Abstract
Of the ca . 500 known hermaphroditic fish species, bidirectional sex change and simultaneous hermaphroditism are currently known in 69 and 57 species, respectively. Both bidirectional sex change and simultaneous hermaphroditism are predicted to evolve when mating opportunities are limited, such as in cases of low‐density distribution and low mobility of individuals. However, the plasticity of sex is adaptive in obtaining mating opportunities, especially when there is mate loss. Pair‐rearing experiments and histological observations of gonads of the goby Lubricogobius exiguus , which has low‐density distributions and low mobility, showed bidirectional sex change. The male‐role individuals in pairs had gonads in which only the testis was functional (male‐phase), whereas the female‐role individuals had two types of gonads: only the ovary was functional (female‐phase) or both testis and ovary were functional (simultaneously hermaphroditic phase, SH‐phase). In addition, single‐rearing experiments showed SH‐phase gonads in all individuals, and some cyclic spawning but no self‐fertilization occurred. These results revealed that L. exiguus has an unusual sexual pattern among hermaphroditic fishes because they undergo bidirectional sex change and some maintain SH‐phase gonads. These findings indicate that the low‐density distribution and low mobility of L. exiguus in their natural habitat may have influenced the evolution of this unique sexual pattern.