POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



rakers over the ceratobranchial bone were 

 first observed in 17-mm-SL specimens from the 

 western Atlantic. It was not until 23 mm SL 

 that all specimens, including the Mediter- 

 ranean samples, had the diagnostic maximum 

 count. Maximum counts over the hypobranchial 

 bone (including the hypo-ceratobranchial carti- 

 lage) of 7 to 9 rakers were first observed on a 

 34-mm-SL specimen from the western Atlantic, 

 but not before 45 mm SL did all juveniles, 

 including the Mediterranean samples, attain 

 the maximum count. One exception was noted 

 at 71 mm SL with 6 hypobranchial rakers. 

 The maximum total count for the first gill arch 

 of 36 to 41 rakers was first observed in a 62-mm- 

 SL specimen; all juveniles in the 90- to 94-mm- 

 SL size range will probably have the maximum 

 total count. As in T. atlanticus, the maximum 

 total gill arch count is reached in some T. 

 thynnus specimens at a smaller size than the 

 maximum epibranchial count. This discrepancy 



is again explained by the range in number of 

 rakers over the ceratobranchial and hypo- 

 branchial bone. Gibbs and Collette (1967) give 

 34 to 43 rakers as the total number over the 

 first arch for the western Atlantic T. thynnus 

 thynnus. Our difference is due to my smaller 

 sample size and population variance. 

 Thunnus spp. (Table 11). Juvenile T. albacares 

 and T. obesus could not be separated and were 

 grouped together under Thunnus spp. Lack of 

 enough specimens in a proper size range pre- 

 vented me from making observations on gill- 

 raker ontogeny. There is considerable over- 

 lap between the Thunnus spp. complex and 

 T. alalunga in number of rakers and their 

 development over the three bones of the first 

 arch. The maximum counts of 14 to 16 rakers 

 over the ceratobranchial bone are identical for 

 Thunnus spp. and T. alalunga. At 15 mm SL, 

 one Thunnus spp. specimen had the maximum 

 ceratobranchial count and at 20 mm SL all 



Table 9. — Distribution of gillrakers over the branchial bones of the first gill arch for various size groups in Thunnus alalunga 



juveniles, n = 118. 



(Total gillraker counts for western Atlantic adults from Gibbs and Collette, 1967, Table 2*.) 



Epibranchiai 



Ceratobranchial 



Hypobranchial 



Total gillraker count 



6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 



1 — — 1 — 1 



2 5 2 2 2 



2 12 7 4 11 4 8 1 1 



12 5 5 8 3 1 



12 4 12 1 



1 — 2 — 1 



2 2 1 

 1 

 1 



1 1 10 20 15 6 2 



581 



