Sex-specific organs are one of the anotomical boundaries tha t a sex-changing fish must deal with. The gonads are subject to some of the most drastic change during the hermaphroditic process.
Recognizing the importance of histological studies, many studies have been to conducted to examine the structure of the gonads in hermaphroditic and gonochoristic fish (Nakamura et al.,1989; Cole et al.,1994; Godwin et al.,1996).
In Thalassoma duperrey, a protogynous wrasse, six sequential stages of histological change have been identified (Nakamura et al., 1989):
1) Female with no evidence of change 2) Degeneration of stage 3 and stage 4 oocytes (vitellogenic stages) 3) Degeneration of stage 2 oocytes (previtellogenic stage) 4) Proliferation of spermatogonia 5) Beginning of spermatogenesis 6) Production of sperm In protogynous parrotfish, the comparisons of different tissues makes the change easier to follow:
all drawings done by the author 
Immature Female Ovary Male Testis Testis Close-up Transforming Gonad Recently Transformed Male Mature Secondary Male During this process, the gonads experience structural changes of the walls as well as the ducts (Sadovy and Shapiro,1987). The whole process works because the gonads are relatively simple in terms of their structure. In this process the hormones responsible for carrying out this change have been identified, but those instigating this process have yet to be understood (Nakamura et al.,1989).
