FISHERY BULLETIN: VOL. 73, NO. 2 



region extends laterally to just below the skin. The 

 average deep, intermediate, and subcutaneous red 

 muscle temperatures of four K. pelamis (caught in 

 28.5°C surface water) were 35°, 35°, and 33°C. 

 Deep white muscle temperatures in these fish 

 averaged 34° C; brain temperatures were 33° C. 

 The temperatures reported here for T. albacares 

 and K. pelamis are in good agreement with those 

 found for these species by other investigators 

 (Barrett and Hester 1964; Stevens and Fry 1971). 



Heat-Exchaneer Structure and 

 Red-Muscle Distribution 



Thunnus albacares 



The distribution and structure of the lateral 

 heat exchangers found for T. albacares in this 

 study agree fully with those described by Gibbs 

 and Collette (1967) and are summarized here with 

 new notes on variations related to size. Epaxial 

 and hypaxial arteries and veins subdivide from 

 their respective trunks at about vertebrae no. 10 

 and extend along the body to about two-thirds of 

 the way from the second dorsal fin to the tail (ver- 

 tebrae no. 29 or 30) where they are rejoined by a 

 commissure. One row of retial vessels originates 

 from the lateral edge of each artery and vein, and 

 this is consistent with the observations of 

 Kishinouye (1923, as Neothunnus). Thunnus al- 

 bacares' lateral retia are long and strongly curved 

 towards the center of the body. Retial curvature 

 was not observed in specimens smaller than 3 kg. 

 Cutaneous vessel diameters increase dramatically 

 with increased size, ranging from 0.5 to 1.0 mm 

 (artery and vein) in a 1.1-kg specimen to 6.0 to 8.0 

 mm in a 42-kg fish. Retial vessels ranged from 0.05 

 to 0.1 mm in diameter. 



The central heat exchanger in T. albacares ex- 

 tends from the first to the second dorsal fins (ver- 

 tebrae no. 8 or 9 to 20) and is situated immediately 

 below the vertebrae in the haemal arch. This 

 structure is composed of the dorsal aorta, the 

 posterior cardinal vein, and their small vessels 

 that form two "wing-shaped" retia (Figure 2). 

 Diameters of the dorsal aorta and posterior car- 

 dinal vein only increase slightly with increasing 

 size, ranging from 1.5 to 3.0 mm in a 2.7-kg fish to 

 3.5 to 4.0 mm in a 42-kg specimen. This contrasts 

 markedly with the large weight-related change in 

 the diameters of the lateral blood vessels. The 

 central retia originate as thick bundles in the 

 haemal arch, then extend supralaterally and pass 



through vertebral foramina into the red muscle. In 

 the muscle these vessels flatten into broad con- 

 tinuous sheets of alternating veins and arteries 

 (0.1 to 0.2 mm in diameter) that are only one layer 

 thick (Figure 2). This layer penetrates far into the 

 muscle, from 18 mm in a 2.7-kg fish to 40 mm in a 

 42-kg fish. 



Red muscle in T. albacares appears in thin bands 

 along each side of the fish at the level of the ver- 

 tebrae (Figure 2). Only red fibers from the hypa- 

 xial muscles actually reach the vertebrae, but 

 epaxial and hypaxial muscle both extend well 

 toward the fishes' side. Longitudinally, red muscle 

 extends from behind the transverse septum (ver- 

 tebrae no. 6 or 7) to as far as the fifth finlet (ver- 

 tebrae no. 28 or 29) and is fairly uniform in thick- 

 ness and shape (cf. Kishinouye 1923, Plate XVII, 

 as Neothunnus). As was found for E. lineatus 

 (Graham 1973) and, as would be expected, there is 

 good agreement in the lineal distribution of red 

 muscle and the lateral and central heat exchangers 

 of T. albacares. 



Katsuwonus pelamis 



Except for its higher position in the body, the 

 central exchanger of K. pelamis (Figure 3) is very 

 similar to that of Euthynnus and Auxis, consist- 

 ing of the closely associated dorsal aorta and 

 posterior cardinal vein and a thick vertical rete, all 

 in the haemal arch (Kishinouye 1923; Godsil 1954; 

 Graham 1973). Just posterior to the pectoral fins in 

 a 580-mm (about 4 kg) specimen, the following 

 vessel diameters were measured: dorsal aorta, 

 2.0 mm; posterior cardinal vein, 4.0 mm; retial 

 vessels, 0.05 to 0.1 mm. At its center (Figure 3), 

 vessels in the central rete of this fish were 8.0 mm 

 long. 



Lateral heat exchangers are better developed in 

 K. pelamis than in either Euthynnus or Auxis 

 (Figure 1). Both epaxial and hypaxial sets of cu- 

 taneous vessels, with retia, are present, but they 

 are further apart than in T. albacares (Figure 1), 

 reflecting the laterally thicker wedge of red 

 muscle in K. pelamis (see below). The cutaneous 

 vessels are smaller than in Thunnus. The most 

 developed retial vessels occur anteriorally but are 

 variable in their position, length, and the direction 

 they penetrate red muscle (cf. Godsil and Byers 

 1944, Figure 15). 



Red muscle in K. pelamis is thicker than in T. 

 albacares but does not appear to extend as far into 

 the tail. In a transverse section (Figure 3), both 



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