Evolution of Sexual Expression in Fishes: Relation between Sexual Difference and Sex Change

1.在冲绳岛静子岛上，对黑线鱼(Halichoeres Blackurus)和二线鱼(Labroids Dimidiatus)两种鱼类的性别差异和性别变化进行了野外和水族馆实验。在黑腹鱼中，不仅在身体大小和颜色上存在性别差异，而且在腹鳍和尾鳍长度上也存在性别差异。由于雄性相互领地保卫产卵地点，因此体型对雄性交配成功最为有效。雌性更喜欢大而明亮的雄性，而不是小而单调的雄性。当雄性被移出它们的领地时，体型最大的雌性会改变性别。性行为先发生变化，然后是性腺，最后是体色变化。雄性丧失后，雌性选择体型最大的雌性作为配偶，雌-雌性产卵经常发生在不受精的情况下释放卵子。最大的雌鸟表现出完全的雄鱼行为，但当雄鸟很快被送回时，它们可以作为雌鸟产卵。同样，在体色上没有性别差异的情况下，当雄性被移除时，最大的雌性与较小的雌性一起产卵。两个物种中体型最大的雌性都应尽快扮演雄性角色，以保持其配偶，直到完成性腺性别转换。这表明，择偶机制不仅可能影响性别差异的进化，而且可能影响性别变化的过程。

Field and aquarium experiments were carried out on sexual difference and sex change in two labrid fishes, Halichoeres melanurus and Labroides dimidiatus, on Sesoko Island, Okinawa.1. In H. melanurus, sexual differences were found not only in body size and color, but also in ventral and caudal fin length. Since males were territorial each other to defend spawning sites, body size was most effective on males' mating success. Females preferred large bright males than small drab ones.2. When males were removed from their territories, the largest females changed sex. Sexual behavior changed first, then gonad, and lastly body color changed.3. After male loss, females chose the largest female as their mate, and female-female spawning often occurred releasing eggs without fertilization. The largest female performed complete male behavior, but could spawn as female when the male was returned soon.4. Also in L. dimidiatus that had no sexual difference in body color, the largest female spawned with smaller females in male role when their male was removed. The largest females in both species should take male role as soon as possible to keep their mates until completion of gonadal sex change.5. It is suggested that mate choice mechanisms may affect not only on the evolution of sexual difference but also on the process of sex change.