Biological parameters of the fish, gonadal state, and ultrastructure of oocyte envelopes and spermatozoa have been studied in Japanese goatfish Upeneus japonicus from Nha Trang Bay. The females are larger than the males: their body length–body weight ratios are significantly different. The ovarian state indicates continuous oogenesis. The average batch fecundity is 3301 oocytes. The females (50% of the fish) reach sexual maturity at a fork length of 104 mm. The egg envelope includes a single-layer zona radiata and a chorion. In the ovulated oocytes, their thickness is 2.0—2.5 and 0.1—0.3 μm, respectively. The spermatozoon head has a pointed bend in its apical sharpened part. Among other studied species of the family Mullidae, a similar spermatozoon structure is observed in U. tragula and U. heterospinus.
N. G. Emel'yanova (2025) studied this question.