Description of larvae and juveniles of Brycon falcatus (Characiformes: Bryconidae) from the Tapajós River sub-basin, Brazil

Brycon falcatus (Characiformes: Bryconidae) is an Amazonian fish species of great relevance in fisheries and as a seed disperser. However, knowledge about its early development is limited due to the lack of information for accurate identification. This study describes the larval and juvenile period of B. falcatus, using morphological, meristic, and morphometric data, and analyzing larval growth patterns. A total of 116 individuals, with lengths ranging from 4.02 to 72.83 mm, were analyzed, collected in the Tapajós River sub-basin, Amazon basin, Brazil. At hatching, larvae are poorly developed, gradually acquiring morphological and functional traits. In the yolk-sac larval stage, pigmentation is scarce, becoming evident in preflexion, a pattern that continues through flexion and postflexion stages. Juveniles display dark longitudinal stripes on the flanks, a humeral spot, and a spot on the caudal peduncle. The number of myomeres ranges from 42 to 44, and the formation of fin rays follows specific patterns. Growth is predominantly continuous isometric. Larvae differ from congeners by myomere number and pigmentation, juveniles by fin ray number and pigmentation. Finally, a taxonomic key for the identification of Brycon larvae from the Tapajós basin is provided. These data ensure identification and conservation of B. falcatus.