Figure 5. — Skulls of T/ii(?!?H;s species. Right lateral view 

 of postcroventral part of neurocranium showing conforma- 

 tion of posterior parasphenoid margin. Arrangement with- 

 in each species in order of increasing skull length from right 

 to left. ALA— 131, 138, 150, 167 mm. ALB— 103, 107, 

 116, 122, 169, 179 mm. ATL— 51, 80, 88, 97, 102 mm. 

 OBE— Pacific, 97, 141, 143, 146, 165, 189, 210 mm. 

 THY— 76, 139, 230, 335 mm. TON— 57, 61, 122, 127 mm. 



is quite characteristic of somi species. 



In T. thynnus, T. maccoyii, and T. alabmga a 

 decided angle is formed by the posterior parasphe- 

 noid margins. The acuity of the angle and its 

 posterior e.xtent generally increase with size in T. 

 Ihynnns (fig. 5), and extreme development of the 

 angle is found in very large specimens. Within its 

 observed size range (skull length 88-167 mm.), 

 T. alalunga, however, displays a more acute angle 

 than does T. thynnus of similar size. T. maccoyii 

 resembles T. thynnus in this respect. 



In relatively large specimens of T. obesus the angle 

 is apparent but not as acute as in T. thynnus, T. 

 maccoyii, and T. alalunga. Observations of Pacific 

 specimens indicate probable changes with growth. 

 The two smaller eastern Pacific specimens reported 

 by Godsil and Byers (1944) and again by de Sylva 

 (1955) have rounded margins, but other eastern 

 Pacific specimens within the same size range show a 

 definite angle. 



T. albacares, T. atlanticus, and T. tonggol exhibit 

 great variation. Some have unmistakably rounded 

 margins; others are somewhat angulate but have a 

 very short distance from the first vertebral centrum 

 to the apex of the angle, so that the angle itself is 

 never obvious. 



Supraoccipital crest. In T. alalunga (fig. 1) the 

 supraoccipital crest is relatively more slender than in 

 any of the other species of Thunnus and is longer, 

 nearly always reaching at least to the centrum of 

 the third vertebra. In the other six species the 

 crest rarely extends beyond the second vertebra. 



Ventral parasphenoid shaft. In T. atlanticus the 

 anterior portion of the parasphenoid shaft is concave 

 ventrally (de Sylva, 1955). In T. tonggol we found 

 it concave in three small specimens (skull length 

 57-99 mm.) and flat in two larger specimens (skull 

 length 122-128 mm.). It is most commonly flat or 

 slightly convex in the other species, but a degree of 

 concavity has been observed in individuals, especially 

 young, of all except T. obesus. 



Other characters. — Godsil and Byers (1944) cited 

 several additional neurocranial characters that are 

 supposedly useful in distinguishing among the species. 

 In our estimation, none of these is valid for the 

 following reasons. 



The parietal crest in T. albacares was described as 

 extending farther forward than in T. obesus so that 

 a projection of the curvature of the lateral edge of 

 the parethmoid (prefrontal of Godsil and Byers) 

 would he continuous with the parietal crest in T. 

 obesus but woidd run below it in T. albacares. Our 

 material shows both conditions in all species. 



The angle of the long axis of the basisphenoid 

 relative to the parasphenoid is highly variable and 

 not reliable as a specific character. The width of 

 the basisphenoid relative to its height is not only 

 variable within any given size range but also changes 

 with growth. 



The anteriorly directed process at the upper end of 

 the basisphenoid was used by de Sylva as a distin- 

 guishing character (1955: 32-35). He described the 

 process in T. albacares and T. "sibi" (Pacific T. 

 obesus) as being directed obliquely ventrad so that a 

 line drawn through its axis would transect the 

 parasphenoid at or behind the junction with the 

 parethmoid; in T. thynnus, T. alalunga, and T. 

 atlanticus such a line would more nearly parallel the 

 parasphenoid and would not cross it. We find this 

 character variable within a species. Furthermore, 

 in larger fishes, the entire bone becomes relatively 

 shorter and wider, whereas, the process becomes 

 broader and more rounded. 



The head of the vomer in T. alalunga was described 

 as having a thin bony ridge behind the dentigerous 

 anterior portion, a similar ridge being present in 

 some T. thynnus, but not in T. albacares or T. obesus. 

 Actually, all the species may have such a ridge. In 

 T. alalunga teeth are generally restricted to the 

 anterior end; the posterior end is very thin. The 

 other species usually, but not always, bear teeth 

 along the entire ridge, and the posterior portion is 

 wider. In T. atlanticus the ridge is usually absent. 



ANATOMY AND SYSTEMATICS OF TUNAS 



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