FISHERY BULLETIN: VOL. 11. NO. 2 

 Table 12. — Number of lateral line scales for selected sizes of juveniles ofThunnus. 



posteriorly is the 4th or 5th, never the 3rd as 

 in T. thynnus. If the Hnes of the curve that show 

 descent and ascent were to subtend an angle, it 

 would be larger than 90^. Some specimens of these 

 species have also acquired four or more scales 

 by 19 mm SL. They can be distinguished from 

 T. thynnus by their 4th scale that stays in line 

 on a descent with the 3rd. At 21 mm SL, all 

 specimens have four or more scales. Slight dif- 

 ferences in the curvature of the lateral line were 

 noted between T. alalunga, T. atlanticus, and 

 Thunnus spp., but these differences were not 

 distinct enough for separating the species. 



IDENTIFICATION 



(Table 13) 



The generic external characters of transforming 

 and juvenile Thunnus have been adequately docu- 

 mented by a number of workers as pointed out in 

 the introduction. Richards and PotthofF(in press) 

 concluded that if accurate identifications to 

 species are necessary, the only choice is to use 

 osteological characters, even though this involves 

 the time-consuming task of clearing and staining. 



Only three characters are available to separate 

 the species from 8 to 14 mm SL: the number of 

 precaudal and caudal vertebrae, vertebral posi- 

 tion of the first closed haemal arch, and the 

 pterygiophore pattern under the second dorsal 

 fin. Assuming a specimen is not a variant in 



any of the three characters, it can be identified 

 as follows: 



Character 



Number precaudal + caudal 



vertebrae 

 First closed haemal arch 



Pterygiophore pattern 



Species separated 



T. atlanticus from T. thynnus, 



T. alalunga, Thunnus spp. 

 T. atlanticus + Thunnus spp. 



from T. thynnus + T. 



alalunga 

 T. alalunga from T. thynnus + 



Thunnus spp. 



Although the vertebrae are not yet developed 

 posteriorly at 8 mm SL, they nevertheless can be 

 counted by noting chondrified neural and haemal 

 spines which are present above and below the 

 notochord. Care should be taken not to count 

 the parhypural bone as a haemal spine. The 

 parhypural belongs to the hypural plate but 

 resembles a haemal spine during early develop- 

 ment. The position of the first closed haemal arch 

 is difficult to determine in small specimens but 

 it can be done with diligence and patience. The 

 pterygiophores may not be entirely ossified at 

 8 mm SL, but they are present as chondrified 

 struts. Varying the substage light, by moving the 

 mirror below the microscope, will bring them 

 into view. 



The vertebral position of the first haemal post- 

 zygapophysis becomes available as a character on 

 specimens from 15 to 20 mm SL. In this size 

 range, only T. atlanticus acquires the diagnostic 



584 



