nine-tenths, after 34 hours (table 10). The 

 total radioactivity decreased rapidly during 

 the first day after the larvae were transferred 

 to nonradioactive sea water (fig. 15). Between 

 17 and 19 percent of the Zn"^ was retained 

 within the body 14 days after feeding (no cor- 

 rection was made for Zn®^ decay). 



These findings suggest to us that digestion 

 rate in relation to time since last feeding prob- 

 ably contributed to the low incidence of food 

 found in larvae captured at Indian River Inlet 

 during dark nights, and especially in those 

 captured toward dawn. 



Table 10. — Distribution of Zn^^ in Atlantic menhaden 

 larvae after feeding on copepods labeled with this 

 radionuclide 



[Levels represent the mean of individual lots of larvae, adjusted 

 for the dosage given] 



Elapsed time 

 since feeding 



Activity level 



Alimentary 

 tract 



Remainder 

 of body 



Houra 



0.0 



1.5 



8.5 



34.0 



89.0 



Percent 



Percent 



TIME (days) 



Figure 15. — Activity level of Zn^s jn the body of At- 

 lantic menhaden larvae fed labeled copepods and re- 

 turned to nonradioactive sea water. Solid and open 

 circles represent two larvae. 



Effects of Capture and Preservation of Larvae on Their 

 Alimentary Tract Contents 



All larvae transferred by the dip net defe- 

 cated all or a portion of their alimentary tract 

 contents when removed from the water, but 

 none transferred in sea water defecated any 

 of their tract contents. 



Active larvae placed in the various Formalin 

 solutions had violent spasms, accompanied by 

 complete or partial defecation of their tract 

 contents. If defecation were incomplete, the re- 

 maining organisms were restricted to the end 

 of the posterior alimentary tract. Sometimes 

 the entire contents were ejected in a single, 

 violent spasm. Generally, larvae voided their 

 tract contents violently and completely in 10 

 and 50 percent Formalin and died within 10 to 

 30 seconds. In 0.4 percent Formalin they quiv- 

 ered for as long as 10 minutes and often defe- 

 cated all or part of their tract contents. 



Larvae placed in the chloral hydrate solu- 

 tion reacted as in the stronger Formalin con- 

 centrations, although they voided only about 

 one-fourth to one-half of the tract contents. 



Larvae in the tricaine methanesulfonate 

 gradually lost equilibrium in 3 to 5 minutes 

 and had no spasms. They did not defecate any 

 portion of their alimentary tract contents dur- 

 ing transfer by net or immersion in Formalin 

 or chloral hydrate. 



These observations suggest that larvae may 

 defecate much of their alimentaiy tract con- 

 tents during capture with a net or at the time 

 of preservation, thereby leading one to the er- 

 roneous conclusion that they feed little or di- 

 gest their food rapidly. So both the high inci- 

 dence of empty alimentary tracts and the low 

 nimibers of food organisms per tract which we 

 found in larvae collected at Indian River Inlet 

 probably was largely due to the harsh treat- 

 ment to which larvae were subjected during 

 retrieval of the net from the water onto a 

 bridge (about 10 m. above the surface) and 

 their preservation in Formalin. Capturing lar- 

 vae with a net attached to a floating pen and 

 anesthetizing them prior to presei^vation 

 should provide specimens with alimentary 

 tract contents that reflect actual conditions at 

 the time of capture. 



FOOD OF YOUNG ATLANTIC MENHADEN IN RELATION TO METAMORPHOSIS 



509 



