Table 4. — Constants for the allometric equation Y=bX'' 

 Jor larvae and juvenile wahoo collected in the central Pacific 

 Ocean, 1960-62 



1 5 6 7 8 9 10 

 STANDARD LENGTH (MM I 



Figure 4. — Scatter diaj^ram of body-part measurements 

 plotted against standard length of larval and juvenile 

 wahoo. Straight-line segments are regression lines 

 fitted by the least squares method to respective 

 groups of points. Roman numerals indicate curve 

 segments. 



of distinct growth patterns or gradients (Simp- 

 son et al., 1960). The subdivision of the wahoo 

 data provides a satisfactory description of the 

 growth relation. 



Certain points should be kept in mind when 

 interpreting the curves fitted to the wahoo data. 

 The curves are based on a small number of 

 observations, and some curves are based on a 

 narrow range of standard lengths. Since there 

 is little variability in the wahoo data, the small 

 number of observations appear adequate. The 

 narrow range in standard lengths for some 

 curves appears to be a phenomenon of the 

 growth of many fishes. In addition, it is known 

 that shrinkage of fish larvae occurs in pre- 



sei-ved specimens. It is known also that maxi- 

 mum shrinkage occurs within the first few 

 months of preservation and that shrinkage is 

 small after 1 or 2 years, provided there is no 

 drastic change in the amount and concentration 

 of the preservative. As all wahoo specimens 

 had been preserved more than 4 years and were 

 treated in the same manner, the effects of 

 shrinkage are thought to be negligible. 



A difficulty in dealing with changes in body 

 form in wahoo larvae is that the snout grows 

 much more rapidly than do other parts of the 

 body; this differential grovvd:h influences com- 

 parisons based on standard length. To de- 

 termine how snout length influences standard 

 length, I computed regressions of body parts on 

 standard length and on standard length-less- 

 snout length for the four body parts shown in 

 figure 4. Both regression lines showed allo- 

 metric growth in all four body parts. Since the 

 regressions based on standard length fit the 

 data better than do those based on standard 

 length-less-snout length, standard length was 

 used as the basis of comparison. 



Head and Its Components 



The most striking change in body form oc- 

 curs in larvae longer than 3.8 mm., in which 

 head length increases very rapidly. In larvae 

 of this size, head length is nearly one-third 

 of the standard length, but, in larvae 6.6 mm. 

 long, it represents one half the standard length. 

 This large increase in head length is due mostly 

 to rapid growth of the premaxillary. 



Five separate curves are required to describe 

 premaxillary growth in wahoo 2.8 to 23.7 mm. 

 long (fig. 4 and table 4). The finst curve 



308 



MORPHOLOGY AND DISTRIBUTION OF LARVAL WAHOO 



