POTTHOFF: OSTEOLOGICAL DEVELOPMENT AND VARIATION IN YOUNG TUNAS 



specimens had it. Therefore, the juveniles of the 

 Thunnus spp. complex can only be separated 

 from T. alalunga by the vertebral position of the 

 first haemal arch, by the shape of the first 

 haemal spine, by the vertebral position of the 

 first ventral postzygapophysis, and by the 

 pterygiophore pattern of the second dorsal and, 

 perhaps, anal fins. However, caution should be 

 used in applying any separating character be- 

 cause of variability. Percentages for variabili- 

 ties can be found in the respective tables. 



LATERAL LINE SCALES 



(Figure 13, Table 12) 



All species develop the first lateral line scale at 

 16 to 18 mm SL. The scale originates near the 

 dorso-posterior edge of the pectoral girdle. It is 

 rectangular in shape, with strong ossifications on 

 the dorsal and ventral sides. These strong ossifica- 

 tions develop into two plates that project ver- 

 tically to the base of the scale. As growth proceeds, 

 more scales are added posteriorly, forming a dis- 

 tinct broken line, which at first slopes ventrad 

 then dorsad and levels off in a posterior direction. 

 At about 60 mm SL, lateral line scales cannot 

 be counted because the scales fuse to form a con- 

 tinuous line. Fusion of the scales occurs first 



on the anterior portion of the lateral line and 

 proceeds posteriorly. 



Sella (1924) and Watson and Mather (1961) 

 noted specific differences in the shape of the 

 lateral line. I have examined this character in 

 detail and found that T. thynnus can be separated 

 from all the other Thunnus species. Eighty-one 

 T. thynnus from a total of 159 were undamaged 

 and large enough to show the lateral line contour. 

 In all 81 specimens the first three scales formed 

 a posteriorly descending straight line, whereas 

 the following scales formed a posteriorly ascend- 

 ing line at an angle approximately 90"" to the first 

 line. At the 8th or 9th scale the line curved 

 in a posterior direction, parallel to the body axis. 

 At 19 mm SL, some T. thynnus can be separated 

 on the basis of the lateral line contour because 

 at this size some have acquired four scales. The 

 4th scale in T. thynnus is always aligned at a 

 90° angle to the preceding three. At 22 mm SL, 

 all T. thynnus have acquired four or more scales 

 and are therefore totally separable from the other 

 species. 



In the remaining species a descent and ascent in 

 the lateral line is also present, but it forms a 

 smooth curve instead of a right angle. In all 

 specimens (29 T. alalunga, 46 T. atlanticus, and 

 14 Thunnus spp.) the last scale to descend 



Table 11. — Distribution of gillrakers over the branchial bones of the first gill arch for various size groups in Thunnus spp. 



juveniles, n = 40. 

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



Ceratobranchlal 



Hypobranchial 



6 7 



9 10 11 12 13 14 15 16 



1 — 



2 — 



Total gillraker count 



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 32 33 



1 — 4 



1 



1 



6 11 33 37 30 

 13 15 17 4 



583 



