FISHERY BULLETIN: VOL. 12. NO. 



smaller specimens. The species of Th annus can 

 be separated into two groups by the vertebral 

 position of the first haemal postzygapophyses. 

 Thunnus thynnus and Thunnus spp. have the first 

 haemal postzygapophyses most often on the 7th 

 vertebra; T. alalunga and T. atlanticus on the 8th. 

 Thunnus thynnus, T. atlanticus, and Thunnus 

 spp. develop the haemal postzygapophyses on 

 their respective diagnostic vertebrae at about 13 

 to 14 mm SL; T. alalunga develops them at 

 about 15 to 16 mm SL. 



I examined seven prepared skeletons from 

 young adults for all species from 400 to 700 mm 

 SL and found that the position of the first 

 ventrally directed haemal postzygapophyses 

 was one vertebra posterior to those of juveniles. 

 I attribute this difference between juveniles and 

 adults to the lateral movement of the structures 

 during growth and also to differential growth 

 between the centrum and the haemal postzyga- 

 pophyses. The elongate haemal postzygapophyses 

 that characterize the adult T. atlanticus, i.e., 

 the longest haemal postzygapophysis is equal to 

 or longer than the centrum (Gibbs and Collette, 

 1967), develop only gradually in juveniles of that 

 species, and no specimens below 80 mm SL can 

 be separated on the basis of this character. The 

 same is true for adult T. albacares, which 

 approach the condition of T. atlanticus. I was 

 unable to follow this through on juveniles in the 

 Thunnus spp. complex because I lacked speci- 

 mens in the larger sizes. All my Thunnus spp. 

 specimens had haemal postzygapophyses no 

 larger than those of all the other species in their 

 comparable size groups. 



Haemal Prezygapophyses 



(Figures 3 to 7; Table 3) 



The haemal prezygapophyses develop almost at 

 the same time as the haemal postzygapophyses, 

 but later than the parapophyses. They first 

 show up as minute bony projections on the two 

 anterior parts of the haemal arches near the 

 centra in about 10- to 13-mm-SL fish. Develop- 

 ment, in order of appearance, proceeds from the 

 anterior to the posterior vertebrae and varies 

 slightly with species and size. Young T. atlanticus 

 develop them at 10 mm SL, the remaining species 

 between 12 and 13 mm SL. High magnification 

 (lOOx) should be used on specimens that just 

 develop this structure. Most specimens of T. 

 thynnus and T. alalunga have their first haemal 

 prezygapophyses under the 15th or 16th verte- 

 bra, T. atlanticus under the 16th and 17th, and 

 Thunnus spp. under the 14th. There is however 

 considerable overlap with apparent bimodal 

 tendencies for the various species. 



In small juvenile Thunnus, all haemal prezyga- 

 pophyses arise from the haemal arches. Only in 

 specimens larger than 80 mm SL are the haemal 

 prezygapophyses on the centra and then only 

 posterior from about the 30th vertebra. In adult 

 Thunnus the haemal prezygapophyses arise from 

 the centra posterior from about the 25th vertebra 

 (Gibbs and Collette, 1967). 



The position of the anterior haemal prezy- 

 gapophyses on the haemal arches or centra varies 

 for the species of the adults of Thunnus. Adult 

 T. alalunga have their more anterior haemal 



Figure 5. — Relationship of the axial skeleton to the fin supports and fins in Thunnus atlanticus, 23 mm SL. 



568 



