FISHERY BULLETIN: VOL. 72. NO. 2 



Figure 1. — Thunnus atlanticus, 85 mm SL. Specimen was cleared and stained later for species determination. 



the anterior tip of the upper jaw to the posterior 

 edge of the hypural plate was taken. After 

 determining length measurement the right oper- 

 cular plates were removed and the specimens were 

 then cleared and stained (Taylor, 1967). All counts 

 were made on cleared and stained material that 

 was preserved in 100% glycerin, using a binocular 

 microscope with 100 x magnification. On a few 

 of the larger specimens 25 x magnification was 

 used. Counts on the gill arches, fins, and vertebral 

 column were made on the right side of the speci- 

 men, except for the pectoral finrays and lateral 

 line scales, which were counted on both sides. The 

 side with the higher count was used for tabula- 

 tion. Small gillrakers and finrays that were just 

 beginning to appear in their first stage of develop- 

 ment were always included in the counts. 

 Structures such as the pterygiophores, neural 

 and haemal spines, zygapophyses, etc., were 

 always counted, although in smaller specimens 

 some of these structures were very small and 

 showed very weak ossification. The hypural 

 complex (parhypural, hypural plates, and ural 

 centrum) was considered as the last vertebra. 

 Iwas unable to make every count on all of the 

 specimens because of damage or their develop- 

 mental stage. For these reasons, the number of 

 specimens used for the various counts may vary. 

 The osteological terms used in this paper are 

 mainly from Eaton (1945) and Gibbs and Collette 

 (1967). The terms transforming and juvenile are 

 defined for scombrid fishes as follows: specimens 

 of the transforming stage can be identified with 

 the aid of larval pigment characters but have 

 attained most adult gross anatomical features, 



such as number of vertebrae and median fins; 

 juveniles attain the juvenile pigmentation which 

 obliterates the larval pigmentation. The transi- 

 tion from larval to transforming to juvenile stages 

 is gradual in scombrids and allows for individual 

 subjective judgment. In my opinion, the larval 

 stage for Thunnus lasts to about 9 mm SL, the 

 transforming stage to about 13 mm SL, and the 

 juvenile stage to sexual maturity. 



MATERIAL 



Most of the Thunnus used in this study were 

 collected with a dip net, using a strong light at 

 night. A few were collected by plankton nets and 

 some were taken from fish or bird stomachs. The 

 numbers and standard lengths of all specimens 

 examined for this study are shown in Figure 2. 

 Their numbers and size ranges for general capture 

 areas are shown in Table 1. 



VERTEBRAL COLUMN 



Number of Vertebrae 



(Figures 3 to 6; Tables 2, 13) 



The species of Thunnus usually have 39 verte- 

 brae, including the hypural plate: 18 precaudal 

 and 21 caudal vertebrae for all species except the 

 western Atlantic T. atlanticus, which usually has 

 19 and 20, respectively. In juveniles the centra 

 in the anterior two-thirds of the vertebral column 

 are already ossified at 8 mm SL, but are still de- 

 veloping in the posterior third. About 9 mm SL, 

 ossification of all centra is completed. Pleural ribs 



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