HOWELL: SEASONAL CHANGES IN YELLOWTAIL FLOUNDER OVARIES 



frequency distribution was seen between early and 

 late maturing oocytes from October through Decem- 

 ber. The continued growth of late maturing oocytes 

 caused the distributions to become discontinuous by 

 February (Fig. 8). 



Late maturing oocytes were present from October 

 through June (Table 4). Their abundance increased 

 steadily from September through January, remained 

 fairly constant through April, and then declined sharp- 

 ly in May and June (Fig. 5). Their mean diameter in- 

 creased from October through June (Fig. 7). This 

 increase was reflected in their progressively larger 

 size-frequency distributions (Fig. 8). 



Hyaline oocytes were present in relatively small 

 percentages from April through June (Table 4). 

 Their mean diameter was about 400 ju.m (Table 5). 

 Corpora atretica (Regressing Type I) were seen from 

 November through May in very small percentages 

 (Table 4). There was a slight tendency for them to be 

 more abundant in January and February. 



DISCUSSION 



The developmental events observed in yellowtail 

 flounder oocytes are very similar to those described for 

 most other teleosts (see review by Wallace and Selman 

 1981). Development can be divided into two broad 

 phases. In the first, or previtellogenic phase, growth is 

 slow and comparatively few cytoplasmic changes oc- 

 cur. The second, or vitellogenin phase is characterized 

 by rapid growth and the deposition of large amounts of 

 yolk in the cytoplasm. The previtellogenic phase in- 

 cludes the oogonia, early perinucleolus, resting, and 

 late perinucleolus developmental stages. While 

 oogonia were found throughout the year, their abun- 

 dance tended to be somewhat higher from August 

 through October. Similar patterns of year-round 

 presence, with peak abundances in postovulatory 

 fish, have been reported by many others (Barr 1963; 

 Crossland 1977; Htun-Han 1978; Khoo 1979). Since 

 oogonia represent the initial stage in the process of 

 oogonesis, and thus the reserve from which all 

 oocytes will eventually develop, the timing and loca- 

 tion of their production are of considerable interest 

 (see review by Tokarz 1978). Braekevelt and 

 McMillan (1967), studying the brook stickleback, 

 Eucalia inconstans, suggested that they arose mitoti- 

 cally from residual oogonia that remained in the 

 ovary from year to year. Bowers and Holliday (1961) 

 concluded that in the herring {Clupea harengus), 

 oogonia were derived annually from primary germ 

 cells, while others including Wheeler (1924), Yama- 

 moto (1956a), and Foucher and Beamish (1980) 

 working with Pleuronectes (= Limanda) limanda, 



Liopsetta obscura, and Merluccius productus , respec- 

 tively, concluded that at least some oogonia arose 

 from follicle cells following ovulation. In addition to 

 the site of production, the life history stage during 

 which production occurs may differ from species to 

 species. Many investigators (Barr 1963; Shirokova 

 1977; Htun-Han 1978; Monaco et al. 1978) have ob- 

 served mitotic activity in oogonia of mature fish, sug- 

 gesting that a new stock of oogonia arises during each 

 reproductive cycle. Hickling (1935) and Yamamoto 

 (1956b) saw no evidence of mitotic activity and con- 

 cluded that the total reserve stock of oogonia had 

 been produced prior to sexual maturity. While no 

 mitotic divisions were apparent in this study, the fact 

 that oogonia were usually present in small groups is 

 an indication that such divisions were occurring but 

 were overlooked due to the very small size of the 

 oogonia. Furthermore, if the total reserve fund of 

 oogonia, representing all future oocytes, were pres- 

 ent in the ovary of a fish as fecund as yellowtail floun- 

 der (Howell and Kesler 1977), it seems likely that 

 their abundance would have been considerably high- 

 er than observed. Because of these observations, and 

 the seasonal changes in the abundance of oogonia, it 

 seems reasonable to conclude that a new stock of 

 oogonia is produced each year in yellowtail flounder, 

 primarily in the months following spawning. 



The sharp increase in abundance of early perinu- 

 cleolus oocytes following spawning indicates that 

 oogonia were rapidly being transformed into early 

 perinucleolus oocytes at this time. Since few inter- 

 mediate types were observed, it must be assumed 

 that the transition was rapid. 



The coincidental decline in the percentage of early 

 perinucleolus oocytes and the increase in resting 

 oocytes seen in September indicate that some early 

 perinucleolus oocytes are converted into resting 

 oocytes at this time. This is further indicated by their 

 overlapping size-frequency distributions and their 

 cytological similarity. This transformation was ac- 

 companied by a division of the cytoplasm into two 

 concentric zones. Similar cytoplasmic zonation has 

 been noted in a variety of species including Clupea 

 harengus (Bowers and Holliday 1961), Gadus mor- 

 hua callarias (Shirokova 1977), Gadus merlangus 

 and G. esmarkii (Gokhale 1957), Liopsetta obscura 

 (Yamamoto 1956a), and Pleuronectes (= Limanda) 

 limanda (Wheeler 1924). Recent studies (see review 

 by Guraya 1979) suggest that this apparent zonation 

 may be due to aggregates of ribonucleoprotein par- 

 ticles having been extruded through the nuclear 

 membrane. When these aggregates become sur- 

 rounded by cytoplasmic organelles (not seen in this 

 study) they are variously known as "yoke nuclei" or 



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