each side ; they are absent in the other species. 



A post-cardinal vein joins the right cutaneous vein 

 in T. albacares, T. obesiis, T. atlanticus, and T. tonggol; 

 it is absent in T. thijnn us, T. maccoyii, and T. alalunga. 



OLFACTORY ORGAN 



As this manuscript was being completed, Iwai and 

 Nakamura (1964b) described their use of the olfac- 

 tory rosette to distinguish species of Thunnus. In 

 each nasal cavity beneath the anterior naris is an 

 olfactory rosette, consisting of numerous laminae 

 arranged radially around a central axis. According 

 to Iwai and Nakamura, T. alalunga is unique in 

 having a pair of fleshy labia surrounding the short 

 laminae. The laminae in T. obesus are described as 

 smooth, greatly expanded distally with adipose tissue, 

 and often partly fused distally. In T. thynnus the 

 laminae in small specimens are smooth and of uni- 

 form thickness to the distal edge, whereas in larger 

 specimens the laminae are smooth and distally ex- 

 panded, but with little evident adipose tissue. 

 Thunnus maccoyii is differentiated from T. thynnus 

 by the presence, in some specimens, of slight fringing 

 in the distal ends of the laminae. In T. albacares 

 and T. tonggol the laminae are entirely fringed on 

 their distal edges; and in large T. albacares the basal 

 half of the rosette is densely spotted with pigment; 

 howe\'er, Iwai and Xakamura admit that tlie rosettes 

 of T. tonggol and small T. albacares resemble each 

 other, and, from their figure 3, also resemble those of 

 T. maccoyii. On the basis of admittedly insufficient 

 material, Iwai, Nakamura, and Matsubara (1965) 

 indicate that the nasal rosettes of T. atlanticus closely 

 resemble those of T. tonggol except that the laminae 

 of the former tend to ha\'e the distal ends folded 

 inward. 



We have examined very few nasal rosettes, but oiu- 

 observations indicate a need for more careful scrutiny 

 of this character, with respect to normal variation 



and growth changes, before it is used widely. A 

 specimen of T. alalunga and two of T. obesus agreed 

 with Iwai and Nakamura (1964b), but in one T. 

 obesus the laminae were lightly pigmented and had 

 short, flat fimbriae along their entire length. Two 

 specimens of T. albacares were distinctive in the 

 abundance of pigment in the laminae, but while in 

 one the fimbriae were \'ery pronounced and finger- 

 like, in the other they were less developed and flatter. 

 Rosettes of one T. allanlicus and one T. tonggol were 

 virtually identical and were similar to those of T. 

 albacares, with dense, short, flat fimbriae on the 

 laminae, but had less abundant pigment; folding of 

 the distal ends of the laminae was not apparent in 

 either species. In a larger specimen of T. thynnus 

 from Cape Town, South Africa, the laminae were 

 almost uniform in thickness to the distal edge and 

 bore prominent flattened fimbriations along most of 

 their length. These observations are enough at 

 variance with those of Iwai and Nakamura that they 

 clearly show the need for further study. 



MERISTIC CHARACTERS 



The species of Thunnus are essentially identical 

 in the number of fin rays (table 1). The number of 

 gill rakers is the only meristic character that we have 

 found valuable in separating species of Thunnus 

 (table 2). 



Species Differences. — T. atlanticus has fewer gill 

 rakers (19-25) than any other species of Thunnus in 

 the Atlantic, and T. tonggol has fewer (19-26, rarely 

 to 28) than any other Thunnus in the Indo-Pacific 

 (table 2). T. thynnus and T. maccoyii have the 

 greatest number of gill rakers in the genus (31-43). 

 The other three species fall between these two groups 

 with a combined range 23-35. The overlap be- 

 tween species with low, medium, and high numbers 

 of gill rakers is very slight. The T. thynnus- 

 maccoyii complex shows differences of some magni- 



T.^BLE 1. — Range of variation in fin-ray counts in the specie.s of Thunnus 

 [Based on original and published data] 



ANATOMY AND SYSTEMATICS OF TUNAS 



91 



