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

本研究以日本冲绳县濑子岛的两种唇形科鱼类--黑尾海猪和半唇鲷为研究对象，进行了野外和水族箱实验。In H.黑尾鱼的性别差异不仅表现在身体大小和颜色上，还表现在腹鳍和尾鳍的长度上。由于雄鱼为了保护产卵地而互相争夺地盘，所以体型大小对雄鱼的交配成功率影响最大。雌性喜欢大而明亮的雄性，而不是小而单调的雄性。当雄性被赶出它们的领地时，最大的雌性改变了性别。性行为先发生变化，性腺再发生变化，体色最后发生变化.在失去雄性后，雌性选择最大的雌性作为它们的配偶，并且雌性-雌性产卵经常发生在不受精的情况下释放卵子。最大的雌性表现完全的雄性行为，但当雄性很快返回时可以作为雌性产卵.在L。在体色没有性别差异的dimidiatus中，当雄性被移除时，最大的雌性与较小的雌性在雄性角色中产卵。两个物种中体型最大的雌性应尽快承担雄性角色，以留住配偶，直至性腺性别转换完成.这表明，择偶机制不仅影响性别差异的演化，而且影响性别转换的过程。

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.