isthmus nor directly anterior to the 

 anus, 

 f One or several melanophores ap- 

 pear along the dorsal and ventral 

 sides of the body, 

 g The most anterior of the chro- 

 matophores on the dorsal 

 side is located posterior to 

 the origin of the second dor- 

 sal fin. 



bluefin tuna, Thunnus thynnus 



g The most anterior of the chro- 



2 



matophores on the dorsal 

 side is located anterior to the 

 origin of the second dorsal 

 fm. 



long-tailed tuna, Thunnus tonggol 



f One of several melanophores 

 appear along the ventral side 

 of the body. 



bigeye tuna, Thunnus obesus 



c No melanophores appear on the sides of 



the body. 



h Chromatophores appear at tip of lower 



jaw. In the profile of the head, the 



center of the eye is clearly located 



higher than the tip of the snout. 



yellowfin tuna, Thunnus albacares 



hj No chromatophores appear on tip of 

 lower jaw. In the profile of the head, 

 the center of the eye is about on the 

 same level as the tip of the snout or 

 slightly higher. 

 albacore, Thunnus alalunga 



Briefly, larval Auxis can be distinguished from other 

 larvae of tuna and tunalike fishes by the presence of a 

 dotted line of melanophores along the dorsal, lateral, 

 and ventral center lines of the caudal peduncle 

 (Ueyanagi 1964). The standard pattern is three lines, 

 but sometimes only two — the dorsal and ventral — are 

 seen. In small specimens, however, the line on the 

 ventral side is not confined to the vicinity of the caudal 

 peduncle but extends farther forward. Other character- 

 istics include the appearance of melanophores on the 

 isthmus and just anterior to the anus, late appearance of 

 chromatophores on the forebrain (they do not appear 

 until the fish attains a body length of about 8 mm), and 

 poorly developed chromatophores on the first dorsal fin 

 (they do not appear until the fish becomes about 10 mm 

 long). The outline of the anterior surface of the head 

 gives an impression of roundness and the separation 

 betweeen the first and second dorsal fins becomes quite 

 apparant at lengths of 10 mm or greater. 



Additional observations on the morphology of larval 

 Auxis showed that the snout is shorter than in other 

 tuna and tunalike larvae of comparable size (Jones 

 1963). Jones also noted that larval Auxis has 39 

 myotomes, a large chromatophore at the symphysis of 

 the pectoral girdle, and that fin ray development is later 



than in other tuna and tunalike larvae. Jones and 

 Kumaran (1964) added that compared with larval 

 Euthynnus affinis. larval Auxis exhibits later develop- 

 ment of the first dorsal fin and has relatively few chro- 

 matophores on the first dorsal fin membrane, and its fin 

 membrane is relatively narrow. 



The variations in pigmentation among larval Auxis 

 has been used as a basis for separating the larvae into 

 two possible types. Matsumoto (1959), who described 

 the morphology of larval and postlarval Auxis on a 

 worldwide basis, pointed out the possibility that this 

 variation may well be due to the presence of two species 

 of frigate tuna; therefore, he provided two separate de- 

 scriptions calling them Type I and Type EI, as shown in 

 Figure 22. 



Type I and Type II larvae show only minor differ- 

 ences and the most obvious one is the variation in pig- 

 mentation near the caudal peduncle (Matsumoto 1959). 

 Type I, which resembles the form described as A. 

 thazard (Matsumoto 1958), has three equally developed 

 rows of pigment whereas in Type 11 the middorsal edge 

 of the caudal peduncle usually contains only one or two 

 chromatophores and the midlateral line has none. 



Subsequently, Type I larvae, which are stouter than 

 Type n. were provisionally identified as A. thazard and 

 Type II, which are relatively elongate, were identified as 

 A. rochei (Jones 1963). Further observations revealed 

 that compared to A. thazard, larval A. rochei has a 

 relatively shallow body, shows later development of the 

 spinous dorsal, and has less intense pigmentation on the 

 caudal peduncle (Jones 1963; Gorbunova 1969). A later 

 study confirmed Jones' observation; Yabe and Ueyanagi 

 (1962) published the results of a study on larval tuna in 

 which they positively identified specimens of A. rochei. 



A controversy over the identification of larval Auxis 

 erupted in 1963 when Jones (1963) questioned the re- 

 sults obtained by Mito (1961), who described the egg 

 and larval development oi Auxis (Fig. 23). Jones pointed 

 out that because A. rochei is the most common frigate 

 mackerel in Japanese waters, the eggs and larval stages 

 described by Mito may be this species; but he also noted 

 that there were striking differences between larvae 

 described by Mito and those described by Matsumoto 

 (1959) and Jones (1961). These differences, he believed, 

 should be explained by Mito. The breakthrough in arti- 

 ficial rearing accomplished by the Japanese should make it 

 possible to comptire specimens of artificially reared larvae 

 and those obtaind from net tows and night lighting to ob- 

 taln positive identification. 



3.23 Adolescent phase 



Juvenile Auxis can be distinguished from other tunas 

 by certain morphological features (Schaefer and Marr 

 1948; Wade 1949; Mead 1951; Matsumoto 1962). They 

 can be distinguished from other tunas by the wide gap 

 between the first and second dorsal fins and the virtual- 

 ly colorless appearance of the first dorsal. Compared to 

 Katsuwonus, however, the first dorsals are somewhat 

 similar, both having a lightly pigmented or colorless fin. 



27 



