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Aim: To analyze temporal distribution of larvae and juveniles and the early development and of Moenkhausia cf. gracilima. Methods Samples were taken quarterly in twenty-five sites in the upper Paraná River floodplain between August 2013 and May 2015. The samples were taken under the water surface at night using 0.5 mm mesh plankton nets. In the laboratory, samples were sorted, identified and separated into larval (preflexion, flexion and postflexion) and juvenile periods. Results A total of 248 individuals was collected, mainly in the Saraiva Lagoon, suggesting that the entire life cycle of this species occurs in this environment. The reproductive period takes place between December and April, since the postflexion larvae were found until May. However, the occurrence of juveniles between February and May indicates probable batch spawning. Among the 95 individuals used for ontogenic description, 82 were larvae and 13 juveniles. Larvae may be characterized by irregular pigmentation in the upper region of the head, mouth, and body, increasing throughout development; upper lobe of the caudal fin more pigmented than the lower lobe, only visible in postflexion larvae; terminal mouth; anal opening located anterior to the median region of the body and total number of myomers ranging from 34 to 40 (15 to 20 pre and 16 to 23 postanal), while juveniles have characteristics similar to adults. The total number of fin rays is: P. 11-16, V. 7-11, D, 9-11 and A. 21-23. Conclusions According to the distribution of developmental periods it is possible to conclude that this species reproduces in the summer, preferably in lagoons. Growth analysis indicated important alterations in larval morphology (metamorphosis) that may be associated with the ecomorphological characteristics of the species. The morphological separation of larvae of M. cf. gracilima from other larvae of small characids, especially at preflexion and flexion stages may be complicated by the overlap of traits, suggesting the use of other variables, mainly morphometric, for the separation of the species. Resumo Objetivo: Analisar a distribuição temporal de larvas e juvenis e o desenvolvimento inicial de Moenkhausia cf. gracilima. Métodos Foram amostrados trimestralmente vinte e cinco estações distribuídas na planície de inundação do alto rio Paraná entre agosto de 2013 e maio de 2015. As coletas foram na subsuperfície, no período noturno, utilizando redes de plâncton com malha 0,5 mm. Em laboratório, as amostras foram triadas, identificadas e separadas em períodos larval (pré-flexão, flexão e pós-flexão) e juvenil. Resultados Foram capturados 248 indivíduos, sendo a maioria na Lagoa Saraiva, sugerindo que todo o ciclo de vida desta espécie aconteça neste ambiente, e que o período reprodutivo ocorre entre dezembro e abril, uma vez que larvas em pós-flexão foram encontradas até maio. A ocorrência de juvenis entre fevereiro e maio indica provável desova parcelada. Entre os 95 indivíduos utilizados na descrição ontogênica, 82 eram larvas e 13 juvenis. As larvas podem ser caracterizadas pela pigmentação irregular na região superior da cabeça, na boca e no corpo, aumentando ao longo do desenvolvimento; lobo superior da nadadeira caudal mais pigmentado do que o inferior, só visível em larvas em pós-flexão; boca em posição terminal, abertura anal localizada anteriormente à região mediana do corpo e número total de miômeros variando de 34 a 40 (15 a 20 pré e 16 a 23 pós-anal), enquanto os juvenis apresentam características semelhantes ao adulto. O número de raios das nadadeiras é: P. 11-16, V. 7-11, D, 9-11 e A. 21-23. Conclusões De acordo com a distribuição dos períodos de desenvolvimento, é possível concluir que esta espécie se reproduz no verão, de preferência em lagoas. A análise do crescimento indicou alterações importantes na morfologia larval (metamorfoses) que podem estar associadas às características ecomorfológicas da espécie. A separação morfológica das larvas de M. cf. gracilima de outras larvas de caracteres pequenos, especialmente nos estágios de preflexão e flexão, pode ser complicado pela sobreposição de traços, sugerindo o uso de outras variáveis, principalmente morfométricas, para a separação das espécies.

Aim: This study uses the abundance of fish eggs and larvae to evaluate the importance of the main channel of the Paraná River and the adjacent areas of the floodplain, in the last dam-free stretch in the Brazilian territory, for the spawning and development of fish of different reproductive guilds, in order to obtain subsidies to assist in the management and conservation policies of this area, focusing on the maintenance of dam-free areas. Methods Data were taken quarterly from August 2013 to May 2015, in 25 sites, grouped into three biotopes: main channel, tributaries and lagoons. Possible spatial variations in fish spawning and development as well as composition and structure of larvae were evaluated. Results Higher densities of eggs were found in tributaries (Paracaí and Amambai rivers) and greater densities of larvae were observed in lagoons (Saraiva). Significant differences in composition and structure of larvae were detected only between sampling stations. As for taxonomic composition, 29 taxa were recorded, mostly non-migratory. However, long-distance migratory were also widely distributed, such as Brycon orbignyanus, Pseudoplatystoma corruscans, Prochilodus lineatus, Piaractus mesopotamicus and Rhaphiodon vulpinus, as well as invasive species Platanichthys platana and Hemiodus orthonops. In turn, Salminus brasiliensis presented low occurrence. Conclusions This study evidenced that different species spawn in the region, mainly in tributaries, and their eggs and larvae are transported to the main channel of the Paraná River and adjacent lagoons, to complete their early development. The capture of larvae of important migratory species suggests that this environment still exhibits suitable conditions for their reproduction, mainly due to the presence of dam-free tributaries. Also, they emphasize the importance of the integrity of these environments for the maintenance of the regional fish fauna, and it is extremely important the monitoring of reproduction of the most endangered species, as well as of invasive species. Resumo Objetivo: Este estudo utiliza a abundância de ovos e larvas de peixes para avaliar a importância do canal principal do rio Paraná e as áreas adjacentes da planície de inundação, no último trecho livre de barramentos em território brasileiro, para a desova e desenvolvimento dos peixes de diferentes guildas reprodutivas, a fim de obter subsídios que auxiliem nas políticas de manejo e conservação desta área, focando na manutenção de áreas livres de represamentos. Métodos Os dados foram coletados trimestralmente entre agosto de 2013 a maio de 2015, em 25 estações, agrupadas em três biótopos: canal principal, tributários e lagoas. Foram avaliadas possíveis variações espaciais na desova e desenvolvimento dos peixes, assim como na composição e a estrutura das larvas. Resultados Maiores densidades de ovos foram registradas nos tributários (rios Paracaí e Amambai) e de larvas nas lagoas (Saraiva). Diferenças significativas na composição e estrutura das larvas foram observadas apenas entre as estações de amostragens. Quanto a composição taxonômica, registrou-se 29 táxons, sendo a maioria não migradores. Entretanto, também foram amplamente distribuídas as larvas de migradores de longa distância como Brycon orbignyanus, Pseudoplatystoma corruscans, Prochilodus lineatus, Piaractus mesopotamicus e Rhaphiodon vulpinus, assim como as invasoras Platanichthys platana e Hemiodus orthonops. Já Salminus brasiliensis apresentou baixa ocorrência. Conclusões Neste estudo fica evidente que diferentes espécies desovam na região, principalmente nos tributários, e seus ovos e larvas são carreados para o canal principal do rio Paraná e lagoas adjacentes, para completarem seu desenvolvimento inicial. A captura de larvas de importantes espécies migradoras, sugerem que este ambiente ainda apresenta condições adequadas para sua reprodução, principalmente, pela presença de tributários livres de represamento. Ainda, reforçam a importância da integridade desses ambientes para a manutenção da ictiofauna regional, sendo de extrema relevância o monitoramento acerca da reprodução das espécies mais ameaçadas, bem como das espécies invasoras.

Aim: To analyze temporal distribution of larvae and juveniles and the early development and of Moenkhausia cf. gracilima. Methods Samples were taken quarterly in twenty-five sites in the upper Paraná River floodplain between August 2013 and May 2015. The samples were taken under the water surface at night using 0.5 mm mesh plankton nets. In the laboratory, samples were sorted, identified and separated into larval (preflexion, flexion and postflexion) and juvenile periods. Results A total of 248 individuals was collected, mainly in the Saraiva Lagoon, suggesting that the entire life cycle of this species occurs in this environment. The reproductive period takes place between December and April, since the postflexion larvae were found until May. However, the occurrence of juveniles between February and May indicates probable batch spawning. Among the 95 individuals used for ontogenic description, 82 were larvae and 13 juveniles. Larvae may be characterized by irregular pigmentation in the upper region of the head, mouth, and body, increasing throughout development; upper lobe of the caudal fin more pigmented than the lower lobe, only visible in postflexion larvae; terminal mouth; anal opening located anterior to the median region of the body and total number of myomers ranging from 34 to 40 (15 to 20 pre and 16 to 23 postanal), while juveniles have characteristics similar to adults. The total number of fin rays is: P. 11-16, V. 7-11, D, 9-11 and A. 21-23. Conclusions According to the distribution of developmental periods it is possible to conclude that this species reproduces in the summer, preferably in lagoons. Growth analysis indicated important alterations in larval morphology (metamorphosis) that may be associated with the ecomorphological characteristics of the species. The morphological separation of larvae of M. cf. gracilima from other larvae of small characids, especially at preflexion and flexion stages may be complicated by the overlap of traits, suggesting the use of other variables, mainly morphometric, for the separation of the species.