The initial ontogeny of black prochilodus (Prochilodus nigricans Spix & Agassiz 1829) was described through morphological, meristic and morphometric characters. The biological material used was from captivity. 41 eggs, 151 larvae and 10 juveniles were analysed. The eggs were spherical, transparent, nonadhesive and pelagic, with a wide perivitelline space and an average diameter of 2.62 mm. Hatching occurs between 17 and 18 h after fertilization. The standard length of larvae varied from 3.28 to 26.73 mm and that of juveniles from 28.99 to 42.90 mm. The larvae showed an elongated and fusiform body, a convex dorsal profile, a long intestine, extending beyond the median region of the body, a terminal mouth and fleshy lips. The complete sequence of fin rays formation was caudal, dorsal (I,11), anal (I,11), pelvic (9) and pectoral (I,16–17). The number of myomeres ranged from 44 to 46, with mode 45 (n = 122). Growth analyses indicated early and late developmental metamorphosis. P. nigricans presents embryonic and larval development characteristic of fish with telolecithal eggs, in which the newly hatched larva does not yet have all the organic systems fully formed, is limited to a few organs and is still dependent on the yolk for its nutrition.

Os recursos hídricos próximos à área urbana de Santarém/PA vêm sofrendo grandes impactos, podendo ser alterados por diversos fatores, seja por elementos naturais (geologia, vegetação, solos e clima) e principalmente pelo conjunto de atividades antrópicas, alterando sensivelmente os processos biológicos, físicos e químicos dos recursos hídricos. Devido à perda de quantidade e qualidade das águas com ocupação solo, disposição inadequada de resíduos e a destruição de sua vegetação ripária. Alguns estudos sobre a água feita em áreas urbanas mostram interferência antrópica de forma qualitativa e quantitativa nas suas características químicas, físicas e biológicas dos sistemas hídricos. Um dos principais tipos de poluição aquática é a que caracteriza-se pela presença de matéria orgânica na água, com isso, quanto maior a quantidade de material orgânico disponível, maior será a população de organismos que a decompõe, portanto, maior será a quantidade de oxigênio consumido. O estudo tem como objetivo determinar os parâmetros-físico químico e bacteriológicos indicadores de qualidade da água do canal principal de onze microbacias hidrográficas da área urbana e periurbana do Município de Santarém.

The initial development of the red-bellied pacu Piaractus brachypomus is described using morphological, meristic and morphometric characteristics. A total of 127 individuals were analysed (47 in the yolk-sac, 35 in pre-flexion, five in flexion, 20 in post-flexion and 20 in juvenile) with standard length varying between 2.92 and 48.61 mm. The larvae are born poorly developed and have a discoidal yolk at $6.33 mm standard length. During early ontogeny, the mouth passes from terminal to subterminal and the anal opening reaches the vertical line over the midline region of the body. There are changes in body shape from long and moderate to deep, head length from small to large, and eye diameter from moderate to large. Dendritic chromatophores were present in the ventral, dorsal and upper part of the swim bladder in the early larval stages. Rounded spots are evident all over the body in juveniles. The total number of myomeres ranges from 39 to 41 (20-23 pre-anal, 17-20 post-anal). Through the morphometric relationships, it was evidenced that the greatest changes during the initial ontogeny of P. brachypomus occur in the transition from the post-flexion stage to the juvenile period, indicating changes in behaviour, foraging and physiology.

We evaluated the structure of the ichthyoplanktonic assemblage (taxonomic composition, abundance, developmental stages, and relationship with local environmental variables) in marginal lagoons located in the clearwater Tapajós and Trombetas rivers, Amazon Basin, Brazil during periods of high water (rising waters and flooding). Samples were collected by means of horizontal drags in the subsurface of the water with a plankton net (300 µm mesh). Sorting, counting, identification of biological material, and classification of embryonic and larval stages were performed under a stereoscopic microscope. A total of 122 eggs and 2,813 fish larvae were captured and distributed in nine orders, 18 families, 20 genera, and 26 species. Of these, more than 75% are considered sedentary or short-distance migrants and more than 50% are of commercial interest. The results revealed fluctuating patterns of abundance and species composition between the lagoons of the two rivers. The ichthyoplankton community in the lagoons consisted mainly of larvae of native pelagic fish belonging to the Engraulidae family and larvae of benthic fish such as Eleotridae, making up more than 85% of the individuals captured in the study area. In addition to these two groups, some larvae of piranha species (Pygocentrus nattereri) stood out due to their high densities in the Tapajós river lagoons and croaker larvae (Plagioscion spp.) in the Trombetas river lagoons. The highest larval densities were recorded during the night with significant variations recorded in species composition. The presence of all stages of larval development indicates that these biotopes enable growth and are essential for the biological recruitment of fish species in the region. The ichthyoplankton showed a low correlation with the limnological variables, however, the patterns of abundance and composition of fish larval assemblages in the lagoons of the Tapajós and Trombetas rivers were preliminarily elucidated, although additional work is needed to assess the effects on survival and recruitment

The early ontogeny of Triportheus albus and T. angulatus, two fish species of Triportheidae, is described using morphological, meristic, and morphometric characters. These species are exploited by subsistence fisheries and have potential as an alternative source of fish, given the decline in the natural stocks of other commercially important fish species in the Amazon. The specimens were collected in the open water limnetic zone, under of the macrophyte stands, and in subsurface areas near sandbars in the Amazon basin. Intra and interspecific morphometric analyzes were performed to evaluate growth models between species. The combination of color pattern, body morphology, morphometric proportions and myomeres number distinguishes the species from each other and from other congeners. Some morphometric relationships related to head as snout length and eye diameter as well as length from the snout to the origins of anal and length from the snout to the origins of pelvic, related with standard length were different between the two species of Triportheus, reflecting different growth models between them. An identification key for larvae and juveniles of some species of Triportheus from the Eastern Amazon is presented.