Histological and ultrastructural characterization of the gonads of the grunting toadfish allenbatrachus grunniens (Linnaeus, 1758) from the pranburi river estuary, Thailand

Information on the reproductive biology of toadfish remains limited. In this study, we examined the structure and development of gonads in the grunting toadfish Allenbatrachus grunniens (Linnaeus, 1758) using morphological, histological and ultrastructural methods. The fish were collected from the Pranburi River Estuary, Thailand, during the annual reproductive period for this species (January to December 2018). The ovary of this species was paired and had elongated sac-like structures parallel to the kidneys and the digestive tract. In females, we identified 4 oocyte differentiation phases in the ovary including oogonia proliferation phase, a primary growth phase that was further classified into 2 steps (perinucleolar and oil droplets-cortical alveolar steps), and a secondary growth phase that also contained 2 steps (secondary growth and full-grown oocyte steps) and post-ovulatory phases, indicating an asynchronous pattern in ovarian development for this species. Transmission electron microscopy showed the 4 layers including the zona pellucida, basement membrane, granulosa cells and theca cells, all of which initially appeared in the oil droplets-cortical alveolar stage. The zona pellucida and the granulosa cells were highly developed during the secondary growth stage. The granulosa cells contained abundant smooth endoplasmic reticulum near the mitochondria. In males, the spermatogenesis was classified into spermatogonium to spermatozoa. Finally, we associated the morphological gonadal developments (stage I-IV) and the gonadosomatic index (GSI value) with the cellular developmental processes in both sexes. These results help integrate various levels of reproductive observations, which will be applied to understanding the reproductive cycle and development for aquaculture.