The occurrence of spermatozoa in the ovary of nulliparous females of Heterandria formosa Agassiz, 1853 (Pisces, Poeciliidae).

The ovaries of more than 100 adult nulliparous females of the poeciliid Heterandria formosa were investigated by light- and electronmicroscopy. Among these females two hermaphroditic specimens were found. The gonads of the hermaphrodites do not differ externally from those of normal females. In semithin and ultrathin sections female and male parts of the gonad can be distinguished. The female parts are very dominant. The oocytes of the hermaphroditic gonads are in the stage of vitellogenesis. They are enveloped by a thin zona radiata. The heads of completely developed spermatozoa are aligned horseshoe-shaped. Oocytes and spermatozoa adjoin each other directly. Heterandria formosa is the only known abnormal hermaphroditic species of fish in which spermatozoa and mature eggs occur simultaneously in the gonad.