FISHERY BULLETIN: VOL. 73, NO. 2 



maccoyii; and the yellowfin tuna group, T. al- 

 bacares, T. atlanticus, and T. tonggol. (The seventh 

 species, T. obesus, has traits in common w^ith both 

 groups and vv^ill be discussed later.) Several of the 

 characters used (Gibbs and Collette 1967, Table 4) 

 to distinguish these groups are related to the 

 presence or absence of complete vertebral circula- 

 tion (both a dorsal aorta and posterior cardinal 

 vein present). The yellow^fin tuna group has a 

 posterior cardinal vein, the bluefin tuna group does 

 not. Another striking difference is the presence 

 of large striations and vascular cones on the livers 

 of fish in the bluefin tuna group. The importance of 

 this is discussed below^. 



There are several structural modifications in the 

 vertebrae of the yellowfin tuna group which per- 

 mit the passage of more or larger blood vessels 

 through the haemal arch. Prezygapophyses arise 

 far more ventrad on the haemal arch, post- 

 zygapophyses are longer, and the inferior 

 foramina are larger (Gibbs and Collette 1967, 

 Figures 10-13). In describing these vertebrae, 

 Gibbs and Collette (1967:80) remarked that the 

 development of the vertebral openings and 

 processes in the yellowfin tuna group is almost as 

 complex as that in Auxis, Euthynnus, and Kat- 

 suwonus. The presence of complete vertebral cir- 

 culation and appropriate modifications in the ver- 

 tebral column suggested to me that other species 

 in the yellowfin tuna group, in addition to T. al- 

 hacares, may have central heat exchangers. I have 

 examined a preserved section of vertebral column 

 from T. atlanticus (collected in the Gulf of Mexico 

 and sent to me by F. G. Carey) and T. tonggol 

 (obtained by G. Sharp) both of which have a cen- 

 tral exchanger like that of T. albacares. 



ADAPTIVE SIGNIFICANCE OF 



DIFFERENT HEAT EXCHANGERS, 



BODY TEMPERATURES, AND 



THERMAL PROFILES 



Heat exchangers in T. albacares differ from 

 those of K. pelamis and E. lineatus, and among 

 these three species, there are marked differences 

 in body temperatures and thermal profiles (Figure 

 4). Thunnus albacares and T. thynnus also have 

 different heat exchangers, different body 

 temperatures, as well as different thermal profiles 

 depending on body size. Are there morphological 

 features related to locomotion, or ecological fac- 

 tors, such as geographical distribution patterns or 

 feeding behavior, that would explain thermal and 



anatomical differences between K. pelamis or E. 

 lineatus and T. albacares or between species of 

 Thunnusl 



Comparisons Within the Genus Thunnus 



The morphologies and locomotion of T. thynnus 

 and T. albacares have not been compared. There 

 are some data; however it is diffuse and mostly 

 anecdotal, and it does not suggest functional 

 differences in these two species or in the bluefin 

 and yellowfin tuna groups of Thunnus. 



If species in the yellowfin and bluefin tuna 

 groups are compared on the basis of existing 

 body-temperature data (cf . Carey et al. 1971, Table 

 1), it is apparent that species in the yellowfin tuna 

 group have lower relative temperatures than those 

 in the bluefin tuna group. Ambient water 

 temperatures are not the same for these 

 different species, and only a general comparison is 

 possible. Still, these differences agree with the 

 known differences in T. albacares and T. thynnus 

 (Carey and Teal 1969b; Carey 1973) and are 

 suggestive of a general trend of body-temperature 

 differences that might in turn reflect a significant 

 functional difference between the two taxonomic 

 groups. 



A feature in the natural history of species in the 

 yellowfin and bluefin tuna groups that clearly 

 separates them, and relates to their anatomical 

 and temperature differences as well, is the water 

 temperature that they normally inhabit. Thunnus 

 maccoyii and T. alalunga of the bluefin tuna group 

 occur only in cool water while T. thynnus, because 

 of its thermoregulatory ability, is wide ranging 

 and may occur in waters from 6° to 30°C but seems 

 most common in the range 16° to 22° C (Gibbs and 

 Collette 1967; Carey and Teal 1969b). Of the 

 yellowfin tuna group, T. albacares usually occurs 

 from 20° to 28°C (Schaeffer et al. 1963), and both 

 T. tonggol and T. atlanticus are strictly tropical 

 species (Gibbs and Collette 1967). 



Several facts suggest that incomplete vertebral 

 circulation in the bluefin group is a specialization 

 for living in cooler water and that central heat 

 exchangers are a primitive character related to the 

 occurrence of the yellowfin tuna group in tropical 

 waters. First, central heat exchangers, being re- 

 stricted to within the haemal arch, are, of necessity 

 small and therefore have limited heat-exchanging 

 capacity. Thus, in cool water, and, given that red 

 muscle is large and located at varying distances 

 away from the vertebrae, a small central heat 



226 



