MUGIYA: DIURNAL GROWTH OF RAINBOW TROUT OTOLITHS 



1.8- 



1.4 



1.0- 



0.6- 



1000 1600 2200 0400 



Time of day (hours) 



1000 



Figure 5. — Diurnal change in the ratio of ■'•^Ca to ^H-glutamic 

 acid activity incorporated into the same otoliths in rainbow- 

 trout. Each plotted value represents mean ± SE of 8-10 sam- 

 ples. *P < 0.01 for 2200 h. 



matrix deposition on otoliths varied diurnally in 

 antiphase, indicate the relative importance of 

 these substances for daily increment formation in 

 otoliths. The accretion zone is formed predomi- 

 nantly by calcium deposition, while the discontin- 

 uous zone results from reduced calcium and sub- 

 stantially increased matrix deposition on the 

 otoliths. These findings coincide with the morpho- 

 logical observation that the accretion zone is a 

 crystalline layer with organic materials and the 

 discontinuous zone is a layer containing more or- 

 ganic materials and less calcium (Mugiya and 

 Muramatsu 1982; Watabe et al. 1982). Watabe et 

 al. (1982) observed that the matrix fibers and 

 their aggi'egates were morphologically similar in 

 the two zones and continuous throughout in 

 Tilapia and Fundulus otoliths. Based on these 

 observations, they have suggested that the ma- 

 trix materials are identical in the zones and their 

 deposition might be an uninterrupted event dur- 

 ing diurnal otolith growth. They also stated that 

 this did not necessarily imply that the rate of 

 organic matrix secretion was diurnally constant. 

 In fact, the present study reveals that diurnal 

 variations in the rate of the matrix deposition, 

 coupled with variations in calcium deposition, 

 play an important role in otolith increment for- 

 mation. 



Although the sacculus contains the otolith, 

 otolithic membrane, and endolymph, a high con- 

 tent of acidic amino acids is characteristic of the 

 calcified otolith (Degens et al. 1969). Therefore, 

 variations in the uptake of glutamic acid by the 

 saccular tissue should be closely related to the 

 activity of the matrix formation of the otolith, 

 even though glutamate is also used as a neuro- 

 transmitter in the saccular macula (Potter et al. 

 1986). Otolith forming cells, yet to be positively 

 identified (Mugiya 1974; Dale 1976; Dunkel- 

 berger et al. 1980; Saito 1984), synthesize the pre- 

 cursor of the matrix and secrete it into the lumen. 

 The precursor may then deposit on the otolith 

 after further biochemical modification. 



The present study showed the presence of a 

 time-lag between the matrix biosynthesis in the 

 saccular tissue and its deposition on the otoliths. 

 However, this does not necessarily mean that 

 these two processes are separated in phase. In 

 this study, calcification and matrix formation 

 were measured in terms of "instantaneous" 

 gi'owth rates (Ottaway 1978). Therefore the max- 

 imum deposition of organic matrix on otoliths at 

 night must be accompanied by the active biosyn- 

 thesis of the matrix in the cellular level and its 

 consecutive secretion into the lumen, which may 

 rather reduce the radioactivity in the saccular 

 tissue. The high radioactivity in the tissue at 

 dusk might result from the accumulation of the 

 newly synthesized matrix owing to the reduction 

 of its transport to the otoliths. These results sug- 

 gest the presence of at least three different phases 

 in otolith matrix formation: the active synthesis 

 of matrix proteins on the cellular level with its 

 reduced deposition on the otoliths, active synthe- 

 sis with active deposition, and inactivity in both 

 synthesis and deposition. 



Mugiya (1984) reported that the profile of diur- 

 nal otolith calcification was antiphasal between 

 the summer and winter solstices in rainbow trout. 

 In the winter experiment, the peak and the nadir 

 of calcium deposition on otoliths came at 1600 h 

 and 0400 h, respectively; while in the present 

 winter experiment the peak at 1600 h decreased 

 to the nadir earlier, at 2200 h, followed by a slight 

 increase at 0400 h. Although there is a difference 

 in the time-related profiles, the results of both 

 experiments mainly showed that otolith calcifica- 

 tion slowed down after the onset of darkness and 

 remained relatively inactive until the next sun- 

 rise. 



Molluscan nacre shows a laminar structure re- 

 sulting from alternate accumulation of organic 



399 



