Histological description of the early gonadal development of Arapaima gigas, paiche

Arapaima gigas, paiche, is the largest freshwater fish species in the South American Amazon. Despite the importance of an accurate understanding of the species' gonadal sexual differentiation mechanisms, the gonadogenesis process is still not well studied. The aim of this study was to determine the histological characteristics of the early gonadal development of paiche fry, belonging to the same “levante” (“levante” is the word used to describe the removal of fry from the male paiche). The fry was sampled during the month of September 2016; from the 5‐day post “levante” (dpl) to the 51 dpl with a minimum size of 2.7 cm and a maximum size of 16.0 cm in total length. The histology showed a large concentration of cells in the dorsal part of the abdominal cavity in 5 dpl fry, being greater in the left side. In addition, the gonadal ridges appeared at 6 dpl and the gonadal primordia appeared at 7 dpl. The two gonadal primordia were observed with different sizes, where the left primordia were larger and more basophilic than the right primordia. Finally, gonad differentiation between presumable females and males were observed with some structural differentiation between presumable females and presumable males since 43 post hatching day.