Morphophysiological development, allometric growth, and early ontogeny of the Amazonian ornamental characid Moenkhausia oligolepis (Günther, 1864)

This study describes, for the first time, the morphophysiological development, allometric growth patterns, and the main initial ontogenetic events of Moenkhausia oligolepis , a species of ecological and ornamental importance in the Amazon region. Hatching occurred 16 hours after fertilization, under an average temperature of 25.36 ± 0.64°C with a pH of 7.28 ± 1.11. From hatching onward, the larvae were sampled at regular intervals (0–6 hours post-hatching with collections every 2 h; 6–22 h every 4 h; and subsequently, daily collections) for morphological, morphometric, meristic, and histological analyses. Larval development spanned from the vitelline stage to the post-flexion stage (0–30 days after hatching—DAH), with a well-defined sequence of events: mouth opening and initiation of exogenous feeding at 3 DAH, cannibalistic behavior at 4 DAH, notochord flexion at 15 DAH (mean CP = 6.33 ± 0.56 mm), and the larval–juvenile transition at 30 POE with the development of fin rays and the appearance of scales. Allometric analyses revealed positive cephalic growth relative to standard length (b = 1.389 for HL/SL), indicating prioritization of the development of structures associated with feeding and sensory perception. Notochord length and preanal distance showed growth close to isometry relative to standard length, while body height exhibited negative allometry. Histological analysis demonstrated progressive differentiation of the digestive tract, with the presence of a buccopharyngeal cavity, esophagus, segmented intestine (anterior, middle, and posterior regions), and mucosal cells at 3 DAH. Cytoplasmic vacuolization was observed in hepatocytes starting at 23 DAH, associated with the presence of digested material in the intestinal lumen. The secondary lamellae of the gills exhibited evident buds at 4 DAH, and the swim bladder was divided into two regions at the end of flexion (22 DAH). Gonadal evaluation indicated undifferentiated gonads up to 50 DAH and ovarian differentiation at 60 DAH, characterized by the presence of primary meiotic oocytes; testicular differentiation was not observed during the analyzed period. Based on the observed morphological and ontogenetic characteristics, four larval stages were described (yolk sac, pre-flexion, flexion, and post-flexion). The results expand knowledge of the species’ early biology, providing relevant insights for taxonomic, ecological, and conservation studies, as well as for improving management and larviculture strategies for native ornamental fish.