increased variability at these larger sizes. In T. 

 thijrmus this is particularly evident; specimens over 

 2,000 mm. fork length (not shown in fig. 28) appear 

 randomly distributed over a range of body depths 

 from 22-29 percent of fork length, which is almost 

 the entire range of all species combined. 



The greatest body depth is found in individuals of 

 T. obr.vis at all sizes over fiOO mm., but the .species 

 overlaps with T. atlanticus, T. thynnus, or T. alahinga 

 throughout its known size range. 



Eye size, in combination with other characters, is 

 a useful species criterion, but the negative allometry 

 must be considered. Because we measured the bony 

 orbit, our data are not comparable with most other 

 published data. We recommend that future work- 

 ers use iris diameter. 



Fig. 29 compares the species. Both T. alalungn 

 and T. atlanlicus exhibit wide variation in eye diame- 

 ter, making categorical statements difficult. At less 

 than fiOO mm. fork length the smallest orbit diameter 

 is found in T. thijnnus and T. (oiiggol, the largest in 

 T. atlanticus and T. alalungn, and intermediate in 

 T. albacarcs; T. obesus is not represented. 



At sizes greater than (100 mm., T. obc.sus clearly has 

 the largest orbit diameter. Variation in T. alalunga 

 covers the range from largest to smallest. The other 

 species have so much overlap with one another that 

 species distinctions are impossible. 



COLORATION 



Colors and color patterns of tunas have limited vise 

 in tuna systematics because they show great indi- 

 vidual and age variation, and because they may be 

 lost after death and preservation. Nevertheless, 

 there are some excellent color characters, in spite of 



J i_ 



400 &00 



FORK LENGTH 



Figure 29.— Diameter of bony orbit relative to fork length 

 in Thuitnus species. Only our data used. 1 — T. ntlaritictis; 

 2—T. tonggol; 3— T. albacares; A—T. obcsus; b—T. aUi- 

 lunga; 6 — T. thynnus. 



the difficulty in verbal expression of many of them. 

 Many of the descriptions are taken from Mather 

 (19fi4). 



Body. Most Thunnus species are iridescent dark 

 blue above and silvery below. T. albacares is the 

 most brilliantly colored, with a shining golden lateral 

 band. T. atlanticus also has a prominent gold lateral 

 band, but its body is usually very dark compared 

 with other species. T. obcsus may display a trace of 

 a gold band, but the Imnd is apparently absent in 

 T. thi/nnus and T. tonggol and is replaced by an 

 iridescent blue band in T. alalunga. 



Small specimens of all species may dis'jilay a 

 pattern of white spots or streaks ventrolaterally. 

 In T. tonggol these markings consist of horizontally 

 elongated spots. The other six species have rounded 

 spots that are either randomly distributed or t«nd to 

 l)ecome arranged in vertical rows that alternate with 

 vertical white lines; horizontally elongated spots are 

 sometimes seen on the caudal peduncle but rarely 

 farther anteriorly. This pattern is usually lost in 

 large individuals, although it may be retained in 

 s])ecimens of T. albacarcs and T. thynnus up to 

 1,. 500-1, 600 mm. 



In T. maccoyii, alone among the species of 

 Thunnus, the caudal keels are an unmistakable 

 bright yellow. In the fish markets of Japan, we 

 were able to recognize T. maccoyii from a consider- 

 able distance on the basis of this charac^ter. How- 

 ever, we suspect the keels may lo.se their yellow in 

 larger adults. 



Fins. The color of the first dor.sal fin is variable. 

 It may be entirely white, or there may be a yellow 

 suffusion, and the distal margin may be black. Too 

 few ob.servations have been made to enable us to 

 characterize the species. The second dorsal and anal 

 fins usually have yellow tips in all but T. alalunga 

 and T. atlanticus, which have dark fins with white 

 distal margins. The dorsal and anal finlets are 

 usually bright yellow with black margins in all except 

 T. alalunga and T. atlanlicus. In T. albacarcs the 

 black margin is usually very narrow, while in T. 

 obcsus it is wider. T. alalunga may have yellow in 

 the dorsal and some anal finlets, but the anal finlets 

 are commonly all silvery or dusky. Both the dorsal 

 and anal finlets of T. atlanticus are almost invariably 

 dusky with white margins; yellow has been ob.servod 

 in these finlets only in frozen specimens. June 

 (19.j2b) reported black dorsal and anal finlets in an 

 unusual specimen of T. albacarcs from the central 

 Pacific. 



96 



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