LAUSR

The initial development of the red-bellied pacu Piaractus brachypomus is described using morphological, meristic and morphometric characteristics. A total of 127 individuals were analyzed (47 in the yolk-sac, 35 in preflexion, 5 in flexion, 20 in postflexion 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 approximately 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 highly, 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; 17-20 postanal). Through the morphometric relationships, it was evidenced that the greatest changes during the initial ontogeny of P. brachypomus occur in the transition from the postflexion stage to the juvenile period, indicating changes in behaviour, foraging, and physiology. This article is protected by copyright. All rights reserved.