HEAT EXCHANGE IN THE YELLOWFIN TUNA, THUNNUS ALBACARES, 



AND SKIPJACK TUNA, KATSUWONUS PELAMIS, AND THE 



ADAPTIVE SIGNIFICANCE OF ELEVATED BODY TEMPERATURES 



IN SCOMBRID FISHES 



Jeffrey B. Graham' 



ABSTRACT 



Thunnus albacares and Katsuwonus pelamis are warm-bodied fish and use retia mirabilia as counter- 

 current heat exchangers. Both species have four sets of lateral exchangers, two epaxial and two 

 hypaxial, each consisting of a large cutaneous artery and vein and rete. Katsuwonus pelamis has a 

 central exchanger, located within the haemal arch, which consists of the dorsal aorta, the posterior 

 cardinal vein, and a large vertical rete. The central heat exchanger in T. albacares, while also in the 

 haemal arch, is simpler, consisting of two small "wing-shaped" retia on either side of the dorsal aorta 

 and cardinal vein. 



The adaptive significance of the specialization for heat conservation is discussed. Body temperatures, 

 thermal profiles, and the natural histories of difl'erent warm-bodied species are compared, and warm 

 fishes are contrasted with scombrids that do not conserve heat. The skipjack tunas, Euthynnus and 

 Katsuwonus, have well-developed central heat exchangers and are much warmer than T. albacares. 

 Higher body temperatures in skipjacks seems related to their requirement for a higher basal swimming 

 speed and their faster burst speed. 



Comparisons on the basis of existing knowledge about the two phyletic groups of Thunnus reveal few 

 differences in swimming ability or factors related to locomotion. The bluefin group, consisting of T. 

 thynnus, T. maccoyii, and T. alalunga, however does contrast with the yellowfin group (T. albacares, T. 

 aflanticus, and T. tonggol) by maintaining generally higher body temperature differentials, having 

 incomplete vertebral circulation through the absence of a posterior cardinal vein, and occurring at 

 higher latitudes. 



Scombrids (mackerels, bonitos, and tunas) are 

 pelagic, oceanic fishes that are highly adapted for 

 continuous swimming. Some of the more advanced 

 scombrids (principally frigate mackerels, Auxis; 

 skipjack tunas, Euthynnus and Katsuwonus; and 

 tunas, Thunnus) have evolved the capacity to 

 conserve heat generated by the continuous met- 

 abolic activity of their swimming muscle and thus 

 maintain body temperatures that are warmer 

 than ambient seawater (Carey et al. 1971; Carey 

 1973). There has been convergent evolution for 

 this specialization in mackerel sharks (Isuridae) a 

 highly active, continually swimming group (Carey 

 and Teal 1969a). 



Warm-bodied fish retain heat by using retia 

 mirabilia (= wonderful network) as counter- 

 current vascular heat exchangers. The principal 

 advantage of a high and fairly constant body 

 temperature is facilitation of continuous swim- 



'Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, 

 Canal Zone. 



Manuscript accepted August 1974. 

 FISHERY BULLETIN: VOL. 73, NO. 2, 1975. 



ming by increasing the frequency of muscular 

 contractions, thus increasing available swimming 

 power (Carey et al. 1971). Also, warm-bodied fish 

 probably achieve a marked independence from 

 environmental temperature permitting them to 

 make rapid vertical and latitudinal migrations 

 vdthout the necessity of thermal acclimation. 



In their extensive review of warm-bodied fish, 

 Carey et al. (1971) described two types of heat 

 exchanger, lateral and central. Lateral heat 

 exchangers (Figure 1) are present in many warm- 

 bodied species but are best developed in the genus 

 Thunnus where they consist of four sets of longi- 

 tudinal subcutaneous arteries and veins (two 

 epaxial and two hypaxial), each with adjoining 

 layers of retial vessels that penetrate the red 

 muscle near the midplane (Gibbs and Collette 

 1967; Carey et al. 1971). Large, highly developed 

 central heat exchangers (Figure 1) are found in 

 Euthynnus, Katsuwonus, and Auxis. These are 

 located below the vertebral column, in the haemal 

 arch, and consist of a large vertical rete formed 

 from branches of the dorsal aorta and the posterior 



219 



