100 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



nearly continuous. Even during brief periods of 

 rest, vertical stability was maintained with the 

 dorsal surface up. 



Measurements of larvae are given in table 2. 

 The larvae nearly doubled in length during the 

 absorption of the yolk; however, 80 percent of 

 this increase occurred during the first 27 hours. 



The late larva continued to lengthen slightly 

 after the 27-hour stage (fig. 18). Between 40 and 

 60 hours the most apparent change was the 

 shrinking yolk sac (figs. 19 and 20). At the 62- 

 hour stage, eye pigment developed, and the 

 mouth opened (fig. 19). Swimmingvvascontinuous 

 and directed as tlie larva moved across a 6-inch 



culture bowl with apparent ease. It constantlj' 

 counteracted tlie bouyancy of the yolk by swim- 

 ming in a head -down position. Prior to the devel- 

 opment of eye pigment, larvae appeared landondy 

 distributed in the culture bowl and did not react 

 to the approach of the pipette used to collect 

 samples. After the appearance of pigment, larvae 

 oriented away from the source of light and swam 

 from tlie approaching pipette. 



Pigmentation of yolk-sac larvae was Hmited to 

 widely spaced, small chronn^tophores along the 

 sides and on the finfold. The chromatophores 

 appeared as faint speckling at a magnification of 

 20 X and as distinct structures at 100 X. 



Figure 16. — Newly hatched larva 2.8 millimeters long. 



Figure 17. — Sixteen-hour larva 4.0 mm. long. 



Table 2. — Measurements ' of yellowfin menhaden yolk-sac larvae, in millimeters 

 (N=10 specimens at each age) 



 Measuremrnts of yolk-sac larvae were of preserved material. Ahlstrom and Ball (1954) estimated as much as 20 percent shrinkage due to formalin preser- 

 vation. Investigators examining fresh larvae should interpret the measurements accordingly. 



