Figure 22.— Auxis larvae Type I. 7.05 mm (A) and T>pe II, 7.2 mm (B) (Matsumoto 1959). 



Katsuwonus, nevertheless, possess more pigment in the 

 first dorsal, which is also more or less brown or tan dis- 

 tally, whereas in Auxis, a few scattered chromatophores 

 are found along several of the anterior rays in the distal 

 portion of the first dorsal. Compared with Euthynnus, 

 Auxis are rounder in cross section, relatively less com- 

 pre.ssed laterally, and the length of the head and the 

 caudal region shorter in comparison to total length. 

 Juvenile Auxis also pKJssess a black sptot at the isthmus 

 which is seen only in one other genus, Euthynnus. In 

 young Euthynnus, however, the first and second dorsals 

 are continuous. The large interspace between the dor- 

 sals, the low ray count of the first dorsal, and the 

 absence of an elaborate "trellis" of the vertebrae also 

 help to separate Auxis from Euthynnus and Kat- 

 suwonus. 



The pattern of pigmentation is also a good identify- 

 ing character. Schaefer and Marr (1948) wrote "In the 

 smaller specimens the prominent areas of pigmentation 

 are on the upper and lower jaws, above the snout, 

 around the postero-ventral margin of the orbit, on the 

 upper operculum, between the orbits, along the mid-line 

 of the body, along the bases of the dorsal and anal fins 

 including the finlets, and around the posterior end of the 



urostyle. Large chromatophores in the peritoneum show 

 through the body wall along the upper half of the body 

 cavity. None of the fins or finlets bears pigment spMsts, 

 with the exception of the first dorsal. The first dorsal 

 bears a few scattered chromatophores, largely distrib- 

 uted along the spines." 



Several features of the axial skeleton also are useful in 

 separating the juveniles of partially digested tunas. Pot- 

 thoff and Richards (1970), who studied regurgitated 

 food of terns and noddies from the Dry Tortugas, Fla., 

 observed that damaged or partly digested Auxis and 

 those <30 mm could easily be separated from Euthyn- 

 nus if judged on fin position and gill raker counts. Pot- 

 thoff and Richards revealed that extreme care must be 

 used to examine Auxis specimens because the spines of 

 the first dorsal in some juveniles are subcutaneous and 

 come to the surface in the fin interspace. Furthermore, 

 they noted that the number of gill rakers is evenly dis- 

 tributed over the range for Auxis whereas they are more 

 normally distributed in other species. Counts were as 

 follows: 19 in 38% of the ceratobranchials, 20 in 25%, 21 

 in 25' c, and 22 in 12%. Potthoff and Richards suggested 

 that this kind of distribution may be attributable to the 

 presence of two types of Auxis £md their intermediates. 



28 



