Why Be a Both‐ways Sex Changer?

Sex change is a rather common phenomenon among aquatic animals, but only a few have been reported to change sex in both directions. In laboratory experiments we demonstrated that the coral goby, Paragobiodon echinocephalus , changed sex in both directions with the same likelihood. When the goby lost its mate in the field, it preferred changing sex in either direction over moving a long distance in search of a heterosexual mate. Change in social rank, which is likely to occur in many other hermaphroditic fish, corresponded exactly with the direction of sex change. This constitutes a new condition for the evolution of both‐ways sex change among plants and animals.