Abundant males in populations of a selfing hermaphrodite fish, Rivulus marmoratus, from some Belize cays

The killifish Rivulus marmoratus is the only known selfing hermaphroditic vertebrate, and males of the species are usually exceedingly rare or non-existent in nature. Collections on several Belize cays in 1988 and 1989 yielded 13.5–24% males. Factors responsible for this unusually high proportion of males are currently not understood. Likewise, the biological significance of males in populations which otherwise consist of selfing hermaphrodites (with internal fertilization) is problematic and awaits further study.