POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



to 12 mm SL. A total count of 23 rays was 

 obtained most of the time for the second dorsal 

 fin and finlets of all the species (Frade, 1931; 

 Bullis and Mather, 1956; Gibbs and Collette, 

 1967). Although the variability was greater 

 than that for the first spinous dorsal fin, it did 

 not exceed 6% in the second dorsal fin. I included 

 the dorsal finlet counts with ray counts of the 

 second dorsal fin because in larvae and juveniles 

 ofThunnus, and perhaps with all other scombrid 

 genera, it is impossible to determine precisely the 

 break between the last posterior second dorsal 

 fin softray and the first anterior dorsal finlet even 

 in cleared and stained specimens. Figures 3 to 

 6 show this gradual intergradation from softrays 

 to finlets. The last ray of the second dorsal fin 

 can be separated from the first finlet by their 

 pterygiophore structure in specimens larger than 

 50 mm SL. Finlet pterygiophores have two clearly 

 visible bony parts from a lateral view and second 

 dorsal fin pterygiophores have only one clearly 

 visible part. Figures 3 to 6 show specimens that 

 have not yet developed the two clearly visible 

 parts in their finlet pterygiophores. 



wide ranges narrow in the 25 to 29 mm SL size 

 groups where adult counts of more than 30 rays 

 are attained (Frade, 1931; Bullis and Mather, 

 1956; Gibbs and Collette, 1967). The rays in the 

 pectoral fins were counted on both sides for each 

 specimen. The side that yielded the highest value 

 was taken for tabulation. In 34% of all specimens 

 counted there was no difference in counts between 

 the two pectoral fins; in 47% of the specimens, a 

 difference of one ray was noted; 11% of the speci- 

 mens had a difference of two rays; 6% had a three 

 ray difference; and 2% differed by four rays. Only 

 one specimen differed by five rays. At 8 mm SL, 

 8 to 12 rays were developed on the dorsal side 

 of the finfold. Development of rays proceeded 

 progressively ventrad until the finfold was 

 completely occupied with rays. At 20 mm SL, 

 very few juveniles have the adult count of more 

 than 30 rays. At 23 mm SL, about one-half of 

 the specimens have adult counts, and at 27 mm 

 SL all have adult counts. Adults and all juveniles 

 >27 mm SL have more than 30 pectoral fin 

 rays, usually 31 to 34, sometimes 35, rarely 36 

 or 37. My data are corroborated by Schaefer 

 and Marr (1948) and Mead (1951). 



Anal Fin and Finlets 



(Tables 4, 13) 



The anal fin develops similarly to the second 

 dorsal fin. At 8 mm SL, all rays in the anal fin 

 are present with three or four of the posteriormost 

 finlets lacking. By 11 to 12 mm SL all finlets 

 are developed. Thunnus atlanticus typically has 

 21 anal elements (rays plus finlets), and the 

 remainder of the species have 22 (Frade, 1931; 

 Bullis and Mather, 1956; Gibbs and Collette, 

 1967). Variability for this character ranges from 

 6% to 9%. The counts of the anal finlets were 

 included in the anal ray counts for the same 

 reasons given previously for the second soft 

 dorsal fin. 



Pectoral Fins 



(Table 5) 



Development of rays in the pectoral finfold had 

 already started in my smallest (8 mm SL) speci- 

 mens. The increase in number of pectoral rays and 

 their sequence of development was similar for all 

 the species. Wide ranges in number of pectoral 

 'fin rays were common for equal size groups. These 



Pelvic Fins 



All the fin elements of the pelvic fins were 

 visible in my smallest (8 mm SL) specimens of 

 Thunnus. A count of six fin elements was obtained 

 for each fin throughout the size range sampled. I 

 could not be certain if the first element was a spine 

 but all have 1,5 as adults. 



Table 5. — Range of variation in pectoral fin ray counts for 

 selected sizes in juveniles ofThunnus of all the species combined. 



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