Abstract
Sexual patterns of the sand diver Trichonotus filamentosus were studied using specimens collected at Hakata Bay, Kyushu, Japan, between 1986-1989. The species was found to be a protogynous hermaphrodite for the following reasons: (1) The males are not only larger in body size than females, but also have a different body form and color, with intermediate types occurring within a range where the male and female sizes overlap. (2) Differences in annuli in otoliths between the sexes suggest that males are older than females. (3) Bisexual gonads occur in 4.58% of the specimens examined histologically. (4) Bisexual gonads and mature testes exhibit secondary structure.
The size frequency distributions of the males and females, and the occurrence of specimens with bisexual gonads indicate that the sand diver of Hakata Bay undertakes protogynous sex change at 80-120mm SL, after the spawning season in summer. It has also been found that the present species is monandric, because all males have secondary testes.
