NOTES Ratty et al : Reproductive biology of Thunnus alalunga 



Figure 5 



Electron micrograph of South Pacific albacore sperma- 

 tozoa (scale indicates 1 ^im). 



The primary spermatogonia are the largest cells with 

 diameters of 12-16 jum. They have a prominent nucleo- 

 lus, are basophilic, and are found in cysts with vary- 

 ing numbers of individual cells. The cysts are usually 

 found singly near the periphery of the testis; a cyst 

 situated on the lobule wall is shown in Figure 4A. 



Primary spermatocytes are oval or spherical with 

 diameters of 8-12 yim. They have no visible nuclear 

 membrane and the chromatin material occupies most 

 of the cell. Figure 4B shows examples of primary sper- 

 matocytes. Secondary spermatocytes are very small, 

 spherical cells with diameters of 4-7 ^m. Unlike 

 primary spermatocytes, the chromatin is found in a 

 clumped condition, similar to the spermatids. The cells 

 occur in groups and are strongly basophilic. Figure 4B 

 shows secondary spermatocytes. 



Spermatids are strongly basophilic spherical cells 

 with diameters of 2-4 fim. As they mature, they become 

 smaller and the chromatin becomes uniformly con- 

 densed. Figure 4C shows that after detaching from the 

 lobule wall, the spermatids remain in dense clusters. 



The spermatozoa have a rounded nucleus and are 

 morphologically subdivided into head, neck piece, short 

 midpiece, and tail; as in other teleosts, there is no 

 acrosome. They measure 1-2 ^m in diameter, excluding 

 the tail. An electron micrograph of an individual sperm 

 is shown in Figure 5. 



Table 3 



Differences in meiotic activity between right 

 and left testes, Thunnus alalunga (n = 106). 



Meiotic activity 



Percent occurrence 



Left < right 

 Left = right 

 Left > right 

 No data 



10 

 26 

 61 



3 



Meiotic activity 



Relative abundances of sperm, spermatids, and sper- 

 matocytes were taken as a measure of meiotic activ- 

 ity. As previously discussed, the albacore testes were 

 generally asymmetrical in size with the right testis 

 more often larger than the left. However, we found 

 that meiotic activity and sperm-duct development 

 showed the opposite relationship. The left testis was 

 meiotically more active than the right in 61% of males 

 examined, the left and right testes had about equal ac- 

 tivity in 26%, and the right was more active than the 

 left in 10% (see Table 3). 



Examination of data for each pair of testes showed 

 a 60% occurrence of greater meiotic activity in the left 

 testis relative to larger size of the right testis. Other 

 combinations of meiotic activity and relative size of 

 testes showed a 15% or less occurrence (see Fig. 6). 



Relative abundance of sperm, spermatids, 

 and spermatocytes 



Absolute counts of the number of spermatogenic cells 

 would be exceedingly difficult to make. Instead we 

 estimated relative abundances of spermatocytes, sper- 

 matids, and sperm present using the categories none, 

 few, sonw, and many. The results show that for all sizes 

 of fish, the relative abundance of sperm was higher in 

 the left testis than the right testis. For example, there 

 were nearly 2 times more many and 2V2 times fewer 

 none categories of sperm observed for the left than the 

 right testis. In both the left and right testes of fish in 

 all sizes, relative abundance of sperm was highest in 

 the caudal portion and decreased towards the rostral 

 end (Figure 7). 



The findings are similar for spermatids and sperma- 

 tocytes, but the differences between testes are not as 

 pronounced as those for sperm. For all sizes of fish, 

 relative abundances of both spermatids and spermato- 

 cytes were higher in the left than the right testis. For 

 the left testis, there were about 20% and 30% more 

 many and 40% and 50% fewer none, categories ob- 

 served for spermatids and spermatocytes, respectively. 



