Ontogenia inicial do Tetra Olho-de-Fogo amazônico Moenkhausia oligolepis (Characiformes: characidae)

Abstract

This study describes, for the first time, the morphophysiological development, allometric growth patterns, and the main early ontogenetic events of Moenkhausia oligolepis, a species of ecological and ornamental importance in the Amazon region. Hatching occurred 16 hours after fertilization under a mean temperature of 25.36 ± 0.64°C and pH of 7.28 ± 1.11. From hatching onward, larvae were sampled at regular intervals (0-6 h post-hatching with collections every 2 h; 6-22 h every 4 h; and subsequently daily) for morphological, morphometric, meristic, and histological analyses. Larval development extended from the yolk-sac stage to post-flexion (0-30 days after hatching – DAH), with a well-defined sequence of events: mouth opening and the onset of exogenous feeding at 3 DAH, observation of cannibalistic behavior at 4 DAH, notochord flexion at 15 DAH (mean SL = 6.33 ± 0.56 mm), and the larva–juvenile transition at 30 DAH with the development of fin rays and the appearance of scales. Allometric analyses revealed positive cephalic growth in relation to standard length (b = 1.389 for HL/SL), indicating prioritization of the development of structures associated with feeding and sensory perception. Structures such as notochord length and pre-anal distance showed growth close to isometry, whereas body depth exhibited negative allometry. Histological analysis demonstrated the progressive differentiation of the digestive tract, with the presence of the buccopharyngeal cavity, esophagus, segmented intestine (anterior, middle, and posterior), and mucous cells at 3 DAH. Cytoplasmic vacuolization in hepatocytes was observed from 23 DAH onward, associated with the presence of digested material in the intestinal lumen. Secondary gill lamellae exhibited evident buds at 4 DAH, and the swim bladder showed division into two chambers by the end of the flexion stage (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 the knowledge of the early biology of this species, providing relevant information for taxonomic, ecological, and conservation studies, as well as for improving management strategies and larviculture of native ornamental fishes.