Testicular structure and development of the male germinal epithelium in the freshwater drum Aplodinotus grunniens (Perciformes: Sciaenidae) from the Usumacinta River, Southern Mexico

The present study describes the morphological changes in the male germinal epithelium of the freshwater drum Aplodinotus grunniens inhabiting southern Mexico. To our knowledge, this is the first study to provide a detailed description of the testicular structure and morphological changes of the germinal epithelium of this species. Based on the histological characteristics of the germinal epithelium, four main reproductive phases were described in A. grunniens males: Developing, Spawning Capable, Regressing and Regenerating. The germinal epithelium varied from a proliferative continuous germinal epithelium during the regenerating and development phases to a sperm storing discontinuous germinal epithelium during the Spawning Capable and Regressing phases. The seminiferous lobules ranged from a simple unrestricted lobular organization to an anastomosing‐like testis that transformed the testis of A. grunniens into a sperm storage organ. Our study provides valuable information about the morphophysiological aspects of the testis of A. grunniens that may be used for further investigations on the testicular physiology of this species or for comparative studies on gonad morphology. The description of the testicular cycle of the freshwater drum provided here will allow the reproductive staging of male individuals for a variety of purposes either for fisheries management or for the incipient aquaculture of this species.