Histological analysis of T. thynnus gonad tissue. Transverse sections of mature T. thynnus gonads at different stages as per Itano et al. and Schaefer designations. (A) Stage 6 mature ovaries displaying fully yolked oocytes (fy), oil droplets (od), nucleus (n) and post ovulatory follicles (po), scale bar = 100 μm. (B) Stage 4 mature ovaries exhibiting a high proportion of unyolked oocytes (uy) and fully yolked oocytes undergoing atresia (at), scale bar = 100 μm. (C) Mature unrestricted spermatogonial testis displaying spermatogonia (sg), spermatocytes (ss), spermatids (sm) and spermatozoa (s), scale bar = 20 μm.