Forward et al.: Swimbladder inflation of Brevoortia tyrannus 



257 



(Fig. 7A). This response level continued 

 into the time of the next light-phase. 

 Mean swimbladder volume of llmmTL 

 larvae varied over time, but the maxi- 

 mum and minimum means over the first 

 solar day were not significantly different 

 due to the large variances (Fig. 7B). 



Discussion 



indicates there was no endogenous rhythm in in- 

 flation in constant high light conditions (Fig. 7A). 

 Placement in darkness induced inflation in -40% 

 of the larvae during the normal light-phase. This 

 percentage increased dramatically to 70^ at the 

 normal time for the beginning of the dark-phase 



Atlantic menhaden inflate their swim- 

 bladders in response to a decrease in 

 light intensity. The smallest size observed 

 with an inflated swimbladder was 

 lOmmTL, which is smaller than the mini- 

 mum size of 13mmTL found by Hoss & 

 Blaxter (1982). This difference may re- 

 sult from the large sample size used in 

 the present experiment, since the per- 

 centage of lOmmTL larvae with an 

 inflated swimbladder was -10%. The per- 

 centage of larvae inflating their swim- 

 bladder in response to a decrease in light 

 intensity increased with size and reached 

 100% at 17mmTL and greater. 



Swimbladder volume increased with 

 size, which is not surprising, since larger 

 larvae have larger swimbladders. However, within any fish 

 size, the mean volume did not vary significantly with light- 

 ing condition. This result disagrees with the qualitative con- 

 clusion of Hoss et al. (1989) and is likely due to the wide 

 variation in volume. Hoss et al. (1989) also found high vari- 

 ances, but failed to compare mean values statistically. 



The percentage of larvae inflating their swimbladders in- 

 creased as the light intensity decreased, which clearly indi- 

 cates that the decrease in light intensity cued the response. 

 However, a step function was observed, in that once light 

 was below a particular absolute level, maximum inflation 

 occurred. Future experiments are needed to determine 

 whether inflation is cued by exposure to light intensity be- 

 low an absolute level or to the rate of change in intensity. 

 This information will allow predictions of the time of infla- 

 tion in the field. 



Hoss et al. (1989) failed to find an endogenous rhythm in 

 swimbladder inflation for larvae held under conditions simi- 

 lar to the present experiment. In contrast, our study showed 

 a clear rhythm during the first day for larvae held under 

 constant light. The percent inflation was low in fish intro- 

 duced to the dark during the time of the light-phase, and 

 nearly doubled at the time the dark-phase began. This high 

 percent response did not return to a low level at the time of 

 the next light-phase. Hoss et al. (1989) did not begin mea- 

 suring swimbladder inflation until after -24 h in constant 

 light, which may be why they failed to detect an endog- 



