Intraspecific variation in the sexual pattern of red-belted anthias, Pseudanthias rubrizonatus.

Recent studies indicate that the sexual patterns of some species can vary with local population size and density, suggesting that protogynous species may shift to gonochorism in larger populations. Red-belted anthias Pseudanthias rubrizonatus, a protogynous hermaphrodite, forms diverse-sized groups in Kagoshima Bay, Japan (31.58°N, 130.66°E), from typical polygynous medium-sized groups with a few males and several females, to an unusually large group exceeding 10,000 individuals. This study aimed to determine the sexual pattern of P. rubrizonatus in different-sized groups. The research was conducted from July 2019 to June 2022. This study revealed the process from sexual differentiation to maturation, and discovered primary males through gonadal histology and otolith growth analysis. All juvenile gonads developed an ovarian lumen before sexual differentiation. Most differentiated to mature females (38.0-77.0 mm standard length [SL], 0-3 years, n = 197), then changed sex (52.0-79.3 mm SL, 2-3 years, n = 14) to secondary males. Sex change occurred year-round. All mature males (50.2-95.1 mm SL, 1-5 years, n = 83) exhibited distinct dimorphic and dichromatic features. Some juveniles underwent bisexuality around 100 days post-hatching; spermatocytes were found with primary-growth stage oocytes (25.5-46.9 mm SL, ≤1 year, n = 24). Primary males (50.2-65.5 mm SL, 1 year, n = 9) were collected only in the large group. These findings indicate intraspecific variation in sexual pattern depending on the group sizes, with diandric protogyny in the large group and monandric protogyny in medium groups. Diandric protogyny is unprecedented in Anthiinae. Although primary males emerge as group size increases, protogyny remains consistently maintained in P. rubrizonatus in Kagoshima Bay, with no evidence of gonochorism observed.