POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



RAY 



INLET 



-*^IIIODLE RADIAL 

 PROXIMAL RADIAL 



PROXIMAL RADIAL 



Figure 8. — Schematic representation of the relationship between vertebrae, pterygiophores, and 

 fin elements in juveniles of Thunnus (lateral view). A. Anterior portion of first dorsal fin, 

 pterygiophores, and vertebrae; B. Last spine of first dorsal fin, anterior portion of second dorsal 

 fin, pterygiophores, and vertebrae; C. Last ray of second dorsal fin, anterior finlets, pterygiophores, 

 and vertebrae; D. Anterior portion of anal fin, pterygiophores, and vertebrae. 



the fin elements. Generally, all species of Thun- 

 nus have 37 dorsal fin elements (XIV- 15-8) but 

 only 36 pterygiophores; the first pterygiophore 

 supports the first two spines. In actuality, the first 

 pterygiophore is serially associated with the 

 second spine but has captured and fused to it 

 the distal radial of the first spine (Figure 8A). 

 The proximal radial of the first spine has been 

 lost. The first spine is therefore only secondarily 

 associated with the first pterygiophore. Pos- 

 teriorly, each fin element is serially associated 

 with a pterygiophore, but each fin element also 

 rests atop the adjacent posterior pterygiophore 

 in a secondary association (Figure 8). Thus, two 

 pterygiophore fin element associations exist. The 

 serial association is most often overlooked. The 

 last fin element (finlet) in the dorsal fin assembly 

 is serially associated with its pterygiophore. It 

 also rests in a secondary association on a very 

 small bone, which I assume to be a reduced 

 proximal radial of a lost finlet without its serial 

 middle and distal radials. The distal radials of 

 the spinous dorsal that, to me, resemble the horns 



of a moose (Kramer (1960) uses the term 

 "alate"), form in part and rigidly support the 

 dorsal groove and become smaller in size in a 

 posterior direction. The distal radials are still 

 present in the second dorsal fin, although their 

 semblance to moose horns has disappeared ("non- 

 alate"). They decrease in size posteriorly (Figure 

 8B) until they disappear from the lateral view at 

 about the 9th ray. Dissection of posterior fin 

 rays and finlets revealed the presence of the distal 

 elements between the bifurcate bases of the rays 

 or finlets (Figure 9B, C). Spines, on the other 

 hand, do not have bifurcate bases (Figure 9A). 

 Instead, the distal radials are located anterior to 

 the base of the spine. 



The anal fin and finlet pterygiophores are 

 structurally similar to the pterygiophores of the 

 second dorsal fin and finlets with one exception: 

 The first anal pterygiophore in Thunnus is 

 derived from two cartilaginous parts which fuse 

 at about 8 mm SL. It has two anal fin elements 

 in serial association and one in secondary (Figure 

 8D). Thus, all Atlantic species of Thunnus, 



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