GRAHAM: HEAT EXCHANGE IN SCOMBRIDS 



exchanger may prove insufficient to maintain a 

 warm temperature. Carey and Teal (1966, 1969b) 

 pointed out the obvious insulative value of having 

 a large lateral heat exchanger between the warm 

 muscle and cool water. Also, in cool water it may 

 not be efficient for heat conservation to pump a 

 large volume of cool blood (from the gills) into the 

 center of the body via the dorsal aorta, and this 

 might explain why the dorsal aorta in T. thynnus 

 is small. Indeed, the lower core temperature found 

 in T. thynnus may result from the small volume of 

 unheated blood that does flow through the dorsal 

 aorta. Another vascular specialization that ap- 

 pears directly related to the cool-water distribu- 

 tion of the bluefin tuna group is the presence of 

 vascular bundles on their livers which enables 

 these fish to warm their viscera, thus facilitating 

 digestion in cooler water. 



A consideration of the bigeye tuna, T. obesus, 

 substantiates the idea that central heat 

 exchangers and ultimately complete vertebral cir- 

 culation are lost as tuna species evolve into cooler 

 habitats. Although T. obesus and T. albacares have 

 practically the same latitudinal distributions 

 (Gibbs and Collette 1967), the former occurs in 

 deeper and therefore cooler water (Kishinouye 

 1923:390 as Parathunnus mebachi, a synonym for 

 T. obesus). This aspect of the distribution of T. 

 obesus thus makes it intermediate, in terms of its 

 thermal habitat, to that of the bluefin and 

 yellowfin tuna groups. Thunnus obesus is also 

 morphologically intermediate to the bluefin and 

 yellowfin tuna groups of Thunnus. It has complete 

 vertebral circulation and vascular bundles on its 

 Hver (Gibbs and Collette 1967) yet, F. G. Carey 

 (pers. commun.) who has extensively studied this 

 species reports that it does not have a central heat 

 exchanger. With respect to body temperatures, 

 thermal profiles, and the structure of its lateral 

 heat exchangers, T. obesus closely resembles T. 

 thynnus (Carey and Teal 1966). Thus for the 

 bigeye tuna, which in terms of adapting to cool 

 water appears to be at an intermediate position 

 between the yellowfin and bluefin tuna groups, a 

 central heat exchanger is not present although 

 complete vertebral circulation persists. With re- 

 spect to the latter, however, and perhaps under- 

 scoring the de-emphasis of vertebral circulation, it 

 is relevant to point out that although T. obesus 

 does have a posterior cardinal vein, Godsil and 

 Byers (1944:114) describe it as "relatively small" 

 and note that it fuses anteriorly with the right 

 cutaneous vein. 



Elevated Body Temperatures and 



Locomotion in Skipjack Tunas and 



T. albacares 



Studies of scombrid locomotion (Fierstine and 

 Walters 1968; Magnuson 1970, 1973) suggest that 

 elevated body temperature in skipjacks, while 

 related to their requirement for a faster typical 

 (basal) speed, primarily contributes to their higher 

 burst swimming speed. 



Magnuson (1970, 1973) pointed out that 

 scombrids are negatively buoyant and that the 

 skipjack tunas, which lack a gas bladder, are even 

 more negatively buoyant than is T. albacares. To 

 compensate for this, and to maintain hydrostatic 

 equilibrium, skipjack tunas must swim more 

 rapidly. Magnuson has argued that the need for a 

 faster basal speed correlates well with a sig- 

 nificantly higher amount of red muscle found in 

 skipjack tunas (about 8% of body weight in Kat- 

 suwonus and Euthynnus, compared with 7.4% in 

 T. albacares of the same size) and with their 

 slightly greater amounts of blood hemoglobin 

 (Magnuson 1973, Table 7). 



The amount of red muscle of course bears an 

 important relationship to body temperature. In 

 warm-bodied fish, retia supply blood to red muscle 

 which is highly aerobic. Red muscle is the principal 

 organ used for basal swimming (Rayner and 

 Keenan 1967), and therefore it is the principal site 

 of thermogenesis. (White muscle mainly functions 

 in burst swimming.) Thus skipjack tunas, to 

 maintain a high basal speed, have a large mass of 

 red muscle, and it could be logically concluded that 

 to augment power output, the capacity to conserve 

 heat and keep swimming muscles warm has 

 evolved in skipjack tunas. The difficulty with this 

 idea however is that other scombrids such as the 

 Pacific bonito, Sarda chiliensis, have minimum 

 speed requirements as high as those of the skip- 

 jack tunas (Magnuson 1973), but are not warm- 

 bodied, nor do they have high hemoglobin levels or 

 large amounts of red muscle. This obviously in- 

 dicates that elevated body temperatures and high 

 amounts of hemoglobin and red muscle in the 

 skipjack tunas, while contributing to the 

 sustenance of a high basal speed, must have other 

 functions as well. 



Further comparison of Sarda with Euthynnus 

 provides valuable insight to the significance of 

 elevated body temperature to burst swimming. 

 Sarda velox and E. lineatus (Figure 5) attain 

 about the same size and are morphologically 



227 



