DIEL CHANGES IN SWIM BLADDER INFLATION OF THE 

 LARVAE OF THE NORTHERN ANCHOVY, ENGRAULIS MORDAX 



John R. Hunter and Carol Sanchez' 



ABSTRACT 



Laboratory and field studies demonstrated that larval anchovy 10 mm standard length and larger 

 inflate their swim bladders each night and deflate them in the day. Maximum night levels of inflation 

 were attained 2 h after the onset of dark and typical day levels occurred about 2 h after the onset of 

 light. Laboratory experiments indicated that larvae fill their bladders at night by swallowing air at the 

 water surface and the vertical distribution of sea-caught larvae suggested that they migrate to the 

 surface each night to fill their swim bladders. Gas is released by passing bubbles through the pneumatic 

 duct into the alimentary canal. The diel rhythm of inflation was viewed as an energy sparing 

 mechanism. Measurements of sinking speed of larvae with and without inflated bladders suggested 

 that the energy saved at night by inflation of the swim bladder would exceed the cost of vertical 

 migration to the surface and that the migratory range over which energy savings are possible would be 

 greater as larvae increased in length. 



Northern anchovy, Engraulis mordax Girard, are 

 more vulnerable to starvation in the larval stage 

 than at any other time of life, consequently, 

 energy sparing mechanisms may be critical to 

 their survival. In a recent paper Uotani (1973) 

 showed that the larvae of several clupeoid fishes, 

 Engraulis japonicus (Houttuyn), Sardinops 

 melanosticta (Temminck and Schlegel), and 

 Etrumeus teres (DeKay) have expanded swim 

 bladders when captured at night in the sea and 

 deflated ones when captured during the day. 

 Energy conservation is certainly one of the possi- 

 ble adaptive advantages of such behavior, but the 

 energy saved must be evaluated in terms of the 

 energy cost of daily filling the bladder. This 

 requires that the mechanism of filling be known. 

 The object of the present study was to determine 

 if the larvae of the northern anchovy display a 

 similar rhythm and to evaluate this behavior as a 

 possible energy sparing mechanism. 



The swim bladder in adult northern anchovy is a 

 tubular vesicle that extends the length of the body 

 cavity. It is connected to the alimentary canal by a 

 pneumatic duct which originates from the dorsal 

 wall of the cardiac stomach; no anal duct exists as 

 it does in some clupeoids (O'Connell 1955). Two 

 tubules on each side of the body extend from the 

 anterior end of the bladder into the cranium where 

 they expand into two pairs of capsules, termed 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, CA 92038. 



prootic and pterotic bullae (O'Connell 1955). The 

 swim bladder of the larva is basically similar to 

 that of the adult. At the time of initial filling of the 

 swim bladder, the pneumatic duct is functional 

 and the bullae become filled with gas. No his- 

 tological evidence exists for gas secretion in adult 

 E. mordax nor for the larvae (O'Connell 1955, and 

 pers. commun.). 



The swim bladder is deflated by passing gas 

 bubbles through the pneumatic duct into the 

 alimentary canal and out the anus. On a number of 

 occasions we have observed this process while 

 examining a live anchovy larva under a dissection 

 microscope. We have also captured larvae with 

 deflated swim bladders that had gas bubbles in the 

 alimentary canal. 



METHODS 



Fertilized anchovy eggs were obtained from a 

 captive population of adults maintained in 

 spawning condition in the laboratory (Leong 1971) 

 and the larvae were reared using the techniques, 

 foods, and tanks described by Hunter (1976). The 

 larvae were reared at temperatures of 16.5° ± 

 0.2°C and 16.9° ± 0.9°C. A 12-h photoperiod was 

 used without a dawn or dusk transition in light 

 intensity. Incident light at the surface was about 

 2,000 Ix in the day and at night no light was 

 provided in the closed room which contained the 

 rearing tanks. 



Larvae reared in the laboratory were sampled at 



Manuscript accepted May 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4, 1976. 



847 



