GRAHAM: HEAT EXCHANGE IN SCOMBRIDS 



somewhat variable rows of retial vessels in each 

 lateral exchanger while T. albacares only has one. 

 Euthynnus and Auxis (Figure 1) have only a small 

 pair of epaxial heat exchangers. 



Thermal Profiles of Fish with 

 Central Exchangers 



Lateral midplane thermal profiles of T. al- 

 bacares and K. pelamis, taken in the red muscle 

 just posterior to the pectoral fins, illustrate 

 general differences in thermal profiles and body 

 temperatures between these species, E. lineatus, 

 and T. thynnus (Figure 4). Katsuwonus and 

 Euthynnus have much warmer core temperatures 

 than T. albacares, but warmest temperatures in 

 Euthynnus are restricted to a fairly narrow zone 

 around the vertebral column. Euthynnus' profile 

 therefore seems related to its poorly developed 

 lateral exchangers (also red muscle is very thick in 

 the center and thinner laterally, Figure 1) and, 

 based on structural similarities, this type of ther- 

 mal profile would be predicted for Auxis. Kat- 

 suwonus on the other hand, with its lateral 

 exchangers has heat widely distributed across its 

 body. 



Thunnus albacares, with a small central 

 exchanger and well-developed lateral exchangers 

 has a widely distributed warm region although it 

 is much cooler than K. pelamis and E. lineatus 

 (Figure 4). The dimensions of T. albacares' cu- 

 taneous vessels increase at a much faster rate with 

 increased body weight than do the dorsal aorta 

 and posterior cardinal vein, and in larger fish a 

 greater proportion of blood flow would occur 

 through lateral vessels which might change the 

 thermal profile. 



COMPARISONS WITHIN 

 THE GENUS THUNNUS 



Heat Exchangers and Thermal Profiles 

 in T. albacares and T. thynnus 



Comparative studies of the vascular anatomy of 

 Thunnus show different levels of structural 

 complexity in the heat-exchanging systems 

 (Kishinouye 1923; Godsil and Byers 1944; Gibbs 

 and Collette 1967) which relate to thermal profiles 

 and body temperatures. In T. thynnus, lateral heat 

 exchangers are used solely (Carey and Teal 1966). 

 Two rows of retial vessels emanate from each cu- 



o 



o 



a. 



E 



36 -I 



35 



34 



33 



« 



o 32 

 n 



■o 



30 



29 - 



Water temperature 29 C 



T. albacares 



— I 



center 



edge 



Relative distance through body 



Figure 4. -Lateral midplane thermal profiles from the center 

 (near the vertebrae) to the edge (subcutaneous) of red muscle in 

 four species of warm-bodied fish. (Data for Thunnus thynnus 

 were provided by F. G. Carey, that for Euthynnus lineatus are 

 from Graham 1973). 



taneous artery and vein (only one row occurs in T. 

 albacares), and these extend axially for a long 

 distance (Carey et al. 1971). Reliance upon cu- 

 taneous circulation is so extensive in T. thynnus 

 that the dorsal aorta is reduced in diameter and 

 the posterior cardinal vein is absent. In warm 

 water T. thynnus is about the same temperature 

 as T. albacares, but its thermal profile (Figure 4) 

 reflects the exclusive presence of lateral heat 

 exchangers in that warmest temperatures are 

 found in the middle of the muscle, while the center 

 of the fish is cooler (Carey et al. 1971). (Again, 

 thermal profiles probably change with body size.) 



Anatomical Features and 

 Phyletic Groupings Related to 



the Presence or Absence of 

 Complete Vertebral Circulation 



In their comprehensive study of the genus 

 Thunnus, Gibbs and Collette (1967) recognized 

 seven species which, on the basis of 18 characters, 

 were separated into two phyletic groups: the 

 bluefin tuna group, T. thynnus, T. alalunga, and T. 



225 



