NOTE Forward et al.: Swimbladder deflation in Brevoortia tyrannus 



645 



through gas diffusion as suggested by Hoss and 

 Phonlor ( 1984) did not occur. 



If larvae were exposed to light near the time of the 

 beginning of the light phase, deflation began within 

 5 minutes and was statistically apparent in 15 min- 

 utes. Rapid deflation occurred as the pneumatic duct 

 opened between the gut and swimbladder and gas 

 passed into the alimentary canal, where it was moved 

 to both the mouth or anus for expulsion. The lowest 

 light intensity that evoked deflation was about 10 12 

 photonscm _2 -s _1 . This threshold is below the lowest 

 light intensity that inhibits inflation at sunset ( 12- 

 16 mm larvae; 6xl0 13 photonscm~ 2 s _1 ; Forward et 

 al., 1993). Thus, larvae appear to be more sensitive 

 to light at sunrise than at sunset. Maximum night- 

 time light intensity from the moon and star light is 

 about 10 11 photonscm^s 1 (McFarland and Munz, 

 1975; Lythgoe, 1979). Because this value is below the 

 threshold intensity ( 10 12 photonscm^s 1 ) for defla- 

 tion, moon and star light probably will not initiate 

 deflation at night. Since surface light levels are about 

 10 1 ' photonscm~ 2 -s _1 at noon (Lythgoe, 1979), an 

 intensity of 10 12 photonscm~ 2 s _1 occurs earlier, prob- 

 ably near sunrise. 



Larvae appear to have an endogenous rhythm 

 in light-cued deflation. If they were maintained 

 in constant darkness, light induced a low per- 

 cent deflation during the night phase and a high 

 percentage during the day phase. This rhythm 

 is the reverse of the inflation rhythm, in which 

 sudden darkness initiates inflation at night but 

 rarely during the day (Forward et al., 1993). The 

 functional significance of the deflation rhythm 

 may be that 1) larvae do not deflate their 

 swimbladder at night in response to any light 

 and 2) they are "prepared" for rapid deflation at 

 sunrise. 



Field studies suggest Atlantic menhaden lar- 

 vae undergo nocturnal diel vertical migration 

 ( DVM ), in which they remain at moderate depths 

 during the day and occur near the surface at 

 night (Govoni and Pietrafesa, in press). Swim- 

 bladder inflation at sunset would increase buoy- 

 ancy and reduce larval sinking rate (Hoss et al., 

 1989), which would maintain larvae closer to the 

 surface. The present laboratory study supports 

 a nocturnal DVM pattern by indicating that the 

 percentage of larvae with inflated swimbladder 

 and swimbladder volumes increased through the 

 night, when larvae have access to the air-water 

 interface. These increases are not predicted if 

 larvae inflate their swimbladder only once at 

 sunset and then sink. Thus, there is probably a 

 cycle during the night, in which larvae sink while 

 remaining motionless and then periodically return 

 to the surface for additional gas. This pattern would 

 retain larvae near the surface, which may be useful 

 for transport from the offshore spawning area to the 

 mouth of an estuary (Hoss et al., 1989). 



A final consideration in the present study is why 

 larvae deflate their swimbladders. Clearly, Atlantic 

 menhaden larvae are adapted for deflation at sun- 

 rise. Their rhythm indicates they are most respon- 

 sive to a light intensity increase at this time, and 

 deflation occurs within 15 minutes. Such a dramatic 

 response suggests deflation has an important func- 

 tional advantage. 



A fully inflated swimbladder may reduce the speed 

 of movement and, thereby, the effectiveness of prey 

 capture. Larvae feed during the day and use vision 

 to find their prey (Blaxter and Hunter, 1982). Al- 

 though a reduction in capture efficiency is possible, 

 larvae with fully inflated swimbladders can still cap- 

 ture prey as observed in the rhythm experiment. Al- 

 ternatively, an inflated swimbladder may increase 

 detection of menhaden larvae by visual predators. 

 Since larvae are relatively transparent, the differ- 

 ence in refractive index between air and water in- 

 creases the contrast between an inflated swim- 



