POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



Figure 4. — Relationship of the axial skeleton to the fin supports and fins in Thunnus alalunga, 22 mm SL. 



arches. The first (anteriormost) ventrally directed 

 parapophysis occurred on the 8th or 9th vertebra 

 on specimens 10 mm SL and smaller. In fish 

 larger than 10 mm SL the first ventrally directed 

 parapophysis occurred on the 6th, 7th, or 8th 

 vertebra. I could not determine specific dif- 

 ferences in the position of the first ventrally 

 directed parapophysis. Differences reported by 

 Watson and Mather (1961), Watson (1964), and 

 Gibbs and Collette (1967) occur only in specimens 

 larger than 80 mm SL. In juveniles less than 80 

 mm SL, the first ventrally directed parapophysis 

 is one to three vertebrae anterior to the adult 

 position. As the fish grow, some of the anterior- 

 most parapophyses move to a lateral position on 

 the centra. 



First Closed Haemal Arch 

 (Tables 3, 13) 



The closed haemal arches are formed quite ' 

 early. Even the smallest (8 mm SL) specimens of 



Table 2. — Precaudal and caudal arrangement of the vertebrae 

 and total vertebral number in juveniles oi Thunnus. 



Thunnus that I examined had their first anterior- 

 most closed haemal arch on the diagnostic verte- 

 bra. The arch is formed by the fusion of the distal 

 ends of the elongated two parapophyses on each 

 centrum. At times it is difficult to determine 

 whether the parapophyses have actually fused or 

 are only lying close together. Care should be taken 

 to determine this fact. The position of the first 

 closed haemal arch is a character of some value, 

 since it forms at very small sizes. Posteriorly, 

 the arches become more elongated in a dorso- 

 ventral direction, forming spines at their distal 

 points (Figures 3 to 6). The vertebral position of 

 the first closed haemal arch separates the species 

 of the genus Thunnus into two groups. In T. 

 thynnus and T. alalunga the first arch occurs 

 on the 10th vertebra; in T. atlanticus and 

 Thunnus spp. it occurs on the 11th. The variability 

 of this character ranges from 17c to 12% for the 

 various species. Thunnus alalunga is the most 

 conservative for this character, and T. thynnus 

 is the most variable. 



First Ventrally Directed Haemal 

 Postzygapophysis 



(Figures 3 to 7; Tables 3, 13) 



The haemal postzygapophyses begin to develop 

 a little later than the parapophyses. In 12- to 

 15-mm-SL fish, they first appear as tiny bony 

 projections on the ventral posterior edge of the 

 centrum. They develop anteriorly and posteriorly 

 from the center of the vertebral column. At about 

 13 to 16 mm SL, the position of the first haemal 

 postzygapophysis becomes a diagnostic character. 

 A 100 X magnification should be used in the 



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