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



607 



age (2.5% ) of immature males were found in size classes 

 greater than 325 mm. Male striped mullet showed 50% 

 maturity at 275 mm, and 100% maturity by 350 mm. 



Oogenesis began in specimens as small as 291 mm 

 (Fig. 7B) and there were 15 females below 325 mm 

 undergoing oogenesis (4.5% of all developing females!. 

 Ovaries were found in three small females (<300 mm). 

 Immature females were not found larger than 400 mm 

 or older than 3 years. All females greater than 400 

 mm were mature, regardless of their age. The majority 

 of females over 425 mm (88.3%) were developing and 

 found only in the fall. No ripe female striped mullet 

 were found. Ovarian atresia was found from December 

 through May. Female striped mullet showed that 50% 

 maturity was reached at 325 mm, and 100% maturity 

 occurred in specimens 400 mm. 



Gametogenesis occurred in each sex between the first 

 and second year (Fig. 8). However, the majority of speci- 

 mens at age 1 (65%) were immature. Males and females 

 showed 50% maturity at 2 years. Males showed 100% 

 maturity at age 4 and females at age 5. Running, ripe 

 males were first observed at age 1 but were found in 

 much greater numbers at ages 4-6. Resting males oc- 

 curred in every age class except age 6 (Fig. 8A). The 

 abundance of males aged 3 and older was far lower 

 (by at least an order of magnitude) than that of 1- and 

 2-year-olds (Fig. 8A). Atretic ovaries were found in all 

 age classes, and resting females were found in every 

 age class except age (Fig. 8B). 



Maturity stages by month showed immature and rest- 

 ing (but sexually mature) male and female striped mul- 

 let occurred in every month (Fig. 9). Developing males 

 were found from August through February, and run- 

 ning, ripe males from October through February. Males 

 (atretic) were found from November through March. De- 

 veloping females occurred from August through April. 

 Mean monthly GSI for males and females showed notice- 

 ably increased gonad size in November and December, 

 and obviously enlarged gonads occurred from October 

 through March (Fig. 10). 



Histological descriptions: undifferentiated juveniles 



The primordial gonad lobes were suspended by mes- 

 entery connected dorsally to the peritoneum and were 

 attached ventrally to the intestines (Fig. 11A). Gonads 

 from specimens <100 mm were identifiable only through 

 histological examination of whole-body cross sections. 



The gonads in specimens less than 50 mm had lobes 

 ranging from 70 to 100 ,um in length (Fig. 11B). Lobes 

 were made up of somatic cells and a peripheral germi- 

 nal epithelium. The lobes were attached along their 

 dorsomedial surface by loose fibrous connective tissue, 

 known as stromal tissue. No defining male or female 

 characteristics were present at this fish length. 



In specimens ranging from 50 to 100 ,i<m gonad lobes 

 had increased to 150 um and appeared more vascular- 

 ized (Fig. 11C). The lobes were attached to the suspen- 

 sory mesentery, which was attached to the peritoneum. 

 A few deuterogonia were visible along the lateral pe- 



S 40 



^m Undifferentiated 

 i i Male 

 ■n Female 



Figure 6 



Sex by age class in years. Age is the number of annuli 

 present on the sagittal otoliths. rc = 6284. 



riphery of each lobe. The remainder of the lobes con- 

 tained somatic tissue. The individual germ cells were 

 approximately 5 ;im in size. In some specimens, somatic 

 tissue was beginning to form bands that would later 

 develop into ductwork. 



In specimens ranging from 100 to 150 mm, the gonad 

 lobes were obviously vascularized and had attained a 

 size of 200 to 300 um (Fig. 11D). Early ductwork was 

 beginning to become evident. Deuterogonia were en- 

 larging and forming nests along the lateral and distal 

 portions of the lobes. Somatic cells made up a large por- 

 tion of each lobe and the stromal tissue was now more 

 stalklike, attaching each lobe to the suspensory tissue. 

 There were only four specimens in this size range that 

 had started to differentiate as males. Gonads destined 

 to be males were identified by duct structures within 

 the gonad lobe as well as by more elongated germ cell 

 nests. These morphologically distinct features resulted 

 in an early demonstration of the corradiating pattern of 

 ducts and lobules seen in more advanced testes. 



The 150 to 200 mm size class showed that 0.2% 

 of females and 37.3% of males began initial differ- 

 entiation, but the majority of all specimens (62.5%) 

 remained undifferentiated. The undifferentiated go- 

 nads had become larger, and lobe size was 600 to 800 

 um (Fig. HE). There was increased vascularization, 

 particularly along the medial portion of the stroma. 

 Germ-cell nests were now more organized, with 4 to 8 

 cells visible in each. 



More than 83% of specimens >200 mm had become 

 sexually differentiated. The undifferentiated gonads 

 in specimens >200 mm were highly vascularized and 

 had both the presence of ductwork, rounded germ cell 

 nests, and lobule-like structures. In some cases, germ 

 cell nests that were characteristic of female precursors 



