ALA 



THY 



OBE 



ALB 



ATL 



TON 



Figure 13. — Left lateral view of vertebrae 20-28 of Thurinus 

 species, showing development of inferior foramina. 



the largest usually tliree times or more as long as the 

 horizontal width of the base of the corresponding 

 haemal spine. In T. ohesus, T. maccoyii, T. thynnns, 

 and T. alalunga the size of the openings is variable, 

 usuallj' small, and the largest opening is rarely more 

 than aliout 1.5 times as long as the width of the 

 adjoining bony neural arch. 



T. atlandcus, T. albacarcs, and T. tonggol are 

 distinguishable by the development of vertebral 

 processes and openings, which approach the ornate 

 trelliswork seen in Auxis, Euthynnus, and Katsu- 

 xvonus; T. alalunga shows the least development, and 

 T. ohesus, T. maccoyii, and T. thynnus are inter- 

 mediate. 



The first haemal prezygapophysis tends to be 



found more anteriorly in T. albacarcs and T. tonggol, 

 most commonly on vertebra 13 or 14 (range 12-15) 

 in T. albacarcs. In T. maccoyii it usually occurs on 

 14 or 15 (range 14-16), in T. alalunga and T. obesus 

 on 15 or 16 (range 14-17), and in T. allanticus on 

 16 or 17 (range 15-18). T. thynnus displays a 

 wider range of variation (12-17), and overlap is 

 considerable between it and the other species. 



While this paper was in press, Nakaniura and 

 Kikawa (1966) described specific differences in the 

 infra-central grooves on the ventral side of the centra 

 of vertebrae 10-30. These infra-central grooves were 

 categorized into three types: type A with two 

 separate infra-central grooves; type B with two 

 grooves connected by a canal; type C witii a single, 

 usually elongate, groove. We have reexamined our 

 skeletal material, which comprises many more speci- 

 mens than the total of 19 used by Nakamura and 

 Kikawa, and we conclude that the infra-central 

 grooves are a useful character, but show variation 

 that often precludes positive species identification. 

 Our conclusions are as follows: 



Thunnus albacai-es (24 specimens, fork length 280- 

 1,430 mm.) has type C infra-central grooves th;it 

 tend to be divided by a thin septum anteriorly but 

 that posteriorly are undivided or occupied by a 

 honeycomb-like network of bony material. In this 

 character, T. albacarcs is distinctive, but is ap- 

 proached by some specimens of T. maccoyii. 



Thunnus maccoyii (16 specimens, 742-1,445 mm.) 

 is the most variable of all the species. Very few 

 have type C (undivided) grooves, as described by 

 Nakamura and Kikawa. Typically, all the verte- 

 brae have two grooves (type A) that are divided 

 by very thin septa or a honeycomb of septa. This 

 condition resembles that of the anterior grooves of 

 T. albacarcs. Only an occasional undi\ide(l groove 

 occurs among the divided ones in T. maccoyii, 

 whereas most of the grooves are undivided in T. 

 albacarcs. Some specimens, particularly tiie si.\ from 

 Australia that were used by Godsil and Holmbcrg 

 (1950), were almost impossible to distinguish from 

 T. thynnus. 



Thunnus thynnus thynnus (7 specimens, .'U()-2,315 

 mm.) and T. t. orientalis (18, 530-1,450 mm.) have 

 type A grooves, two grooves per centrum, that tend 

 to be round or oval anteriorly and become longer 

 and narrower posteriorly. The distance between 

 the two grooves on the anterior centra is highly 

 variable. When the grooves are rtarrowly separated, 

 the specimens resemble some T. maccoyii. 



80 



U.S. FISH AND WII.DI.IKK SKRVICE 



