FISHERY BULLETIN: VOL. 73, NO. 2 



Figure 5.— The bonita, Sarda vehx, (top) and the skipjack tuna, Euthynnus lineatus (bottom). Note differences in body 



shape and pectoral and caudal fin size and shape. 



similar. These species also have similar distribu- 

 tions and, in the Gulf of Panama, they occur in the 

 same areas and eat similar prey (crustaceans, 

 squid, and small fishes; pers. obs.) although Sarda 

 has a bigger mouth and large teeth. The 

 different mouths and other differences suggest 

 that the sv^^imming capability of these species are 

 also different. Sarda has a smaller pectoral fin 

 (Magnuson 1973; Figure 5, this paper) and a low^er 

 caudal fin aspect ratio (Fierstine and Walters 1968, 

 Table 7). Its red muscle is not as well developed as 

 that in Euthynnus (Fierstine and Walters 

 1968:17), and Sarda has much less blood 

 hemoglobin (Klaw^e et al. 1963). Finally, a very 

 striking difference exists in the maximum burst 

 speeds of E. affinia and S. chiliensis (Magnuson 

 1973, Table 6). In fact, the three vi^arm-bodied 

 species listed by Magnuson (Table 6), all have 

 burst speeds nearly double those of 5. chiliensis, 

 suggesting that elevated body temperatures, 

 coupled with morphological adaptations, greatly 

 increase the maximum swimming speed. The 

 principal contribution of high body temperature to 

 burst swimming is probably the maintenance of a 



thermal profile that warms large portions of white 

 muscle. 



For Katsuwonus, Euthynnus, and T. albacares, 

 which are all tropical species, there are differences 

 in several structures related to locomotion such as 

 caudal fin aspect ratio and the amount, distribu- 

 tion, and shape of red muscle (Fierstine and 

 Walters 1968). It is reasonable to assume that 

 these differences, combined with elevated body 

 temperature, must confer different capabilities for 

 acceleration, maneuverability, and sustained 

 swimming on different species. One difficulty with 

 the data presently available however is that T. 

 albacares grows to be much larger than skipjack 

 tunas, and allometric growth is known or thought 

 to occur in several locomotion-related structures 

 (see discussions by Gibbs and CoUette 1967; Mag- 

 nuson 1973). Without quantitative data on growth 

 patterns of these features, their contribution to 

 locomotion cannot be fully evaluated. 



ACKNOWLEDGMENTS 



This study was supported by the Smithsonian 



228 



