FISHERY BULLETIN: VOL. 72. NO. 2 



Caudal Fin 



(Table 6) 



At 8 mm SL, a total of 27 to 31 rays is 

 developed on the caudal fin. If the total was an 

 even number then an equal number of rays was 

 found dorsad and ventrad to the midline. If it was 

 an uneven number then the dorsal side of the 

 caudal fin always carried one more ray. As the 

 larvae grow, additional procurrent rays are added 

 equally on the caudal fin to the dorsal and ventral 

 side. The last rays to develop are the anterior- 

 most. There is a difference in caudal ray develop- 

 ment between T. thynnus and the other species. 

 Thunnus thynnus usually has fewer caudal 

 rays than the other species at all sizes, par- 

 ticularly from 14 to 22 mm SL. At 17 mm SL, 

 few T. thynnus have the maximum caudal counts 

 of more than 48 rays. At 23 mm SL, about one- 

 half of the T. thynnus specimens had maximum 

 counts and only after about 35 mm SL did all 

 but two specimens have maximum caudal counts. 

 Collectively, the remaining species differ from 

 T. thynnus in that, at 15 mm SL, a few speci- 

 mens had already acquired the maximum comple- 

 ment of caudal rays. At 18 mm SL, one-half 

 of the specimens had maximum counts, and all 

 but one specimen had the maximum counts at 

 24 mm SL. I noted three exceptions to the above 

 statements: one 34-mm-SL T. atlanticus had 48 



rays and two T. thynnus had 47 and 44 rays 

 at 40 and 50 mm SL, respectively. From my data 

 I believe that all Thunnus juveniles >24 mm SL 

 have a maximum of more than 48 caudal rays, 

 usually 49 to 51, rarely 52. I also believe the 

 maximum juvenile counts represent the adult 

 complement of caudal rays, although I did not 

 examine any adult fish. Frade (1931) found 46 

 caudal rays as the most frequent number for adult 

 T. thynnus. The difference is likely due to dif- 

 ficulty in counting the anteriormost rays on 

 adults. For this study, I did not attempt a detailed 

 examination of the principal caudal rays or of the 

 hypural complex. A study of the ontogeny of the 

 caudal skeleton in T. atlanticus will be published 

 in the future. 



Dorsal and Anal Fin Supports 



(Figures 8 to 10; Tables 7, 13) 



The spines, rays, and finlets of the dorsal and 

 anal fins are supported within the body by ptery- 

 giophores. The pterygiophores are made up of 

 two or three bony parts referred to as radials. 

 Spines and rays have a proximal and a distal 

 radial; finlets have an additional middle radial. 

 The distal radials of the posterior rays of the 

 second dorsal fin and finlets and of the anal fin 

 and finlets cannot be seen from a lateral view 

 because they are hidden by the bifurcate bases of 



Table 6. — Range of variation in total caudal fin ray counts for selected sizes 

 and species groups of juveniles of Thunnus. 



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