FISHERY BULLETIN: VOL. 73, NO. 2 



Figure 3.-Central heat exchanger of 

 Katsuwonus pelamis. (Top right, scale 

 = 5.6 cm): Transverse section show- 

 ing the position of the central heat 

 exchanger (che) and red muscle (rm). 

 (Top left, scale = 1.1 cm): Close view 

 of the rete (r), the dorsal aorta (da), 

 and the posterior cardinal vein (pcv) 

 in the haemal arch. (Bottom, scale = 

 1.9 cm): Red muscle and the central 

 heat exchanger. 



COMPARISON OF KATSUWONUS, 



EUTHYNNUS, AUXIS, AND 



T. ALB AC ARES 



Differences in Heat Exchangers 



Central heat-exchanger differences can be 

 summarized as follows: Katsuwonus, Euthynnus, 

 and Auxis have only a single vertical rete w^hereas 

 T. albacares has two much smaller retia. Thunnus 

 albacares' central exchanger is immediately below 

 the vertebral centrum (Figures 1 and 2) while in K. 

 pelamis it is lower, about midway between the 

 vertebrae and the coelomic cavity, and in Euthyn- 



nus and Auxis it is quite low, occurring just above 

 the coelom (Kishinouye 1923; Godsil 1954; Graham 

 1973). In E. lineatus and E. alletteratus, and K. 

 pelamis that I have examined, and in Auxis (God- 

 sil 1954), the dorsal aorta is actually embedded in 

 the dorsal side of the posterior cardinal vein and is 

 surrounded by a vast network of retial vessels 

 which in effect bathes the aorta in venous blood. 

 This structure has been interpreted as allowing 

 the rete to occupy a full arc over the vessels, thus 

 maximizing its heat-exchanging area (Graham 

 1973). 



Both K. pelamis and T. albacares have two pairs 

 of lateral exchangers. Katsuwonus has two 



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