TANAKA ET AL.: EFFECTS OF PHOTOPERIOD AND FEEDING ON TILAPIA NILOTIC A 



measured every 3 h clearly showed that the ring is 

 formed periodically every 24 h. This is supported 

 with the observation that the number of rings 

 (incremental zones), except those in the core re- 

 gion, is in good correlation with the chronological 

 age in days after hatching of the fish. This also 

 shows that the core region is formed during the 

 embryonic stage. Several crystallized spherules 

 found in the center of the core region indicate that 

 the otolith rudiment is formed by fusion of many 

 small primordia. Their presence is helpful to iden- 

 tify the plane that exactly passes through the 

 center of the otolith. 



The rate of otolith growth, judged from a change 

 of the completeness of current increment, de- 

 creased or stopped for at least a few hours before 

 and/or after the beginning of a light period in all 

 the experiments. This may indicate that the 

 growth of aragonite crystals at the margin of the 

 otolith stopped or slowed down during this period 

 of a day. On the other hand, growth of the otolith 

 was apparent the rest of the time, showing that 

 crystals deposited more rapidly. The structural 

 difference observed between the surface of a com- 

 plete crystalline layer and the outer surface of the 

 growing otolith, flat and smooth in the former and 

 rough with stacked crystals in the latter, also 

 shows a difference in growth phases. These obser- 

 vations confirmed that in the fast growth phase an 

 incremental zone is formed in the margin of the 

 otolith and in the resting phase a discontinuous 

 zone is formed. In relation to the structure of in- 

 cremental and discontinuous zones discussed 

 above, a cyclic deposition of organic materials or 

 calcium or both seems to occur according to a daily 

 photoperiod. Recently, Mugiya et al. (1981) re- 

 ported that "^^Ca uptake by goldfish otoliths slowed 

 down or stopped at sunrise and resumed in 3 h. 



When a light and dark cycle was suddenly re- 

 versed, it took at least 6 d for the rhythm of the 

 otolith growth to be adapted to the new photocon- 

 dition. This indicates that otolith growth is 

 primacily controlled by an endogenous rhythm 

 synchronized with the environmental photo- 

 period. Taubert and Coble (1977) also presented a 

 hypothesis that the formation of otolith rings is 

 controlled by an internal, diurnal clock when fish 

 are exposed to a 24-h photoperiod. 



It was clearly shown that the otolith resumes its 

 growth a few hours after lights-on, leaving a new 

 discontinuous zone behind it. This pattern of 

 growth was not affected by the change of the 

 lengths of light and dark phases such as 18L-6D 



and 6L-18D. This shows that the stimulus of 

 lights-on entrains the rhythm of otolith growth to 

 the photoperiods and thus is important for the 

 formation of the otolith rings. The importance of 

 this stimulus as a cue for the ring formation is 

 supported by the experiment in which light and 

 dark phases were reversed, where changes in 

 phase of otolith growth started after the first 

 stimulus of lights-on, but not lights-off, in the 

 reversed cycle (Figure 4). 



Since T. nilotica, as a typical diurnal fish, begins 

 to feed immediately after lights-on, feeding was 

 also expected to be one of the factors which control 

 the daily rhythm of otolith growth. However, a 

 difference of 6 h in daily feeding did not affect the 

 phase of the rhythm. Therefore, feeding time is not 

 critical for the formation of otolith rings when the 

 fish is exposed to a 24-h photoperiod. Taubert and 

 Coble (1977) also showed that the feeding cycle of 

 24 h did not cause the formation of daily rings on 

 the otolith of T. mossambica held under constant 

 light. 



ACKNOWLEDGMENTS 



We thank Norimitsu Watabe and John M. Dean, 

 University of South Carolina, for their discussion 

 and advice in preparing the manuscript. This 

 work was supported bv a fund from the Japan 

 Society for the Promotion of Science awarded for a 

 cooperative research under the Japan-United 

 States Cooperative Science Program. 



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