POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



EPIBRANCHIAL 



CERATOBRANCHIAL 



HYPOBRANCHIAL 



TOTAL GILL ARCH 



r. iTLAMTICUS 



T. iLALUHe* 



T. THYHIIUS 



T. ATLAMTICUS 



T. ALALUM6A 



T. THYKKUS 



T. AILAMTICUS 



T. ALALUKSA 



T. THYHMUS 



T. ATLAMTICUS 



T. ALALUHBA 



T. THYMHUS 



? 



I 1- 





I 1- 



—I 1 — I — I 1— I — I — r— I — I — I I I I I I — I— I 



10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 

 STANDARD LENGTH, MM 



Figure 12. — Representative scheme of gillraker development with increase in size over the three 

 bones of the first gill arch for juveniles of three Thunnus species. Dotted line: developmental 

 stage, one or more rakers present; dashed line: adult gillraker complement present in some 

 specimens; solid line: adult gillraker complement present in all specimens; question mark: 

 assumed, no data available. 



mm SL) of the Atlantic species. Therefore, some 

 juvenile T. atlanticus have their first raker over 

 the hypobranchial at 16 mm SL, and all have at 

 least one raker over the hypobranchial at 20 mm 

 SL. Some T. alalunga and Thunnus spp. juve- 

 niles have their first raker over the hypobranchial 

 at 18 mm SL, and all have at least one raker over 

 the hypobranchial at 22 mm SL. Thunnus 

 thynnus has the highest adult gillraker counts 

 and adds its first rakers to the hypobranchial 

 at the largest size of all the species; they begin 

 to appear on some of the 21-mm-SL specimens 

 from the western Atlantic and on some of the 

 23-mm-SL specimens from the Mediterranean. 

 Not until 28 mm SL do all T. thynnus have at least 

 one raker over the hypobranchial. 



Summarizing the data from Tables 8 to 11, one 

 can conclude that: 



1. The species differ in size at which they 

 attain the maximum gill raker counts on each 

 bone of the first gill arch. 



2. Each species differs from the others in maxi- 

 mum number of rakers that it can have on some 

 or all of the three bones of the first gill arch. 



3. All species of Thunnus first attain the maxi- 

 mum complement of rakers over the cerato- 

 branchial bone. 



4. The range of the maximum number of rakers 



over the ceratobranchial bone shows the greatest 

 interspecific difference. 



Thunnus atlanticus (Table 8; Figures 11, 12). The 

 maximum gillraker count over the epibranchial 

 bone of 7 to 9 rakers was first present in 

 specimens of 58 mm SL. The diagnostic maxi- 

 mum count of 11 to 13 rakers over the cerato- 

 branchial bone was first present in a 9-mm-SL 

 specimen, but it was not until 15 mm SL that 

 all specimens had the diagnostic count. Maxi- 

 mum counts over the hypobranchial bone (in- 

 cluding the cerato-hypobranchial cartilage) of 

 4 to 6 rakers were first observed on 21-mm-SL 

 specimens, but not before 26 mm SL did all 

 juveniles attain the maximum count. The maxi- 

 mum total count for the first gill arch of 23 

 to 27 rakers was first observed in a 43-mm- 

 SL specimen and all specimens larger than 52 

 mm SL had maximum total counts. The maxi- 

 mum total count is attained in some specimens 

 at a smaller size than the maximum epi- 

 branchial count. This discrepancy is explained 

 by the range in number of rakers over the 

 ceratobranchial and hypobranchial bone. Gibbs 

 and Collette (1967) give 19 to 25 rakers as the 

 total number over the first arch for T. atlanti- 

 cus. Because all specimens larger than 34 mm 

 SL had more than 20 gillrakers, I believe that 



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