Larval and Juvenile Development of Rhaphiodon vulpinus (Pisces: Cynodontidae): Morphological Characteristics and Growth Patterns

Abstract

This study investigated the early development and growth patterns of Rhaphiodon vulpinus (Characiformes, Cynodontidae), a top predator relevant to subsistence and sport fishing in the Amazon basin. Morphological, meristic, and morphometric variations throughout early ontogeny were analyzed and discussed in relation to the species' ecomorphology and taxonomy. Larvae and juveniles were collected from riverine (Amazon, Japurá, Solimões, and Tapajós Rivers) and lacustrine systems (Maicá and Uruxi Lakes) in the Amazon basin between 2010 and 2023. Different collection methods were employed to maximize the capture of all early developmental stages. In the laboratory, R. vulpinus larvae and juveniles were sorted, identified, and classified according to developmental periods and stages. Additionally, all individuals were morphometrically measured, described regarding their body characteristics, and the initial growth pattern was analyzed using regression models. A total of 153 individuals were examined, with standard lengths ranging from 3.85 to 59.45 mm. R. vulpinus is altricial, with body shapes varying from elongated to moderate, and small to moderate head and eye sizes. The total myomere count ranges from 67 to 69, a distinctive feature of R. vulpinus among Cynodontidae species, along with a unique morphometric pattern throughout ontogeny. The early growth of R. vulpinus was characterized by breakpoints between the preflexion, flexion, and postflexion stages, indicating considerable body and physiological changes, such as adaptations for locomotion and predation. This study expands the knowledge of the early ontogeny of R. vulpinus, provides insights into morphometric patterns, and updates effective characteristics for identifying R. vulpinus larvae and juveniles.