. Basándonos en estos resultados y los publicados en otros estudios biogeográficos recientes, concluimos que el grupo de especies de peces de Fitzcarrald acumulado durante un período de varios millones de años, se debe posiblemente al resultado de la dispersión del mayor grupo de especies de la Gran Amazonia.(AU)

A new species of Pseudobunocephalus Friel, 2008 (Siluriformes: Aspredinidae) from the lower Tocantins and Mearim river drainages, North and Northeast of Brazil.

Pseudobunocephalus timbira, new species, is described from streams of the lower Tocantins and the Mearim river drainages, in North and Northeast of Brazil. Pseudobunocephalus timbira can be distinguished from all congeners by having the second hypobranchial and the third basibranchial cartilaginous (vs. ossified). Additionally, it can be dintinguished from P. lundbergi by the following putative apomorphic features within Pseudobunocephalus: posterolateral process of premaxilla present (vs. absent); bony knobs in dorsal lamina of Weberian apparatus absent (vs. present); distal end of posterior margin of 5th parapophysis not enlarged (vs. enlarged); number of ribs three (vs. four or five) and infraorbital sensory canal entering neurocranium via frontal (vs. via sphenotic). It is distinguished from P. bifidus and P. iheringii by having a gracile body not surpassing 34 mm SL (vs. robust body, reaching up to 59 mm SL, respectively); by having the posterior margin of cranial fontanel concave (vs. posterior margin somewhat straight with parieto-supraoccipital extending anteriorly); by having a conspicuous knobby ornamentation on dorsal surface of skull (vs. skull knobs slightly pronounced); by having Weberian ventral blade of hemal canal opened (vs. closed) and by the absence of serrations on the proximal portion of the anterior margin of pectoral-fin spine (vs. serrations covering entire anterior margin of the pectoral spine). Additionally, it can be distinguished from P. amazonicus, P. rugosus and P. quadriradiatus, by having the posterolateral mental barbel with at least one fleshy lobe located proximally along the posterior margin (vs. posterolateral mental barbel simple, not having fleshy lobes). It also differs from P. amazonicus and P. rugosus by having five branchiostegal rays (vs. four). It also can be distinguished from P. amazonicus by having the contact of hyomandibula cartilage with neurocranium limited to the sphenotic (vs. extending to both sphenotic and pterotic); by having the ventral blade of Weberian apparatus open (vs. closed) and by anterior exit of hemal canal in abdominal vertebra (vs. in complex vertebra); from P. rugosus by coloration of proximal portion of caudal fin similar to rest of caudal fin (vs. clear patch) and from P. quadriradiatus by the total number of pectoral fin-rays six (vs. five). Variable characteristics within Pseudobunocephalus species are summarized and comments on the phylogenetic relationships and the disjunct distribution of the new species are made. [Species zoobank url: urn:lsid:zoobank.org:act:392F95E0-86E1-4386-8779-C4F71098DBCC].

Phylogenetic relationships and historical biogeography of Oligosarcus (Teleostei: Characidae): Examining riverine landscape evolution in southeastern South America.

The pike-characin Oligosarcus is a group of Characidae composed of 22 species, which have mostly allopatric distributed species in southeastern South America and sympatric occurrence of few species. Oligosarcus shares a similar distribution pattern with other fish genera and therefore, can help us to understand biogeographic events that influenced freshwater fish distribution in the southeastern South America. Our paper presents the most extensive taxonomic coverage for molecular analysis of Oligosarcus and uses various methods to examine the evolutionary history of the genus. Phylogenetic relationships among species of Oligosarcus were examined using a multilocus dataset by Maximum Likelihood and Bayesian methods. A relaxed molecular clock was used to estimate lineage divergence times, which provide a framework to examine the biogeographic history of this clade across the drainage basins of southeastern South America. Oligosarcus was resolved as monophyletic with strong support, and related to lineages currently assigned to the genus Astyanax. Within Oligosarcus, two groups of approximately equal species richness were resolved as monophyletic, mainly restricted to continental and coastal drainages of southeastern South America. Oligosarcus radiation is estimated to the late Neogene, with its origin in the Pliocene and most speciation events occurring in the Pleistocene. Some apomorphic characteristics associated with piscivory (e.g. large caniniform teeth) in Oligosarcus likely have evolved once, and are convergent to similar phenotypes observed in a distantly related clade of Astyanax (formerly Bramocharax). In addition, the presence of morphological convergence within the genus Oligosarcus (e.g. trophic morphology) seems to explain the difference between the present molecular hypothesis and some previous morphological studies. Ancestral geographical range estimation using analytical methods (e.g. DIVALIKE and DEC) demonstrated the effects of different Landscape Evolution Models (LEMs) on diversification of Oligosarcus. The results suggest that the two main Oligosarcus clades evolved in allopatry in continental and coastal drainages, with subsequent range extension and vicariance events that established the modern distributions. LEM analyses indicate the importance of formation of riverine barriers across the watershed of the La Plata basin and the effects of sea-level changes during the Pleistocene for delineating lineage distributions of Oligosarcus.

Geographic distributions, phenotypes, and phylogenetic relationships of Phalloceros (Cyprinodontiformes: Poeciliidae): Insights about diversification among sympatric species pools.

With 22 described species, Phalloceros is the most species-rich genus of Poeciliidae in South America. Phalloceros diversity is characterized by high degrees of endemism and sympatry in coastal and inland drainages in southeastern South America. The taxa are also characterized by pronounced differentiation in sexual characters (i.e., female urogenital papilla and male gonopodium), which might have contributed to their diversification. Here we estimate phylogenetic relationships based on more than 18,000 loci in 93 individuals representing 19 described species and two putative undescribed species. Morphologically defined species correspond to monophyletic species lineages, with individuals within a species clustering together in phylogenetic estimates, with the main exception being P. harpagos, supporting undiscovered diversity in this morphospecies. Shifts in the female and male sexual traits (i.e., urogenital papilla and gonopodium) occurred in concert multiple times along the phylogeny highlighting the role of sexual selection in driving divergence in this genus. Out of 22 valid species, 14 species are found in sympatry with at least one other species of this genus. However, most co-occurrences are observed among non-sister species suggesting that diversification among closely related species involved mostly allopatric speciation, with only two instances of sympatric sister-species observed. A strong mismatch in sexual traits among sympatric taxa suggests that co-existence may be linked to divergent sexual traits that maintain species genetic distinctiveness through mechanical disruptions of interbreeding.

Anatomy and homology of the accessory electric organs of the toothless knifefishes (Rhamphichthyoidea: Gymnotiformes).

We describe the anatomy and histology of the accessory electric organs of several knifefish taxa. Accessory electric organs are observed among Rhamphichthyoidea in the opercular, mental and humeral regions. Within this group, some species of Brachyhypopomus possess an accessory electric organ in the opercular region. Rhamphichthyinae and Steatogenys possess accessory electric organs in the mental region of the body that differs in many aspects, such as general electrocyte shape and its number of caudal ridges. Steatogenys, Hypopygus and Rhamphichthys possess an accessory electric organ in the humeral region that differs in position, electrocyte configuration and shape. Electrocytes of both humeral and mental accessory electric organs in Steatogenys share a number of common features (e.g., electrocyte shape and innervation pattern), which distinguishes them from the electric organs of related groups. Rhamphichthys has an accessory electric organ in the humeral (specifically subpectoral) region, which has not previously been reported in the literature and differs in arrangement and electrocyte shape from those previously described electric organs of other taxa. Homology of these accessory electric organs is discussed in the context of hypothesized relationships among rhamphichthyoid taxa, indicating that accessory electric organs originated multiple times with apparently no subsequent losses.

Molecular phylogeny of Banjo catfishes (Ostaryophisi: Siluriformes: Aspredinidae): A continental radiation in South American freshwaters.

The family Aspredinidae is a moderately diverse and broadly distributed group of freshwater fishes endemic to South America. Commonly known as Banjo Catfishes, Aspredinidae currently includes 44 valid species divided among 13 genera. The first species-comprehensive hypothesis on phylogenetic relationships among aspredinids is presented. The phylogeny is based on DNA sequence data for five gene fragments (mitochondrial 16S and COI; nuclear RAG1, MYH6 and SH3PX3) from 114 individuals representing 31 species in 12 aspredinid genera. Analyses of molecular data support the monophyly of most genera (Bunocephalus excepted) and several higher-level relationships previously proposed by morphological studies. Based on the molecular phylogeny, a new suprageneric classification for Aspredinidae is proposed with the new monotypic subfamily Pseudobunocephalinae as the sister taxon to all other aspredinids.

A new species of Hoplomyzon (Siluriformes: Aspredinidae) from Maracaibo Basin, Venezuela: osteological description using high-resolution computed microtomography of a miniature species

A new miniature species of banjo catfish of the genus Hoplomyzon is described from the Lake Maracaibo Basin in Venezuela. The new species is distinguished from all its congeners by the straight anterior margin of the mesethmoid (vs. a medial notch); a smooth and straight ventral surface of the premaxilla (vs. presence of bony knobs on the ventral surface of premaxilla); absence of teeth on dentary (vs. teeth present on dentary); configuration of ventral vertebral processes anterior to anal fin, which are composed of single processes anterior to anal-fin pterygiophore (vs. paired process); presence of several filamentous barbel-like structures on the ventral surface of head of adults (vs. small papillous structures in the ventral surface of head of adults); and 8 anal-fin rays (vs. 6 or 7). An extensive osteological description is made of the holotype using high-resolution x-ray computed microtomography (HRXCT).(AU)

Se describe una nueva especie miniatura de pez banjo perteneciente al género Hoplomyzon, proveniente de tributarios del Lago Maracaibo en Venezuela. La nueva especie se diferencia de sus congéneres por presentar el margen anterior del mesetmoide recto (vs. con una hendidura central); superficie ventral de la premaxila lisa y recta (vs. superficie ventral de la premaxila con protuberancias óseas); ausencia de dientes en el dentario (vs. dientes presentes); configuración de los procesos anteriores ventrales de la aleta anal, los que están compuestos de procesos simples anteriores al pterigióforo de la aleta anal (vs. procesos dobles); presencia de varias estructuras en forma de barbillas en la superficie ventral de la cabeza (vs. pequeñas estructuras en forma de papilas en la superficie ventral de la cabeza); y 8 radios en la aleta anal (vs. 6 o 7). Una extensiva descripción osteológica del holotipo fue realizada utilizando microtomografía computarizada de rayos X de alta resolución (HRCXCT).(AU)

A new species of Amaralia Fowler (Siluriformes: Aspredinidae) from the Paraná-Paraguay River Basin.

A new species of the banjo catfish genus Amaralia is described from the Paraná-Paraguay River Basin in central-western Brazil, Paraguay and northern Argentina. Amaralia oviraptor is distinguished from its single and allopatric congener, Amaralia hypsiura, by the greater number of dorsal-fin rays (3 vs. 2); by the absence of lateral contact between middle and posterior nuchal plates (vs. middle and posterior nuchal plates contacting each other laterally); and by a longer cleithral process (17.4-19.5 % of SL, mean 18.2 % vs. 14.0-17.2 % of SL, mean=15.5 %). Comments on the peculiar oophagic diet of Amaralia and an extended diagnosis of the genus are provided.

Two new species of the banjo catfish Bunocephalus Kner (Siluriformes: Aspredinidae) from the upper and middle rio São Francisco basins, Brazil

Two new species of banjo catfish of the genus Bunocephalus are described from the upper and middle rio São Francisco basins of Brazil. Bunocephalus harttiis distinguished from all its congeners by the absence of serrations along the anterior margin of pectoral-fin spine in adults (vs. presence of serrations along the anterior margin of the spine). Bunocephalus minerim can be diagnosed from all congeners,except B. larai, by the absence of an epiphyseal bar between the paired frontals (vs.presence of the epiphyseal bar at least in adults). Bunocephalus minerim is distinguished from B. laraiand other congeners, except B. chamaizelus, by having nine principal caudal-fin rays (vs.10 principal caudal-fin rays).

Duas novas espécies de peixe-banjo do gênero Bunocephalussão descritas para as bacias do alto e médio rio São Francisco no Brasil. Bunocephalus harttidifere das demais espécies do gênero pela ausência de ganchos ao longo da margem anterior do espinho da nadadeira peitoral em adultos (vs. presença de ganchos ao longo da margem anterior do espinho). Bunocephalus minerim pode ser distinguido dos demais congêneres, exceto B. larai, pela ausência da barra epifiseana entre os frontais (vs. presença da barra epifiseana ao menos nos adultos). Bunocephalus minerimpode ser distinguidade B. laraie outras congêneres, exceto B. chamaizelus,pela presença de nove raios principais na nadadeira caudal (vs. 10 raios caudais principais).

A novel learning rule for long-term plasticity of short-term synaptic plasticity enhances temporal processing.

It is well established that short-term synaptic plasticity (STP) of neocortical synapses is itself plastic - e.g., the induction of LTP and LTD tend to shift STP towards short-term depression and facilitation, respectively. What has not been addressed theoretically or experimentally is whether STP is "learned"; that is, is STP regulated by specific learning rules that are in place to optimize the computations performed at synapses, or, are changes in STP essentially an epiphenomenon of long-term plasticity? Here we propose that STP is governed by specific learning rules that operate independently and in parallel of the associative learning rules governing baseline synaptic strength. We describe a learning rule for STP and, using simulations, demonstrate that it significantly enhances the discrimination of spatiotemporal stimuli. Additionally we generate a set of experimental predictions aimed at testing our hypothesis.

Aquatic Biodiversity in the Amazon: Habitat Specialization and Geographic Isolation Promote Species Richness.

The Neotropical freshwater ichthyofauna has among the highest species richness and density of any vertebrate fauna on Earth, with more than 5,600 species compressed into less than 12% of the world's land surface area, and less than 0.002% of the world's total liquid water supply. How have so many species come to co-exist in such a small amount of total habitat space? Here we report results of an aquatic faunal survey of the Fitzcarrald region in southeastern Peru, an area of low-elevation upland (200-500 m above sea level) rainforest in the Western Amazon, that straddles the headwaters of four large Amazonian tributaries; the Juruá (Yurúa), Ucayali, Purús, and Madre de Dios rivers. All measures of fish species diversity in this region are high; there is high alpha diversity with many species coexisting in the same locality, high beta diversity with high turnover between habitats, and high gamma diversity with high turnover between adjacent tributary basins. Current data show little species endemism, and no known examples of sympatric sister species, within the Fitzcarrald region, suggesting a lack of localized or recent adaptive divergences. These results support the hypothesis that the fish species of the Fitzcarrald region are relatively ancient, predating the Late Miocene-Pliocene (c. 4 Ma) uplift that isolated its several headwater basins. The results also suggest that habitat specialization (phylogenetic niche conservatism) and geographic isolation (dispersal limitation) have contributed to the maintenance of high species richness in this region of the Amazon Basin.

Redescription and phylogenetic position of the enigmatic Neotropical electric fish Iracema caiana Triques (Gymnotiformes: Rhamphichthyidae) using x-ray computed tomography

We redescribe Iracema caiana, a monotypic genus of the gymnotiform electric fish family Rhamphichthyidae. Iracema is known only from the type series (holotype and three paratypes) collected from the rio Jauaperi, affluent to the rio Negro in the Amazon basin, and was never collected again. Previous morphological studies were limited to features of external morphology. To study the osteology of Iracema we examined two specimens of different sizes using high-resolution x-ray computed tomography, a non-invasive and non-destructive technique to visualize internal anatomical structures. We describe and illustrate the osteology of Iracema caiana, and present data on morphometrics and external morphology. Contrary to previous hypotheses we propose that Iracema is the sister group to Rhamphichthys based on four synapomorphies: intermuscular bones present in the adductor mandibulae, reticulated texture of opercles, fully ossified Baudelot's ligaments, and elongate scales above the lateral line in the posterior portion of the body.

Iracema caiana, um genêro monotípico de peixes elétricos Gymnotiformes da família Rhamphichthyidae, é aqui redescrito. Iracema é conhecido apenas da série-tipo (holótipo e três parátipos) coletado no rio Jauaperi, afluente do rio Negro na bacia amazônica, nunca sendo coletado novamente. Estudos morfológicos prévios de Iracema foram limitados somente à morfologia externa. Neste trabalho, a osteologia do gênero foi examinada com base em dois espécimes de diferentes tamanhos usando-se tomografia computadorizada de alta resolução, uma técnica não invasiva para a visualização de estruturas anatômicas internas. Caracteres externos e morfométricos são apresentados e a osteologia é ilustrada e descrita. Contrariamente a hipóteses prévias, Iracema é proposto como grupo irmão de Rhamphichthys com base em quatro sinapomorfias: presença de ossos intermusculares no adductor mandibulae, textura reticulada do opérculo, ligamento de Baudelot completamente ossificado e escamas alongadas na região posterior do corpo acima da linha lateral.

Differential effects of excitatory and inhibitory plasticity on synaptically driven neuronal input-output functions.

Ultimately, whether or not a neuron produces a spike determines its contribution to local computations. In response to brief stimuli the probability a neuron will fire can be described by its input-output function, which depends on the net balance and timing of excitatory and inhibitory currents. While excitatory and inhibitory synapses are plastic, most studies examine plasticity of subthreshold events. Thus, the effects of concerted regulation of excitatory and inhibitory synaptic strength on neuronal input-output functions are not well understood. Here, theoretical analyses reveal that excitatory synaptic strength controls the threshold of the neuronal input-output function, while inhibitory plasticity alters the threshold and gain. Experimentally, changes in the balance of excitation and inhibition in CA1 pyramidal neurons also altered their input-output function as predicted by the model. These results support the existence of two functional modes of plasticity that can be used to optimize information processing: threshold and gain plasticity.