Description of the Early Development of Hoplias intermedius (Günther, 1864) (Characiformes: Erythrinidae).

In this study, we provide a detailed description of the eggs, larvae, and juveniles of Hoplias intermedius, focusing on their morphological, meristic, and morphometric characteristics. Specimens were obtained through induced spawning using the hypophysation technique. The eggs (n = 82) were large and spherical, with a yellowish yolk, transparent chorion, and a small perivitelline space. During development, 135 larvae and 16 juveniles were examined, with standard lengths ranging from 5.48 to 39.00 mm. Larvae exhibited a terminal mouth, simple nostrils, spherical eyes, a large yolk sac, occupying approximately one-third of the body, and an intestine extending beyond the mid-body region. Pigmentation was initially sparse but became more intense during the preflexion stage. Juveniles already exhibited body morphology similar to that of adults. Fin rays counts varied as follows: pectoral (9-10), pelvic (7-9), dorsal (12-14), anal (10-13), and caudal (18-20). The total number of myomeres ranged from 36 to 44, with a modal distribution of 28 preanal and 16 postanal myomeres. Morphometric analysis revealed variation in body height (from elongated to moderate), head length (from small to large), and eye diameter (from moderate to large). Throughout development, increases were observed in snout length, head height, and distances from the snout to the pectoral, pelvic, dorsal, and anal fins. Growth analyzes indicated that most morphometric variables changed progressively during ontogeny, although some exhibited shifts in growth trajectories (breakpoints), likely associated with major developmental transitions such as yolk absorption and fin differentiation. These findings expand knowledge on Neotropical fish development. Specifically, the convergence of the lower margins of the dentary bones toward the mandibular symphysis provides a reliable diagnostic character to distinguish Hoplias intermedius from its congeners. This study offers essential morphological and meristic criteria for taxonomic identification, with direct implications for aquaculture and biodiversity conservation.