Absence of a functional swimbladdcr in T. longgol 

 may be regarded as a useful diagnostic character, 

 but in most specimens we discovered a tiny swim- 

 bladder, about 4 mm. in diameter, which could 

 easily have been overlooked. A very small swim- 

 bladder (ca. 20 mm. long) also was present in two 

 T. maccoyii (742 and 7()4 mm.). 



Variation in the other five species is considerable 

 (fig. 17). In T. allanticiis the swimbladdcr may be 

 short, oblate, lying far anterior in the body cavity; 

 or it may approach the length of a poorly developed 

 swimbladder of T. albarnrcs. In the latter case, the 

 swimbladder is comprised of two chambers divided 

 by a transverse membrane, with the anterior chamber 

 probably lepresenting the small type of swimbladder, 

 the posterior chamber an addition. In T. albacarcs 

 the swimbladder is moderately long, usually reaching 

 about the level of the 14th or 15 vertebra (range 

 12-17). In T. alalunga it varies from narrow, only 

 moderately long, and deflated, to fully the length of 

 the body cavity and inflated to fill much of the 

 cavit.v. 



In T. Ihynnns and T. maccoyii variation in swim- 

 bladder dimensions may be correlated with growth. 

 The changes in swimbladder dimensions were recog- 

 nized by Serventy (1956a: 10), and were suggested 

 l)ut not recognized by Godsil and Holniberg (1950: 

 21). Smaller specimens (457-954 mm.) have rudi- 

 mentary swimbladders that are slender, deflated, and 

 short (about 6 vertebrae long), about a quarter of 

 the length of the body cavity, and do not reach the 

 depression anterior to the dorsal bulge. In two T. 

 maccoyii the swimbladder was tiny and could easily 

 have been missed in a casual dissection. In a 1,060- 

 mm. specimen of T. I. Ihynnus the organ was inflated 

 and occupied about half the body cavity (fig. 17). 

 In specimens 1,390-mm. and larger the swimbladder 

 extends from the depression anterior to the kidney 

 almost or quite to the posterior end of the body 

 cavity; it is as wide as the body cavity in its anterior 

 half and posteriorly tapers almost to a point. 



Urogenital System 



General Description. — The paired gonads are fi(!- 

 quently visible in ventral view. They lie along the 

 dorsolateral body wall, their posterior ends forming 

 ducts which open on each side of the urinary papilla. 

 The kidney is anterior in position and dorsal to the 

 layer of fibrous connective tissue overlying the swim- 

 bladder. Its anterior margin usually follows the 

 edges of the depression anterior to the hump formed 



by the haemal arches, and lateral extensions reach 

 forward in a semicircle and sometimes nearly meet 

 anteriorly. Depending upon the species, a posterior 

 extension ("tail") may reach about as far as the 

 level of the 16th vertebra; and T. alhacares some- 

 times has accessory masses of kidney ti.ssue posterior 

 to the main mass. In the anterolateral extensions of 

 the kidney, the urinary ducts ("ureters") arise and 

 join witliin or just posterior to the kidney substance, 

 and the resulting single duct proceeds posteriorly. 

 Just before the anus, the duct enters a small but 

 prominent urinary bladder, which may lie within 

 the mesenteries of the gonads or project freely into 

 the body cavity, depending upon the species. The 

 urinary bladder empties through a urinary papilla 

 behind the anus. 



Specific Characters (fig. 18). — The kidney of T. 

 alnbinga is uni(iue in lacking a "tail," the end reach- 

 ing the level of the 7th to 9th vertebra (1 1th in one 

 specimen). In T. thynnus and T. maccoyii the tail 

 is relatively short, reaching the Sth to 1 Ith vertebra. 

 Its configuration varies in Atlantic specimens from 

 tapering to truncate, encompassing all the forms 

 used by Ciodsil and Holmberg (1950) to differentiate 

 T. maccoyii from Atlantic and Pacific T. Ihynnus. 

 None of our T. maccoyii had truncate kidneys. In 

 T. ohesns the tail is slightly longer than in T. thynnus, 

 reaching the 11th to 14th vertebra, is narrower, and 

 is moi'e distinctly delimited from the anterior kidney 

 mass. The kidney of T. alhacares has a long tail, 

 tapering gradually' from the anterior kidney mass 

 and reaching the 12th to 16th vertebra; accessary 

 kidney masses are often present posteriorly. In 

 T. atlanticus the kidney mass is bulky anteriorly and 

 has a long, narrow tail that reaches the r2th to 17th 

 vertebra. In T. tonggol there is a large anterior 

 portion and a long, narrow tail that reaches the 

 15th to 17th vertebra. 



The branching of the ureters varies considerably, 

 but shows some general tendencies that, together 

 with the shape of the kidney, are useful in distin- 

 guishing species (fig. 18). In the tailless kidney of 

 T. alalunga, the two main branches are widely 

 divergent, joining at the posteriormost end of the 

 kidn(>v sulistance. In 7'. Ihynnus and T. maccoyii 

 the junction may occur at the posterior end, at some 

 distance craniad, or just posterior to the kidney. 

 The angle formed liy the branches is small when the 

 junction is well posterior, large \\;hen far anterior. 

 In T. obesus the branches converge to run close 

 together and almost parallel for a considerable dis- 



86 



U.S. FISH AND WILDI.IKK SKRVICE 



