Nanmatsu et at: Reproductive biology of Dexistes nkuzenius 



639 



F=124.1, P<0.0001) and became significantly higher in 

 each successive stage of maturity (Fisher's PLSD test, 

 P<0.05), except for the first two phases (P=0.687). The 

 mature- and spent-phase ovaries were excluded from 

 the test because their values fluctuated depending on 

 spawning times or the degree of POF absorption. 



Ovaries in the late vitellogenic maturity phase, which 

 occurred from May to September, contained oocytes in 

 the tertiary yolk stage, secondary yolk stage, cortical 

 alveoli stage, and late and early perinucleolus stages, 

 but not in the primary yolk stage (Table 3). Ovaries in 

 the premature phase, which occurred from September to 

 October, also revealed two peaks and a hiatus in oocyte 

 developmental composition. As described before, ovaries 

 with POFs also contained maturing oocytes. These re- 

 sults show that this species is a multiple-spawner and 

 has group-synchronous ovaries (Wallace and Selman, 

 1981; Takano, 1989); therefore, fecundity is fixed before 

 spawning starts. 



On the other hand, ovaries in the mid-vitellogenic 

 phase were observed from January to September and 

 contained oocytes in the secondary and primary yolk 



stages, and in the late perinucleolus stage. Cortical 

 alveoli are very small and were present in only 10 of 

 the 309 ovaries observed in our study. It is possible 

 that the duration of this stage is very short. Therefore, 

 in the ovaries oocytes do not divide into two groups, 

 those that spawn in the next reproductive season and 

 those that do not, until they have progressed to the late 

 vitellogenic maturity phase. 



Oocyte composition 



Table 3 shows the annual changes in oocyte composi- 

 tion. One ovary observed in January contained POFs 

 and perinucleolus stage oocytes, whereas the others 

 contained oocytes in the primary and secondary yolk 

 stages. Of those observed from February to April, none 

 contained POFs. Frequency of occurence of ovaries 

 with secondary yolk-stage oocytes increased during the 

 season. From May to August the most advanced oocyte 

 observed was in the tertiary-yolk vitellogenic stage, and 

 the frequency of this stage also increased in number 

 throughout this season. Migratory-nucleus-stage and 



