FISHERY BULLETIN: VOL. 79, NO. 3 



with the long axis nearly perpendicular to the 

 margin of the otolith, while the other is thin ( about 

 0.2-0.5 ixm wide) and appears as a concentric 

 groove in slightly etched planes of the otolith (Fig- 

 ure IB). Most of the grooves are not continuous 

 around the whole area and fade out near the area 

 along the long (anteroposterior) axis of the otolith. 

 Usually, a few grooves in the area around the core 

 region, which is delimited by the earliest clear 

 circular groove, are weakly etched and set at wide 

 intervals (Figure lA). 



On the other hand, examinations of nonetched 

 fracture planes of the otolith revealed several 

 thick crystallized lamellae piled up with narrow 

 (<0.1 iJLTn) interstitial layers between them (Fig- 

 ure IC, D). The surface of a completed crystallized 

 lamella that was fractured tangentially at the 

 boundary to the interstice was observed to be flat 

 and smooth (Figure ID, E), while the growing sur- 

 face of the otolith was rough with stacked crystals 

 (Figure IF). Therefore, we consider that the con- 

 centric grooves in acid-etched planes correspond to 

 the interstitial layers where the growth of crystal- 

 lized lamella was interrupted. The grooved zone is 

 here defined as the discontinuous zone and the 

 crystallized lamella as the incremental zone, both 

 zones constituting a unit increment. 



The core region, defined by the earliest clear 

 discontinuous zone, was about 40 /um in diameter 

 and had several, usually five or more, narrow un- 

 clear rings (Figures lA, 2A). Ten or more 



spherules of crystals, about 1.5 /u.m in diameter, 

 were aggregated at the center of the core region. 

 Some of them had the relatively finely organized 

 core surrounded by coarse granules along the 

 margin (Figure 2B). The peripheral region of the 

 aggregate of the spherules was often deeply etched 

 and appeared as a hole. 



Correlation between the Number of 



Otolith Rings and Age in 



Days after Hatching 



The number of rings (incremental zones) was 

 counted from preparations of otoliths from fish 

 19 and 28 d after hatching. In this case, unclear 

 and narrow rings in the core region were not 

 counted because they seemed to be formed during 

 the embryonic stage. Sixteen otoliths from 8 fish 

 19 d after hatching and 14 otoliths from 7 fish 28 d 

 after hatching showed 18.89 ±0.23 and 

 27.12 ±0.27 (mean ± SE) rings, respectively. The 

 observed close correlations between the number of 

 rings and the age in chronological days indicated 

 that the ring formation proceeded on a daily basis 

 until at least 28 d after hatching. 



Formation of Otolith Rings Under 

 a 12L-12D Photoperiod 



When fish were acclimated to a 12L-12D photo- 

 period of lights-on at 0800 h and lights-off at 2000 



Figure 2. — Scanning electron microscope photographs of otoliths oi Tilapia nilutica 1 d after hatching. A) Core region encircled by the 

 first clear discontinuous zone (arrow). An aggregate of a number of small crystallized spherules is visible in the center. B) Crystallized 

 spherules in the center area of the core region. 



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