Abstract.— Atlantic menhaden 

 Brevoortia tyrannus larvae inflate 

 their swimbladders at night and 

 deflate them during the day. The 

 present study considered the rela- 

 tionship of inflation to light inten- 

 sity, the time-course of inflation, and 

 the presence of an endogenous 

 rhythm in inflation. The percentage 

 of laboratory-reared larvae that in- 

 flate their swimbladders increased 

 upon sudden exposure to a decrease 

 in light intensity. Percentage infla- 

 tion was maximal at an intensity of 

 10 13 photons cnr 2 s _I and lower. For 

 any specific size of larvae, the infla- 

 tion volume did not vary signifi- 

 cantly with light intensity, but vol- 

 ume increased with total larval 

 length. Inflation began within 5 min 

 of introduction into darkness, and 

 maximum percent inflation was evi- 

 dent by 20 min. There was a rhythm 

 in which darkness induced a low per- 

 cent inflation during the day phase 

 and a high percentage at the begin- 

 ning of the dark-phase. This dra- 

 matic increase in inflation at sunset 

 may function for predator avoidance. 



Swimbladder inflation of 

 the Atlantic menhaden 

 Brevoortia tyrannus 



Richard B. Forward 

 Leslie M. McKelvey 



Duke University School of the Environment, Marine Laboratory 

 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 



Manuscript accepted 17 February 1993. 

 Fishery- Bulletin, U.S. 91:254-259 i 1993) 



254 



Diel inflation and deflation of the 

 swimbladder are common to larval 

 clupeoid fishes (Uotani 1973, Hunter 

 & Sanchez 1976, Blaxter & Hunter 

 1982). Both Atlantic {Brevoortia ty- 

 rannus; Hoss et al. 1989) and gulf 

 (Brevoortia patronus; Hoss & Phonlor 

 1984) menhaden larvae inflate their 

 swimbladders during the night and 

 deflate them during the day. Infla- 

 tion occurs by moving to the surface, 

 swallowing air into the alimentary 

 canal, and moving it to the swim- 

 bladder through the pneumatic duct 

 (Hoss & Phonlor 1984). Deflation is 

 presumed to take place by diffusion, 

 because menhaden have no open con- 

 nection between the anal opening and 

 swimbladder (Tracy 1920). 



Inflation/deflation by menhaden 

 larvae is clearly related to the light: 

 dark cycle. Field studies of both At- 

 lantic and gulf menhaden found most 

 larvae had deflated swimbladders 

 and low swimbladder volumes dur- 

 ing the day, with the reverse condi- 

 tions at night (Hoss & Phonlor 1984, 

 Hoss et al. 1989). Since inflation is 

 rapid, occurring within 1 h after sun- 

 set, Hoss et al. ( 1989) suggested that 

 change in light intensity may cause 

 the inflation response. 



The present study was undertaken 

 to determine (1) the relationship be- 

 tween inflation and light intensity for 

 Atlantic menhaden larvae, (2) the 



time-course for inflation, and (3) the 

 presence or absence of an endogenous 

 rhythm in inflation. 



Materials and methods 



Atlantic menhaden Brevoortia ty- 

 rannus were spawned and reared in 

 the laboratory using methods de- 

 scribed by Hettler (1983). After egg 

 hatching, the larvae were held in cir- 

 cular tanks ( 100 L) at about 20°C on 

 a 12:12 h LD cycle. The dark phase 

 began at 1800 h. Lighting during the 

 light phase was provided by daylight 

 fluorescent tubes at a surface inten- 

 sity of 1.6X 10 1S photons cm 2 s 1 (400- 

 700 nm) as measured with a scalar 

 irradiance meter with a 4ii collector 

 (Biospherical Instruments, Inc.). 

 Young larvae were fed rotifers Bra- 

 ck ion us plicatilus cultured in the 

 laboratory. Older fish (13-15 mm) 

 were fed brine shrimp (Artemia sp.) 

 nauplii. Larvae measuring 9-20 mm 

 were used in the experiments. 



Inflation was quantified by mea- 

 suring light-refractive bubbles in the 

 swimbladder and alimentary canal 

 to the nearest 0.02 mm under a mi- 

 croscope. It was assumed that any 

 bubbles in the alimentary canal were 

 being transported to the swim- 

 bladder. Gas bubble volume was cal- 

 culated using the equation of Hunter 



