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Fishery Bulletin 94(3), 1996 



belly between pectoral and pelvic fins as in 

 Euthynnus; pectoral and pelvic fins purple, their in- 

 ner sides black. 



Relationships Auxis is related most closely to 

 Euthynnus (Kishinouye, 1923; Collette, 1979, 1984; 

 Finnerty and Block, 1995). It has been considered 

 (Collette, 1979) to be the most primitive member (ex- 

 cept for Allot hunnus) of the Thunnini, the true tu- 

 nas, because it lacks the frontoparietal fenestra 

 present in Euthynnus, Katsuwonus, and Thunnus, 

 because its subcutaneous circulatory system is more 

 poorly developed than that of the more advanced 

 tunas, and because Auxis shows no suturing of the 

 first vertebra to the skull as is present in higher 

 Thunnini. Tunas have a pair of lateral retia which 

 serve the red muscle and act as heat exchangers en- 

 abling them to maintain body temperatures several 

 degrees warmer than ambient water (3.5-9.5°C, 

 mean 5.9°C, in the case of Auxis, Schaefer, 1985). In 

 addition, Auxis and Euthynnus share a large central 

 heat exchanger below the vertebral column in a se- 

 ries of enlarged haemal arches. Auxis is endother- 

 mic at sizes of 200 mm FL and larger (Dickson, 1994). 

 The liver is similar to that in Euthynnus in having a 

 very long right lobe running the length of the body 

 cavity (Fig. 2). Prominent branches of the hepatic 

 vein are present on the ventral surface of the liver 

 lobes in both genera. Auxis differs from Euthynnus 

 in having a greatly reduced left liver lobe; the left 

 and middle lobes are of about equal length in 

 Euthynnus. A swimbladder is present in larvae but 

 degenerates by 20 mm as in Euthynnus and 

 Katsuwonus (Richards and Dove, 1971). 



Osteological comparisons Several authors 

 (Kishinouye, 1923; Watanabe, 1962, 1964; Yoshida 

 and Nakamura, 1965; Chi and Yang, 1972; Uchida, 

 1981; Zavala-Camin, 1983) have reported osteologi- 

 cal differences between the species. Watanabe ( 1962, 

 1964 ) found that there were two types of Auxis based 

 on long and short parapophyses. The differences were 

 more prominent in specimens larger than 111 mm, 

 especially on the 25th and 27th vertebrae. Auxis 

 thazard had long free parapophyses, whereas A. 

 rochei (as A. tapeinosoma) had short free para- 

 pophyses. In addition, in young A. thazard the first 

 free parapophysis is developed mostly on the 23rd 

 vertebra, and the first inferior foramen occurs on the 

 27th or 28th vertebra. In young A. rochei, however, 

 the first free parapophysis is usually produced on 

 the 21st or 22nd vertebra, and the first foramen oc- 

 curs on the 29th or 30th vertebra. Other differences 

 between the two species were the ratio of the caudal 

 vertebral length to precaudal vertebral length and 



Gall bladder 

 Spleen 

 Stomach 

 Intestine 



Figure 2 



Viscera in ventral view. (A) Auxis t. thazard, 

 USNM 339045, Philippine Is., 357 mm FL. 

 (B) Auxis r. rochei, USNM 339044, Philippine 

 Is., 262 mm FL. 



the ratio of body height at anal-fin origin to length 

 from first dorsal to second dorsal origin. Yoshida and 

 Nakamura ( 1965) found that in A. thazard the length 

 of the anterior branch of the haemal process of the 

 24th to the 28th vertebra was longer and contacted 

 the preceding arch. In A. rochei the anterior branches 

 of the haemal processes were short, more fragile, and 

 were never in contact with the preceding haemal 

 arches. The angle between the temporal crests and 

 a line parallel to the supraoccipital crest ranged from 

 to 2 degrees in A. rochei and from 3 to 7 degrees in 

 A. thazard. In skulls of the same length, skulls of A. 

 thazard were wider than those of A. rochei. 



Larval differences In several geographic regions, 

 workers have been able to divide their material of 

 larval and juvenile Auxis into what appear to be two 



