McDonough et al. Sexual differentiation and gonad development in Mugil cephalus 



611 



immature ovaries, and had distinct smooth 

 muscle layers (Fig. 14H). Any stromal tissue 

 left in the ovary at this point was also greatly 

 reduced and was essentially the mesentary 

 from which the lamellae were suspended. 



Discussion 



Age structure 



o 



The abundance of 1- and 2-year-old striped 

 mullet in South Carolina indicated that imma- 

 ture fish dominate the estuarine population. 

 The importance of proper age validation in 

 order to make comparisons of age and sexual 

 maturity cannot be understated. The most 

 important aspect of age validation is to obtain 

 a degree of precision that allows repeatability 

 in age determinations (Campana, 2001). The 

 periodicity of growth increment formation was 

 validated by marginal increment analysis, and 

 the precision of these age estimates was then 

 tested by comparing age counts of two inde- 

 pendent readers. 



Marginal increment analysis showed that 

 annual growth increments were deposited in striped 

 mullet in July in the entire data set, as well as sepa- 

 rately for ages 1-5. By validating increment periodicity 

 separately for different age groups, a consistent pat- 

 tern for the species can be determined (Campana et 

 al., 1995; Campana, 2001). The percent agreement be- 

 tween the two readers and a r-test for independent age 

 determinations allowed direct comparisons of the two 

 groups of ages for consistency (Campana et al. 1995). 

 However, these two methods were both independent of 

 the age of the species. Therefore, average percent error 

 (APE) was used to compare the different sets of ages 

 because it is not independent of the age of a species 

 (Beamish and Fournier, 1981). The low APE (0.41%) 

 found between the two different age estimates indicated 

 a high degree of precision, which allowed acceptance of 

 these age determinations. 



Sexual differentiation 



Striped mullet are gonochoristic and sex is genetically 

 determined. In contrast to mammals, gender of the 

 mature germ cells of teleosts present in the gonad rather 

 than the gender of the duct system forms the basis for 

 classifying an individual as male or female (Shapiro, 

 1992). Early duct structures of the undifferentiated gonad 

 characteristic of male development regress on female 

 development. Initial duct development, along with germ 

 tissue placement, takes on characteristics of the eventual 

 sex once the process of differentiation begins (Asoh and 

 Shapiro, 1997). Because of the plasticity of their gonad 

 development, striped mullet retain some characteristics 

 of the opposite sex (such as singular oogonia in males or 



Males 



Female 



MAY JUN JUL AUG SEP 



OCT NOV DEC 



Month 



I \\ 111; \l \K M'K 



Figure 10 



Mean gonosomatic index value by month for male and female 

 striped mullet (Mugil cephalus L.) from South Carolina estuaries 

 from 1998 to 2000. n = 455. 



duct-work in females) during the initial stages of differen- 

 tiation. The term that has been used to describe this phe- 

 nomenon is "intersex" (Yamamoto, 1969) but this state 

 could more accurately be defined as the hermaphroditic 

 stage of some gonochoristic species. Numerous descrip- 

 tions of intersex exist for teleosts (Atz,1964). Previous 

 studies have brought up the possibility of hermaphrodism 

 in striped mullet (Stenger. 1959; Atz, 1964; Moe, 1966); 

 however, there is only one example of a simultaneous her- 

 maphroditic striped mullet in the literature (Franks et 

 al., 1998). Once differentiation advances, these secondary 

 characteristics atrophy, and the gonad develops toward 

 the genetically determined sex. 



We found that at first annular increment deposition 

 (15-19 months), most (95%) immature striped mullet 

 were sexually differentiated. Chang et al. (1995), us- 

 ing cultured striped mullet, found that differentiation 

 began only after 12 months of age, and 70% to 90% of 

 immature fish at 15 to 17 months had differentiated 

 sexually. We found only a small percentage (1.2%) of 

 differentiated specimens at 12 months of age. Chang et 

 al.(1995) did not report fish sizes, and Stenger (1959) 

 studied sizes at sexual differentiation without reporting 

 age. Stenger (1959) concluded that striped mullet up 

 to 150 mm generally were not differentiated sexually. 

 We found four specimens in which differentiation had 

 occurred in the 126-150 mm size range, which repre- 

 sented specimens 12 months or less in age. Once our 

 specimens reached the 176-200 mm size range, just 

 over 60% had sexually differentiated, which was also 

 approximately the size range at which the first annulus 

 appeared (Wenner and McDonough 3 ). 



Chang et al. (1995) found that females differentiated 

 earlier than males; we, on the other hand, showed sex 



