POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN ^ OUNG TUNAS 



Table 7. — Number of dorsal and ventral middle radials of the 

 finlets in three species of juveniles ofThunnus (41-117 mm SL). 



separating T. thynnus from T. alalunga and T. 

 alalunga from the Thunnus spp. complex. All of 

 my T. alalunga specimens (n = 116) had their 

 first three second dorsal interneural spaces 

 occupied with only one pterygiophore (Figure 10, 

 Table 13). The following five spaces were filled 

 with two and one with three pterygiophores, and 

 the last posteriormost second dorsal interneural 

 space again had only one pterygiophore. This 

 pattern of pterygiophore insertions under the 

 second dorsal fin is most characteristic of T. 

 alalunga, but may occur in the other species to a 

 lesser degree. Figure 10 and Table 13 also show 

 the percentages of occurrence for the second dorsal 

 pterygiophore insertion sequences which are most 

 characteristic for that species. Ninety-five percent 

 of my 148 T. thynnus specimens had their first 

 two second dorsal interneural spaces occupied 

 with only one pterygiophore. The following five 

 spaces were filled with two and one with three 

 pterygiophores, and the last two posteriormost 

 second dorsal interneural spaces had only one 

 pterygiophore. The remaining 5% of my T. thyn- 

 nus specimens had the identical T. alalunga 



second dorsal pterygiophore insertion sequence. 

 Ninety-eight percent of my 41 Thunnus spp. 

 specimens resembled T. thynnus and 2% T. 

 alalunga. Thunnus atlanticus was the most 

 variable in the second dorsal pterygiophore inser- 

 tion sequence; A&7c of my 92 specimens resembled 

 T. alalunga, 19*^ resembled T. thynnus, and 35% 

 had six interneural spaces occupied with two 

 pterygiophores but did not have a space with three 

 pterygiophores (Table 13). 



Counting from anterior, the first 12 occupied 

 interneural spaces associate in Thunnus with the 

 first dorsal fin. The diagnostic second dorsal 

 interneural spaces are the 13th through the 21st. 

 Following, in the 22nd to 30th spaces are the 

 finlet pterygiophores (Figure 10). The interneural 

 spaces occupied by three or zero pterygiophores for 

 the second dorsal fin and finlets are depicted in 

 Figure 10 on the basis of where they most often 

 occurred, but they could vary as much as two 

 spaces in an anterior or posterior direction. The 

 percentages in Figure 10 refer only to the arrange- 

 ment under the second dorsal fin. 



The distribution of pterygiophores under the 

 anal fin (Figure 10, Table 13) present too much 

 variability to be useful for species separation, 

 except perhaps in Thunnus spp. and T. atlanti- 

 cus. The first five anteriormost interhaemal 

 spaces are occupied by two or three pterygio- 

 phores in T. thynnus and T. alalunga. The 6th and 

 7th spaces, which complete the anal fin associa- 

 tion, always have only one pterygiophore. The 

 anal finlet pterygiophores insert into the 8th to 

 14th spaces, one to a space. The last or 14th 

 interhaemal space that is occupied by the last or 



NUMBER OF DORSAL PTERYGIOPHORES 



1*t Dorsal Fin 



2nd Dorsal Fin 



Dorsal Finlet 



Vertebrae 

 r ALALUHSA 

 T. THYItllUS 

 THUMMUS SPP. 

 T. ATLAMTICUS 



10 



12 



13 



14 



15 



16 



17 



18 



66% 

 84% 

 78% 

 61% 



19 



20 



21 



22 



23 



24 



25 



26 



27 



28 



29 



30 



31 



N 



46 



40 

 140 

 116 



32 33 34 



74 



116 



31 



SS 



Anal Fin 



Anal Finlets 



NUMBER OF ANAL PTERYGIOPHORES 



Figure 10. — Representative arrangement of pterygiophores in relation to the fins and vertebrae for the juvenile Thunnus 

 species. Percentages and number of specimens (N) are for the occurrence of the commonest arrangement under the second 

 dorsal fin only; other arrangements given in Table 12. Modified after Matsui (1967). 



577 



