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Fishery Bulletin 92(3). 1994 



swimbladder inflation at the beginning of the night 

 (Forward et al., 1993). In all experiments each larva 

 was used only once. It was assumed that all test lar- 

 vae could potentially inflate their swimbladders in 

 darkness. However, the maximum percent inflation 

 was around 92% (Fig. 1), which suggests that about 

 8% of the larvae were developmentally incapable of 

 inflation. This low percentage would not alter the 

 overall result of any experiment. 



The first experiment measured swimbladder de- 

 flation in larvae kept in continuous darkness with 

 and without access to air. Deflation was defined as 

 the absence of gas bubbles in the swimbladder and 

 alimentary canal. Three hours after the beginning 

 of night (2200 hours), a subsample of the larvae was 

 removed from the dark and measured for total length, 

 presence of gas bubbles in the swimbladder and ali- 

 mentary canal, and for size of the bubbles (standard 

 measurements). There was no evidence that larvae 

 lost or took up gas during the measurement proce- 

 dure. The remaining larvae were separated into two 

 groups. The first group remained in finger bowls ( 19.3 

 cm diameter) in darkness with access to the air-wa- 

 ter interface and was similarly sampled during the 

 following day at times (0900, 1200, and 1700 hours) 

 that should have occurred during the normal light 

 phase. The bowls ( 19.3 cm diameter) containing the 

 second group of larvae were sealed at 2200 hours, so 

 that larvae did not have an air-water interface. The 

 seal was accomplished by filling the bowl completely 



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Figure 1 



The percentage of larval Atlantic menhaden, 

 Brevoortia tyrannus, with no gas in their 

 swimbladder or alimentary canal (deflated) when 

 maintained over time in continuous darkness with 

 (solid line) and without I dashed line) access to the 

 air-water interface. The dark bar indicates the time 

 of the dark phase for the normal light-dark cycle. 

 The number under each point is the sample size. An 

 asterisk indicates the proportion with deflated 

 swimbladders is significantly (P<0.05; Z-test for com- 

 parison of two proportions) different from that 3 

 hours after the beginning of the dark phase (initial 

 point). 



with water and by placing a Leucite plate over the 

 top so that no air bubbles were present. Larvae were 

 maintained in darkness and were similarly sampled 

 at 0900 and 1700 hours the next day. 



The second set of experiments was designed to 

 determine the relationship between light intensity 

 and swimbladder deflation. Larvae were placed in 

 darkness at the beginning of the dark phase until 

 two hours (0900) after the time for beginning the light 

 phase. Larvae were then separated into two groups. 

 For the control group standard measurements were 

 made after three more hours of darkness. The sec- 

 ond group was irradiated with a constant light in- 

 tensity for three hours after which standard mea- 

 surements were made. White light (cool white fluo- 

 rescent tubes; 1.7xl0 15 photons-cm~ 2 -s _1 ) was used 

 in the initial experiment, because this was the nor- 

 mal daytime light during rearing. For determining 

 the change in response with light intensity, the stimu- 

 lus source was a 300-W incandescent lamp filtered 

 to the blue region with a Corning 4-96 filter. The 

 transmitted wavelengths encompassed the major 

 spectral-sensitivity maxima of most fish (e.g. Munz, 

 1958; McFarland and Munz, 1975). All intensities 

 below the maximum level were controlled by neu- 

 tral density filters. 



The third set of experiments was designed to de- 

 termine the time-course for swimbladder deflation. 

 Larvae were maintained as above in darkness until 

 0900 hours when standard measurements were made 

 on control groups of larvae. The remaining larvae 

 were then exposed to light similar to that used in 

 the normal light phase (cool white fluorescent lamps: 

 intensity=1.7xl0 15 photons ernes') and standard 

 measurements made on subsamples at various time 

 intervals (5, 15, 30, 60, 90, 120 min). 



The fourth experiment tested for the presence of 

 an endogenous rhythm in swimbladder deflation. A 

 large group of larvae was removed from the rearing 

 tank at the beginning of the dark phase and placed 

 in darkness at a constant temperature (22°C) that 

 was similar to the rearing temperature. Standard 

 measurements were then made at time intervals 

 throughout the next 28 hours on subsamples of lar- 

 vae. Each subsample was placed under blue light (in- 

 tensity= 4.5xl0 15 photons) for one hour, after which 

 standard measurements were made. Differences in 

 the effect of light on percent deflation over the 28- 

 hour interval would suggest the presence of an en- 

 dogenous rhythm. 



The percentage of larvae with deflated swim- 

 bladders and the total volume of bubbles in each larva 

 were calculated for each observation within each 

 experiment. Means, standard deviations, and stan- 

 dard errors of percent data were calculated after the 



