NOTE Fitzhugh and Hettler: Postovulatory follicle degeneration in Brevoortia tyrannus 



569 



individuals and the male to female ratio was about 

 equal. In December 1990, only 12 individuals were 

 induced to spawn at the coldest temperature regime 

 (7 were subsequently identified as females). For each 

 group, acclimation and holding temperatures were held 

 constant prior to spawning and for the week following 

 spawning. 



The time of spawning was estimated the following 

 morning when eggs were collected and identified to 

 stage (Ferraro, 1980). Males were identified by the 

 presence of free-flowing milt just prior to the CP in- 

 jection and were marked by clipping the left pecto- 

 ral fin. Females (identified as not having a fin clip) 

 were sampled up to 4 days following spawning and 

 killed in a solution of MS-222. We excised a tissue 

 section from the middle right ovarian lobe from each 

 female and fixed it in 10% neutral buffered forma- 

 lin. Tissue samples were dehydrated, embedded in 

 paraffin, sectioned, stained with Gill's hematoxylin and 

 eosin for histological observation. Our interpretation 

 of the histological states of the postovulatory follicles 

 follows Hunter and Macewicz (1985), and the promi- 

 nent features we observed are summarized in Table 1. 



Results and discussion 



The postovulatory follicle, which is the evacuated 

 follicle remaining in the ovary following ovulation of 

 the hydrated oocyte, is characterized by an outer 



Table 1 



Distinguishing features for the stages of postovulatory fol- 

 licle degeneration' in Atlantic menhaden, Brevoortia 

 tyrannus. 



Stage Characteristics of postovulatory follicles 



1 Granulosa cells are aligned and granulosa-layer 

 nuclei appear linear in orientation. Some 

 lymphocytes and vacuoles may appear in the 

 postovulatory follicle, signaling initial 

 degeneration. 



2 Loss of linear arrangement of granulosa layer 

 nuclei; cell membranes and columnar/cuboidal 

 shape of granulosa layer is no longer distinct. 

 Lumen is still clearly visible. 



3 Although irregular postovulatory follicle shape 

 is still detectable, fewer folds are apparent as the 

 lumen becomes reduced and is no longer distinct. 



4 Linear appearance of the granulosa layer is no 

 longer distinct; postovulatory follicle not readily 

 distinguished from atretic oocytes. 



' Adapted from Hunter and Macewicz, 1985. 



thecal layer and an inner epithelial granulosa layer 

 (Fig. 1). Many species show similar features in 

 postovulatory follicle degeneration (Hunter and 

 Macewicz, 1985) to those that we noted for Atlantic 

 menhaden. Onset of degeneration was evident when 

 vacuoles and eosinophils were detected within the 

 postovulatory follicle (Fig. 1A). The granulosa layer 

 appeared contorted or folded but the granulosa cells 

 and their nuclei imparted a linear or cordlike ap- 

 pearance (Fig. 1A). Initially, the lumen was appar- 

 ent but narrowed and became less distinct with time 

 (Fig. 1, B and C). We also noted that the linear ar- 

 rangement of the granulosa cell nuclei became less 

 distinct with time (Fig. IB). Vacuolization of the fol- 

 licle increased (Fig. 1C), and a point was reached 

 when the lumen was no longer evident. The 

 postovulatory follicle became reduced in size but the 

 folded appearance of the granulosa layer remained 

 evident and aided in identifying the postovulatory 

 follicle (Fig. 1C). We used this point in degeneration 

 to define the postovulatory follicle duration because 

 as the postovulatory follicle aged further (e.g. Fig. 

 ID), there were no exact features that could distin- 

 guish it from old atretic follicles (preovulatory fol- 

 licles) that remain after an intact oocyte undergoes 

 atresia (Goldberg et al., 1984; Hunter and Macewicz, 

 1985). While atresia of oocytes occurs predominately 

 at the end of a spawning season, it is common to ob- 

 serve some oocytes undergoing atresia throughout 

 the spawning season (Hunter and Macewicz, 1985). 

 We also observed some atresia of vitellogenic oocytes 

 in our samples. Since preovulatory follicles can be 

 present and at some point are not distinguishable from 

 postovulatory follicles, we can only estimate the dura- 

 tion that the postovulatory follicles may be identified. 

 The 54 females sampled after spawning from the 

 three temperature regimes ranged in size from 200 

 to 250 mm fork length which is typical of 2 to 3 year- 

 olds (Nicholson and Higham, 1964). All females 

 (rc=35) from the warm regime (19-20°C) that were 

 sampled within about 40 hours of spawning (deter- 

 mined from the collection and identification of stages 

 of fertilized eggs) displayed detectable postovulatory 

 follicles (100% spawned). Eleven of the twelve fe- 

 males sampled from the 17.9-18.2°C series displayed 

 postovulatory follicles (92% spawned). Six females 

 from the cold series (14.8-17.7°C) contained 

 vitellogenic oocytes, indicating an advanced repro- 

 ductive state; one female possessed cortical alveolar 

 stage oocytes — an immediate precursor to the 

 vitellogenic stage (deVlamming, 1983). However, only 

 4 of 7 females from the cold series possessed detect- 

 able postovulatory follicles (57% spawned). The re- 

 duced temperatures appeared to diminish our abil- 

 ity to induce spawning. 



