The present study describes the external morphology and morphometry of the initial development of Atherinella brasiliensis, and contributes toward broadening knowledge on its biology. A total of 88 larvae and 14 juveniles were used to characterize the morphological development and analyze body proportions. Fish with standard lengths (SL) from 1.4 to 77 mm were used in the study. Larvae hatch at an average of 1.4 mm SL. In the preflexion stage, the larval body is enveloped by a finfold, which begins behind the head. Pectoral fins are the first to be formed and preflexion larvae have four characteristic dendritic chromatophores on the head. The flexion stage begins at an average of 4.4 mm SL; dorsal and anal fins already exhibit pterygiophores and a terminal, somewhat up-turned mouth. At 6.8 mm SL, the flexion stage ends. In the postflexion stage, larvae present greater ossification of the dorsal and anal fin rays, exhibit pelvic fin buds and a darkening of lateral pigmentation. At an average of 8.8 mm SL, head pigmentation intensifies and pelvic fins exhibit conspicuous ossifying rays. Larvae at 11.8 mm SL have all fins formed; the second dorsal fin is the last to be formed at an insertion point posterior to the anal fin. The juvenile period begins at approximately 12 mm SL. At this stage, A. brasiliensis has an anal fin located at the median portion of the body and the posterior end of pectorals surpasses the origin of pelvic fins, which are located at the midpoint between the pectoral and anal fins. Scales are present on the dorsal-lateral region behind the head. The morphological features of A. brasiliensis herein described allow an adequate identification of its larvae and differentiate them from hemiramphid and other atheriniform larvae, thus broadening knowledge on the larval biology of the species.

The considerable similarity in the early life stages of different fish species makes egg and larvae identification in fishery biology and ichthyoplankton surveys a difficult task. Knowledge on early larval development of morphologically similar taxa and species-rich orders, such as Clupeiformes, mainly in the Neotropical fresh waters is rather limited. The aim of the present study was to describe morphological and meristic aspects of the larvae and early juveniles of Anchoviella vaillanti, an endemic species of the São Francisco River basin in Brazil. The characterization was based on an ontogenetic series of 132 individuals (1.3-51.0 mm SL). In the larval period, body varies from elongated to very elongated and the head is small, which is typical of Clupeiformes. The finfold is present beginning in the yolk-sac stage, when larvae have a large yolk sac, until the flexion stage. Pectoral fin buds are the first to form during the preflexion stage, when dorsal-and anal-fin pterygiophores and hypural bones are first visible. The total vertebra count ranges from 36 to 39 and the myomere number ranges from 31 to 45. Complete fin formation obeys the following sequence: anal and dorsal fins during flexion stage; and pectoral, pelvic and caudal fins during postflexion stage. Despite being the only freshwater clupeiform representative in the São Francisco River, A. vaillanti may occur sympatrically with A. lepidentostole in the lower stretches of the river basin. Although early larvae characteristics of A. lepidentostole are not known, its late larvae and early juveniles may be distinguished from those of A. vaillanti, by the higher number of dorsal-fin rays (15 or 16 vs. 12 or 13 in A. vaillanti), higher total vertebra count (40 vs. 37 to 40) and shorter pre-pectoral length (14 to 16 vs. 22.8 to 28.9% SL).A grande semelhança entre larvas de diferentes espécies de peixes torna a identificação de ovos e larvas em estudos de biologia pesqueira e ecologia do ictioplâncton uma tarefa difícil. O conhecimento sobre o desenvolvimento inicial de táxons com grande similaridade morfológica no período larval e riqueza taxonômica, como Clupeiformes, em particular daqueles de água doce na região neotropical, é bastante limitado. Neste sentido, o presente trabalho contribui para a redução desta lacuna, através da descrição morfológica e merística de larvas e juvenis iniciais de Anchoviella vaillanti, espécie endêmica da bacia do rio São Francisco. A caracterização foi realizada a partir de uma série ontogênica de 132 indivíduos (1,3-51,0 mm CP). No período larval, o corpo varia de muito alongado a alongado e a cabeça é pequena, características de Clupeiformes. A nadadeira embrionária está presente desde o estágio larval vitelino, no qual as larvas apresentam um saco vitelino grande, até o estágio de flexão. Os botões das nadadeiras peitorais são os primeiros a surgir, no estágio de pré-flexão, quando surgem também os pterigióforos das nadadeiras dorsal, anal e os ossos hipurais. O número total de vértebras varia de 36 a 39 e de miômero...