Ontogeny of gonadal sex development relative to growth in fathead minnow

Abstract

Ovarian differentiation of fathead minnow Pimephales promelas occurred at between 10 and 25 days post‐hatch (dph)(8–11 mm fork length, LF, and 7–12 mg), and was characterized by the presence of meiotic cells in the centre of the gonad, location of the somatic cells at the periphery of the gonad and the formation of an ovarian cavity. In contrast with the developing ovary, in the presumptive testis somatic cells were scattered throughout the gonads and this was evident from 25 dph (fish >10 mm and >11 mg). In males, at 60 dph (15–26 mm and 39–220 mg) the efferent ducts (sperm ducts) were apparent and the testis lobules started to form, but germ cells (spermatogonia) did not enter meiosis until between 90 and 120 dph. Fish of both sexes reached full sexual maturity at between 120 and 150 dph (males: 33–59 mm and 400–2895 mg; females: 24–48 mm and 160–1464 mg). Differences in body size (LF and mass) between males and females were only apparent when the fish were approaching full sexual maturity (120 dph).