Swimbladder deflation in 

 the Atlantic menhaden, 

 Brevoortia tyrannus 



Richard B. Forward Jr. 



Marine Laboratory, School of the Environment, Duke University 

 Pivers Island, Beaufort, North Carolina 28516-9721 



William F. Hettler 

 Donald E. Hoss 



Beaufort Laboratory, Southeast Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 Beaufort, North Carolina 28516-9722 



Larval clupeoid fishes usually have 

 a pronounced cycle of swimbladder 

 inflation and deflation (Uotani, 

 1973; Hunter and Sanchez, 1976; 

 Blaxter and Hunter, 1982). Field 

 and laboratory studies of both At- 

 lantic {Brevoortia tyrannus; Hoss et 

 al., 1989) and gulf (Brevoortia pat- 

 ronus; Hoss and Phonlor, 1984) 

 menhaden found that larvae in- 

 flated their swimbladders during 

 the night and deflated them dur- 

 ing the day. 



Our past studies of Atlantic men- 

 haden larvae found that the cue for 

 inflation is a decrease in light in- 

 tensity at sunset (Hoss et al., 1989; 

 Forward et al., 1993). Inflation oc- 

 curs rapidly and begins within 5 

 minutes of onset of darkness. The 

 process involves moving to the sur- 

 face, swallowing air into the ali- 

 mentary canal, and moving this air 

 into the swimbladder through the 

 pneumatic duct (Hoss et al., 1989). 

 Menhaden have no connection 

 (pneumatic duct) between the 

 swimbladder and anus as do some 

 clupeiods (Tracy, 1920). Deflation is 

 less studied. It is hypothesized to 

 occur by diffusion of gas from the 

 swimbladder throughout the night 

 and perhaps by active movement of 

 gas to the alimentary canal and 



then out through the mouth and 

 anus (Hoss and Phonlor, 1984). 



The present study was under- 

 taken to determine 1) the manner 

 in which swimbladder deflation oc- 

 curs (by diffusion or active gas 

 movement) in Atlantic menhaden 

 larvae, 2 ) the relationship between 

 deflation and light intensity, 3) the 

 time-course for deflation, and 4 ) the 

 presence or absence of an endog- 

 enous rhythm in deflation. 



Materials and methods 



Atlantic menhaden, Brevoortia 

 tyrannus, were spawned and reared 

 in the laboratory (Hettler, 1983) on 

 a 12:12 hour light-dark cycle with 

 the dark phase beginning at 1900 

 hours. Lighting during the light 

 phase was provided by daylight 

 fluorescent tubes at a surface inten- 

 sity of 1.6xl0 15 photonscm~ 2 -s _1 

 (400-700 nm) as measured with a 

 scalar irradiance meter with a 4tc 

 collector (Biospherical Instru- 

 ments, Inc.). Our previous study 

 found that swimbladder inflation 

 began when larvae were 10 mm to- 

 tal length (TL) but the percentage 

 with inflated swimbladders was 

 low (Forward et al., 1993). Between 



11 and 16 mm TL, swimbladders 

 were inflated during the night and 

 deflated during the day. Above 16 

 mm TL, most fish always had some 

 gas in their swimbladders. Since 

 the percentage of larvae with de- 

 flated swimbladders varies be- 

 tween day and night for 11-16 mm 

 TL larvae, they were used in the 

 present experiments. 



Deflation was quantified by de- 

 termining the proportion of larvae 

 with deflated swimbladders. In ad- 

 dition, inflation was quantified by 

 measuring the size of the light-re- 

 fractive bubbles in the swimbladder 

 and alimentary canal to the near- 

 est 0.02 mm under a microscope. It 

 was assumed that bubbles in the 

 alimentary canal were transported 

 either toward or away from the 

 swimbladder and thereby contrib- 

 uted to the swimbladder volume. 

 Gas bubble volume d>) was calcu- 

 lated by using the equation of 

 Hunter and Sanchez (1976): V=(AI 

 3)7i ab'~, where 6=half the bubble 

 width and o=half the bubble length. 

 Swimbladder volume was the total 

 volume of all bubbles. Since 

 swimbladder volume increases 

 with larval length (Forward et al., 

 1993), this relationship is pre- 

 sented when volume is considered. 

 Only larvae with inflated swim- 

 bladders were used to calculate the 

 mean volume at each larval length. 

 In contrast, the percentage of lar- 

 vae with deflated swimbladders 

 was calculated for all larvae (11- 

 16 mm TL), because the previous 

 study found that the proportion of 

 larvae inflating swimbladders did 

 not vary significantly with larval 

 length (Forward et ai., 1993). 



Four sets of experiments were 

 conducted, all of which began by 

 removing larvae from rearing tanks 

 and by placing them in darkness at 

 the time of the beginning of the 

 dark phase. Our previous study 

 showed that darkness cued initial 



Manuscript accepted 22 February 1994. 

 Fishery Bulletin 92:641-646 1 1994). 



64 1 



