HUNTER and SANCHEZ: CHANGES IN SWIM BLADDER INFLATION 



swim bladders had been collected, they were 

 macerated in the tube and the gas withdrawn with 

 a Pasteur pipette and inserted into the syringe gas 

 analyzer (Scholander et al. 1955). In the analyzer, 

 the carbon dioxide was absorbed with alkaline 

 citrate, oxygen by pyrogallol, and the volume of 

 gas determined before and after each treatment. 

 The remaining gas was considered to be nitrogen. 

 The volume of gas was read under a dissecting 

 microscope using an optical micrometer. Reading 

 error was about ± 0.09 ii\ or from 1 to 2% depend- 

 ing on the volume of the sample. 



RESULTS 

 Diel Rhythm in Swim Bladder Inflation 



The volume of the swim bladder of larvae cap- 

 tured at night in the sea was greater than that of 

 larvae collected in the day (Table 1). Similarly, the 

 volume of the swim bladder of larvae reared in the 

 laboratory was greater at night than in the day. To 

 illustrate these daily changes for laboratory- 

 reared larvae, the mean volume of the swim 

 bladder for 2-h intervals was calculated for each of 

 three length classes (10.0 to 11.9 mm, A^ = 121; 12.0 

 to 13.9 mm, N = 202; 14.0 to 15.9 mm,N = 129). No 

 evidence existed for anticipation of the onset of 

 dark at 2200 h nor for the onset of light at 1000 h 

 (Figure 2). In all three length classes the mean 

 volume did not return to the daytime level until 

 about 2 h after the onset of light nor did they reach 

 the maximum at night until about 2 h after the 

 onset of dark. 



The swim bladder of larvae at night was 

 frequently so inflated that it constricted the gut 

 (see fig. 8 in Uotani 1973). Larvae in the dark with 

 filled swim bladders were motionless or slowly 

 sinking. The body was oriented head down at an 

 oblique angle to the water surface. After sinking a 

 short distance, the larvae swam back to the water 



Table 1.— Swim bladder volume (mm^) of preserved northern 

 anchovy larvae from standard CalCOFI oblique plankton tows 

 taken at night and in the day in southern California inshore 

 waters. 



0.06 1- 



DAY 



NIGHT 



DAY 



I0.0-Il.9nnm 



II II I 11 1 I I I I yr ' \" \ I I I I I I I I I 



19 21 23 I 3 5 7 9 II 13 15 17 

 TIME OF DAY (hours) 



Figure 2.-Mean swim bladder volume ± 2 SE of mean for 2-h 

 class intervals plotted at midpoint of each 2-h class. Data shown 

 for three length classes of laboratory-reared northern anchovy 

 lar\-ae; the onset of dark was at 2200 h and onset of light at 1000 h. 

 No transitional level of illumination existed between night and 

 day. 



surface, a behavior closely resembling that of 

 yolk-sac larvae (Hunter 1972). 



Specimens with obviously inflated swim blad- 

 ders occurred occasionally in day samples from the 

 sea and laboratory but these were only a few 

 percent of the larvae examined if the first 2 h after 

 the onset of light are excluded. On the other hand, 

 the occurrence of larvae with deflated bladders at 

 night was more common. About 10% of the wild 

 larvae and 20% of the laboratory-reared (12.0 to 

 12.9 mm) larvae had swim bladder volumes at 

 night comparable to those in the day (Figure 3). 

 The proportion of larvae with deflated bladders at 

 night decreased with larval length. 



849 



