Structural changes in the ovotestis of the bidirectional hermaphrodite, the blue-banded goby (Lythrypnus dalli), during transition from ova production to sperm production

Hermaphroditism is widespread in fishes and has been extensively studied in many groups. The process whereby the gonad changes morphology in sequential hermaphrodites (i.e., female to male sex change or the reverse) is well documented. However, gonadal changes in bidirectional hermaphroditic fishes, in which gamete production can shift repeatedly, has received little attention. Here we examine the transition process in the bidirectional sex-changer Lythrypnus dalli, as it goes from producing eggs to producing sperm. In the ova-producing phase, the majority of the ovotestis consists of vitellogenic oocytes. Gonad transition is initiated with the break-down and reabsorption of mature oocytes. This is then followed by the proliferation of spermatogenic tissue from the dorsal and ventral regions of the ovotestis. The proliferation of spermatogenic tissue continues until it makes up the majority of the ovotestis with the reminder of the gonad comprising previtellogenic oocytes. We were able to define five transitional stages characterized by the relative area of the ovotestis made up of oogenic, spermatogenic, and atretic tissue. In only one other species of bidirectional sex-changing fish, Eviota epiphanes , has similar examination of transitional morphology been performed. Lythrypnus dalli and E. epiphanes are both in the teleost family Gobiidae, and closely related. It has been hypothesized that these two lineages have independently evolved the ability to change sex. Here we provide a comparison of the transition process between L. dalli and E. epiphanes, which highlights the conserved and novel elements and provided insights into differences in their life histories.