POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



start on the 3rd precaudal vertebrae attached 

 to the parapophyses and to the tips of the haemal 

 arches. Caudal vertebrae lack pleural ribs and 

 their haemal arches continue as one haemal 

 spine. In transforming specimens, it is sometimes 

 difficult to distinguish pleural ribs from haemal 

 spines, but one can use the first large anal 

 pterygiophore as a demarkation point because it is 



always found in the interhaemal space anterior 

 to the first haemal spine. 



Knowledge of the variability of precaudal and 

 caudal arrangement and number of vertebrae is 

 important in the identification of specimens, par- 

 ticularly in the 8- to 14-mm-SL size range where 

 fewer characters are available. However, great 

 differences in vertebral variability exist from 



THUNNUS ATLANTICU8 



STANDARD LENGTH (mm) 



STANDARD LENGTH (mm) 



S 



u 



UJ 20' 



THUNNUS AIALUN6A 

 n:119 



Z 15- 



5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 SC as 90 



19 24 29 34 39 44 49 54 59 64 69 

 STANDARD LENGTH (mm) 



74 79 a4 89 94 



THUNNUS THYNNU8 

 n:1S9 



I I I I 



15 20 25 30 35 40 45 50 55 60 65 70 75 aO aS 90 95 100 105 110 115 



I , I I I I I I I I I I I I I I ) I t I ) I 



14 19 24 29 34 39 44 49 54 59 64 69 74 79 a4 a9 94 99 104 109 114 120 



STANDARD LENGTH (mm) 



Figure 2. — Length distribution by species of all specimens examined for this study. 



565 



