TAXONOMIC STUDY OF THE TUNAS
Tamotsu IWAI, Izumi NAKAMURA and Kiyomatsu MATSUBARA
(Department of Fisheries, Faculty of Agriculture Kyoto University, Maizuru)
The taxonomic status of tunas was evaluated on the basis of previous descriptions and new materials taken from various localities of the world. A number of tunas were carefully examined on many characteristics, i, e., proportinal measurements, skeletal system, sensory organs and visceral organs. From the data obtained, seven valid species, all referable to the one genus Thunnus, were recognized: T. alalunga (BONNATERRE) , T. thynnus (LINNAEUS), T. maccoyii (C>ASTELNAU), T. obesus (LOWE), T. albacares (BONNATERRE), T.atlanticus (LESSON), and T. tonggol (BLEEKER). The bluefin tunas inhabiting the Atlantic, Mediterranean, Pacific, several regions of the Indian Ocean, etc. belong to the single species, T. thynnus. But, the problem whether the Atlantic and Pacific populations should be classified to respective subspecies requires further comparative study. In comparison with the bluefin tuna, the southern bluefin tuna, T. maccoyii, taken chiefly from the eastern Indian Ocean and east seas of Australia, is distinguishable in having some distinctive features. e. g., yellow caudal keel, larger eye, and longer pectoral fin.
In general, none of external features, taken singly, can be uesd for separation of species of young tunas except for T. thynnus and T. maccoyii, because of their resemblance in proportional measurements and meristic characters. The comparative study revealed that olfactory resettes are valid feature distinguishing species of genus Thunnus. As it is generally understood, number of gill rakers and structural diagnoses of liver are also useful features in tuna systematics. In this respect, a combined key of these features may ensure the identification of seven species of tunas.