depth increases slightly faster 

 than the body. 



(a 



1.180) 



Distance from Snout to Anus 



Snout-to-anus distance shows positive allo- 

 metry, but this phenomenon results from the 

 sharp increase in snout length relative to stand- 

 ard length. If the snout length is subtracted 

 from both snout-to-anus distance and standard 

 length, a linear relation. Y - -0.09 4 0.623X. 

 is evident (fig. 6). Thus the snout-to-anus dis- 



2 4 6 8 10 12 



STANDARD LKNGTIl I.E.SS S.NOUT(.MM.l 



16 18 20 22 24 



STA.VDARD LENGTH (MM.) 



Figure G. — Regressions of snout-to-anus distance less 

 snout on standard length less snout (upper line), and 

 orbit diuiiK'ter on standard length (lower line). Lines 

 fitted by the method of least squares. 



tance, less snout length, increases 0.623 mm. 

 for each 1.0-mm. increase in standard length, 

 less snout length. A linear relation between 

 snout-to-anus distance and standard length also 

 exists in the Pacific mackerel, Pneumatophoriis 

 diego (Kramer, 1960). 



The low rate of increase in the snout-to-anus 

 distance of wahoo larvae is probably related to 

 the shape of the digestive tract and the position 

 of the anus. The digestive tract in wahoo lar- 

 vae, unlike that in larvae of yellowfin tuna and 

 skipjack tuna, is elongate, and the anus is situ- 

 ated near the origin of the anal fin, i.e., between 

 the 26th and 29th myomeres, in larvae as small 

 as 2.8 mm. (table 7). The anus maintains this 

 relative position throughout the larval and 



juvenile stages. In yellowfin tuna and skipjack 

 tuna less than 7.0 mm. long, however, the diges- 

 tive tract is compact and the anus is located at 

 a point midway between the posterior edge of 

 the head and the origin of the anal fin, i.e., 

 between the 9th and 11th myomeres; further- 

 more, the anus shifts posteriorly as the larvae 

 increase in size, until it is situated near the 

 origin of the anal fin in larvae 11.0 to 13.0 

 mm. long. 



In one respect the digestive tract resembles 

 that of larvae of yellowfin tuna and skipjack 

 tuna : it forms a complete loop, which is clearly 

 visible in larvae 2.8 to 5.8 mm. long (fig. 2A-D) . 

 The loop is diflficult to see in larvae larger than 

 6.8 mm. because of the heavier abdominal mus- 

 culature and the increased pigmentation over 

 the digestive tract (fig. 3A-D). The 4.0-mm. 

 and 5.0-mm. Pacific mackerel larvae illustrated 

 by Kramer (1960) have a similar loop in the 

 digestive tract. 



Orbit Diameter 



Throughout the size range examined, the 

 relation of orbit diameter to standard length is 

 linear, Y = -0.019 + O.OIOIX, the orbit in- 

 creasing 0.101 mm. in diameter for l.O-mm. in- 

 crease in standard length (fig. 6). The eye is 

 cleft ventrally. 



OSSIFICATION OF SKELETON 



Myomeres, vertebrae, teeth, branchiostegal 

 rays, fin rays, and spines were counted (table 

 7), and the ossification of bones was studied 

 after the larvae had been stained with alizarin. 

 A bone is considered as ossified if it absorbs 

 the stain. O.ssification proceeds more slowly in 

 wahoo than it does in some other scombrids. 

 The sequence of ossification is summarized in 

 table 8. 



Pectoral Girdle 



The cleithrum is one of the first bones to 

 ossify. The process begins in larvae about 2.8 

 mm. or smaller and is complete at 3.1 mm. or 

 larger. 



Upper and Lower Jaws 



In the upper jaw, the development of the 

 maxillary and premaxillary is similar to that 

 described for Trachuiits (Berry, 1964) . The 



312 



U.S. FISH AND Vi^ILDLIFE SERVICE 



