644 



Fishery Bulletin 92(3). 1994 



sity (Fig. 3), as the percent deflation increased as 

 light intensity increased. The lowest light intensity 

 to evoke a significant (P<0.05; Z-test) increase in de- 

 flation (threshold) was 9xl0 12 photonscm~ 2 s _1 . 



<v 

 ■a 



■o 

 o 



E 



3 

 m 



TO 



0> 



01 



Q 



80 



60 



40 



jz 20 



10 



10" 



10' 



io 1 - 



10 



2,.-lN 



io- 



Light Intensity (photons cm" s") 



Figure 3 



The percentage of larval Atlantic menhaden, Brevoortia 

 tyrannus, with deflated swimbladders upon exposure to dif- 

 ferent light levels and darkness. The number below each point 

 is the sample size. The asterisks indicate the lowest light 

 level at which the proportion deflated was significantly 

 (P<0.05; Z-test) greater than the level in darkness. 



80 



t 60 



ill 



40 



O 



20 



* T/ 



20 40 60 80 



Time (mm) 



100 



120 



Figure 4 



The percentage of larval Atlantic menhaden, 

 Brevoortia tyrannus, with deflated swimbladders 

 after different times in light. Means and standard 

 errors are plotted. The average number of replicates 

 was four. The asterisk indicates the first time at 

 which the mean percent was significantly (P<0.05; 

 <-test) greater than the initial level (time 0). 



Timing of swimbladder deflation 



The time course of swimbladder deflation in response 

 to white light was measured upon transfer from dark- 

 ness to 1.7xl0 15 photonscm~ 2 -s _1 . By producing the 

 maximum rate of light intensity change, it was 

 assumed that the maximum rate of deflation 

 would be evoked. An increase in the percent 

 deflated was evident after 5 minutes and was 

 significantly (P<0.05; f-test) greater than the 

 initial level after 15 minutes (Fig. 4). Micro- 

 scopic examination indicated this rapid defla- 

 tion was accomplished by passing bubbles from 

 the swimbladder into the gut and then out 

 through both the anus and mouth. 



Endogenous rhythm in swimbladder 

 deflation 



The percent deflation remained low if larvae 

 were kept in continuous darkness ( Fig. 1 ), which 

 indicates there was no endogenous rhythm in 

 deflation without exposure to light. However, 

 light-cued deflation was not constant over time 

 ( Fig. 5 ). This experiment was conducted twice and 

 involved maintaining larvae in darkness and 

 measuring a subsample after exposure to light 

 for one hour at different times during the solar 

 day. The consistent cycle in both trials was that 

 light-cued deflation was low during the normal 

 dark phase and high during the normal day phase. 

 An interesting observation was made during these 

 experiments. At the last sampling time, larvae had 

 been in darkness for 27 hours, over which time they 

 continued swimbladder inflation. Exposure to light 

 at 2200 hours caused minimal deflation (Fig. 5). The 

 water containing these larvae also had brine shrimp 

 nauplii from the rearing tank. Larvae are visual 

 predators and had, in effect, been starved for 27 hours 

 in darkness. Under these conditions, once exposed 

 to the light, they began to feed. While measuring 

 swimbladder volumes, their digestive tracts were full 

 of nauplii. Thus, larvae can feed with an inflated 

 swimbladder. 



Discussion 



Swimbladder deflation by larval Atlantic menhaden 

 is cued by an increase in light intensity. There was 

 no cycle in which larvae inflated their swimbladders 

 at sunset (Forward et al., 1993) and then deflated 

 them gradually over time. In darkness at the end of 

 the night, larvae, with and without air, had inflated 

 swimbladders, and their volumes were not signifi- 

 cantly reduced through the night. Thus, deflation 



